At the recent NAVDF congress held this past April in Orlando, our colleagues Drs. Flavia Clare and Clarissa Souza provided a comprehensive overview of sporotrichosis, a rare and largely unknown fungal dermatosis.
Sporotrichosis represents a zoonotic subcutaneous mycosis that primarily affects the dermis, subcutaneous tissues, and sometimes the lymphatic system in various animal species and humans. This fungal condition, classified among anthropozoonoses, is initially an animal pathology accidentally transmissible to humans.
The etiological agent *Sporothrix brasiliensis* is considered the primary cause of a major zoonotic sporotrichosis epidemic that emerged in Brazil at the end of the 20th century. This emerging mycosis raises growing concerns in veterinary and human public health, particularly due to the central role played by domestic cats in its transmission.
Microbiological and ecological characteristics of Sporothrix brasiliensis
Fungal properties and thermal dimorphism
Sporothrix brasiliensis belongs to a group of geophilic fungi exhibiting characteristic thermal dimorphism. At ambient temperature (25-28°C), this microorganism adopts a saprophytic mycelial form, while at body temperature (36-37°C), it transforms into pathogenic yeasts recognizable by their distinctive cigar-shaped morphology. This species integrates into the pathogenic clade of the Sporothrix genus, alongside S. schenckii sensu stricto, S. globosa, and S. luriei, with geographic predominance in South America.
Comparative genomic analyses have revealed a remarkable ecological transition in the Sporothrix lineage, characterized by a shift from plant association to mammalian parasitism. This evolution enhances our understanding of mechanisms by which environmental interactions modulate fungal virulence.
Taxonomy and evolution of the Sporothrix complex
For over a century, the scientific community considered Sporothrix schenckii as the sole etiological agent of sporotrichosis. This monospecific conception prevailed until the last two decades, when molecular technique development enabled identification of multiple distinct species within a taxonomic complex. Currently, mycologists recognize the existence of the Sporothrix schenckii complex, comprising several species with varying degrees of pathogenicity and specific geographic distributions.
Among clinically significant species, S. brasiliensis, S. schenckii sensu stricto, and S. globosa represent the most frequently isolated agents in human and animal infections. These three species present distinct virulence factors that influence clinical severity, tissue fungal burden, histopathological damage, and ultimately associated mortality. S. brasiliensis and S. schenckii are particularly distinguished by their increased virulence compared to other complex members, explaining their predominant epidemiological importance.
The geographic distribution of S. globosa remains largely circumscribed to the Asian continent, where this species historically constituted the predominant etiological agent. S. globosa presents low virulence and poor thermotolerance. Nevertheless, the recent emergence of feline transmission involving S. schenckii in Asia has considerably modified the regional epidemiological landscape, with a progressive shift toward this latter species at the expense of S. globosa. This change illustrates the determining impact of transmission modalities on sporotrichosis epidemiological dynamics.
Transmission modes and epidemiological particularities
Unlike other Sporothrix species that depend primarily on sapronostic transmission, S. brasiliensis is distinguished by its preferential transmission via cats. A fundamental distinction between these transmission modes lies in the inoculated morphotypes: hyphae and conidia in sapronoses, versus yeasts in cases of feline transmission. This morphological difference is critically important in understanding pathogenesis and respective virulence of these infection pathways.
Classic or environmental transmission of sporotrichosis occurs through traumatic inoculation of the fungus present in soil, vegetation, or decomposing organic matter. This infectious modality has earned sporotrichosis various historical designations, notably “gardener’s disease,” “horticulturist’s disease,” or “rose disease.” Professions exposing to frequent contact with the external environment – farmers, gardeners, hunters, foresters – traditionally present increased occupational risk. Inoculation typically occurs following cutaneous trauma from plant thorns, splinters, or any other contaminated material.
Sporotrichosis infection can occur through inter-animal transmission (cat-cat or cat-dog) or zoonotic transmission (cat-human), primarily associated with scratches or bites from infected animals. This predominance is likely explained by the high concentration of fungal organisms present in feline lesions. Additionally, since the fungus naturally colonizes soils and plant surfaces, transmission can also result from contact with contaminated plant structures following trauma from punctures or cuts in animals or humans.
Zoonotic transmission and feline epidemiological role
The emergence of feline transmission has fundamentally transformed sporotrichosis epidemiology, creating a new infectious paradigm designated as “cat-transmitted sporotrichosis.” Infected cats can transmit the pathogen through multiple routes: scratches, bites, contact with lesional exudates, and possibly through respiratory droplets in cases of severe nasal involvement. Remarkably, some documented cases demonstrate transmission through simple licking, without apparent cutaneous trauma, highlighting the efficiency of this propagation mode.
The exceptional transmissive capacity of cats is explained by several convergent biological and behavioral factors. First, feline sporotrichosis lesions contain a considerably higher fungal load than observed in any other animal species, facilitating inoculation during even minor contacts. Second, natural feline behaviors – territorial roaming, interspecific fights, mating, hunting potentially infected rodents, defecation in soil – multiply opportunities for pathogen contact and subsequent transmission. Third, the close proximity between domestic cats and humans creates optimal conditions for zoonotic transmission.
Cases of asymptomatic carriage have also been documented, where apparently healthy cats harbored Sporothrix in their claws and oral cavity, thus constituting unsuspected infectious reservoirs. This observation underscores the importance of increased vigilance when handling any cat from endemic areas, regardless of their clinical presentation.
In Brazil, S. brasiliensis is frequently associated with feline infections (96.9%) and manifests increased virulence during epizootics, also confirmed by murine infection models. A notable characteristic of S. brasiliensis infection lies in its propensity to provoke epidemic foci among feline populations, with considerable zoonotic potential. This exacerbated virulence in mice is also found in human hosts, where S. brasiliensis is associated with atypical and severe forms of the disease, including disseminated cutaneous infections in immunocompetent individuals and systemic involvement.
Fomite transmission and environmental persistence
Recent research has established Sporothrix‘s ability to persist on various inanimate surfaces for extended periods, thus identifying a concerning additional transmission route. Viability studies have demonstrated that the fungus remains infectious for 10 days on wood, 12 days on tiled surfaces, 16 days on textiles, and up to 25 days on stainless steel. These data raise major concerns regarding potential contamination of veterinary environments, particularly clinics, surgical suites, and medical equipment.
This prolonged environmental persistence necessitates establishing rigorous disinfection protocols in veterinary establishments likely to receive infected animals. Surgical instrument contamination represents a particularly concerning issue, with several documented cases of iatrogenic post-operative transmission despite apparently adequate sterilization procedures. These observations suggest that conventional sterilization protocols might prove insufficient for complete eradication of fungal spores, possibly necessitating reinforced decontamination approaches.
Geographic distribution and epidemiological scope
Situation in Brazil: a hyperendemic country
Feline sporotrichosis epidemics caused by Sporothrix brasiliensis have been extensively documented in southeastern Brazil, particularly in Rio de Janeiro, São Paulo, Minas Gerais, and Espírito Santo, with similar patterns emerging in southern Brazil and isolated cases in the northeast region. The Rio de Janeiro epidemics, initiated in the 1990s, show continuous growth with over 4,500 human cases and nearly 5,000 feline cases recorded. Zoonotic transmission involving S. brasiliensis remains largely confined to Brazil, with rare cases in Argentina, while S. schenckii has been documented worldwide, including the United States, Mexico, and Malaysia. These epidemics position S. brasiliensis as a growing threat to human and animal health in Brazil and beyond.
Since the first documented cases of S. brasiliensis transmitted by cats in Rio de Janeiro around 2000, over 12,000 human cases have been recorded in this state through 2017. In 2018, cases of cat-transmitted sporotrichosis had spread further, highlighting the need for thorough understanding of this particular zoonosis. Brazil is now considered a hyperendemic country for this mycosis, a situation resulting from the domestic cat’s entry into the transmission chain, transforming sporotrichosis into a rapidly spreading epizootic disease.
Chronological analysis of Brazilian sporotrichosis emergence reveals alarming progression. During the 1950s-1960s, Brazil presented sporotrichosis incidence comparable to other world regions, with sporadic cases primarily attributable to classic environmental transmission. This stable epidemiological situation persisted until the 1990s, a pivotal period marking the beginning of the current health crisis. The increasing involvement of domestic cats in transmission coincided with an unprecedented epidemic explosion, incidence curves demonstrating exponential growth from this period.
The progressive substitution of S. schenckii by S. brasiliensis as the predominant species constitutes a remarkable epidemiological phenomenon. Until the 1990s, S. schenckii represented the quasi-exclusive etiological agent of Brazilian sporotrichosis cases. Identification of S. brasiliensis in the late 1990s and early 2000s marked a decisive turning point. The superior virulence of this emerging species conferred a determining competitive advantage, leading to its establishment as the dominant agent. Today, the vast majority of Brazilian sporotrichosis diagnoses identify S. brasiliensis, testifying to its absolute epidemiological hegemony.
International expansion and emerging cases
The geographic expansion of S. brasiliensis gradually extends throughout Latin America. Many South and Central American countries have begun identifying cases not only in cats but also in humans. Recent cases have been reported in the United Kingdom following importation of a cat from Brazil, demonstrating the risk associated with international movement of potentially infected animals.
The British case perfectly illustrates dangers inherent to contemporary international animal mobility. A Brazilian family immigrating to the United Kingdom brought their apparently healthy cat, which subsequently developed sporotrichosis lesions. The animal infected both owners and the consulting veterinarian through scratching, creating a zoonotic transmission focus in a region previously free of S. brasiliensis. This situation highlights the inadequacy of current sanitary barriers to prevent introduction of emerging pathogens via pet trade and movement.
In the United States, although cat-transmitted sporotrichosis has long been considered rare, an increase in cases is observed. Documented situations in Kansas and Oklahoma have involved cats infecting veterinarians, technicians, and owners through scratches, illustrating the real zoonotic potential of this fungus. While the United States has historically encountered sporotrichosis cases primarily linked to environmental transmission, the increasing involvement of cats in transmission marks a concerning epidemiological evolution.
Detailed epidemiological cases in the United States
The documented Kansas case in 2022 offers a paradigmatic example of feline transmission dynamics. A two-year-old pregnant cat with an indoor-outdoor lifestyle developed cutaneous lesions of initially undetermined etiology. Molecular analyses conducted by the CDC formally identified the causative agent as Sporothrix schenckii sensu stricto, not S. brasiliensis. Local veterinarians’ unfamiliarity with feline sporotrichosis led to inappropriate initial antibiotic prescriptions without therapeutic effect. Lesional progression continued inexorably for several weeks, extending to various body regions with progressive clinical deterioration.
Parturition occurred during this period, producing a litter of apparently healthy kittens. Approximately one month after the first clinical signs appeared, cytological examination finally revealed the presence of characteristic yeast-like structures of Sporothrix. Initiation of itraconazole treatment was complicated by the animal’s gestational state, raising legitimate concerns about potential teratogenicity. Nevertheless, parturition having likely occurred during the final gestational stages at treatment initiation, the newborns showed no apparent abnormalities.
Despite initial clinical improvement under itraconazole, the animal never achieved complete resolution. Addition of terbinafine to the therapeutic protocol did not substantially modify the unfavorable clinical evolution, ultimately leading to euthanasia. This fatal outcome contrasts remarkably with the second case from the same household, another cat that also developed lesions several months later. This second animal, diagnosed early thanks to increased veterinary awareness, responded favorably to itraconazole monotherapy, achieving clinical cure after four months of continuous treatment.
Comparative analysis of these two cases suggests several determining prognostic factors. Early diagnosis with rapid therapeutic initiation in the second animal, contrasting with the first case’s diagnostic delay, likely constitutes a decisive element. The gestational state and associated immunological modifications during gestation in the first animal could also have compromised therapeutic efficacy. These observations highlight the critical importance of early clinical recognition and rapid diagnosis to optimize chances of therapeutic success.
The zoonotic dimension of this case manifested when the veterinary technician handling the first cat sustained a scratch. Initially minor, this injury evolved after one to two weeks into an erythematous crusting lesion, rapidly progressing to a deep ulcer accompanied by regional lymphadenopathy and ascending lymphangitis characteristic of the classic lymphocutaneous form in humans. Zoonotic sporotrichosis diagnosis was confirmed, requiring itraconazole treatment for eight to nine months before complete resolution. This situation strikingly illustrates the substantial occupational risk to which veterinarians and their staff are exposed when handling potentially infected cats.
A similar case occurring in Oklahoma the same year also involved a cat transmitting sporotrichosis to both the veterinarian and owner through scratching, reinforcing the observation of increasing incidence of this transmission modality on U.S. territory. These reports, while numerically limited, signal a concerning trend suggesting modification of the North American epidemiological landscape.
More recently, a suspected case in Boston involved a patient bitten by a cat of Brazilian origin, subsequently developing lesions compatible with sporotrichosis. The patient was bitten by a sick cat in Minas Gerais state in Brazil before traveling to Massachusetts. Although formal identification of S. brasiliensis was not established by molecular analyses, the epidemiological context – cat imported from Brazil, zoonotic transmission – strongly suggests involvement of this highly pathogenic species. This situation raises the alarming possibility of S. brasiliensis introduction on U.S. territory, with the considerable public health implications this entails.
As of January 2026, there is no confirmed evidence of environmental or enzootic presence of S. brasiliensis in the United States. All cases identified on American soil (like the Boston case) are linked to travel history or importation.Endemic feline sporotrichosis in the USA (notably in California) is caused almost exclusively by S. schenckii.
Worldwide distribution and dispersal factors
Sporotrichosis presents a truly cosmopolitan distribution, with cases reported on all inhabited continents. This geographic ubiquity reflects both worldwide environmental distribution of the fungus and multiplicity of possible transmission routes. Nevertheless, certain regions manifest particularly marked endemicity, creating high incidence foci.
In Asia, S. globosa historically constituted the predominant species, particularly in China, Japan, and India. This distribution likely reflected specific ecological adaptations of this species to Asian environmental conditions. However, the last decade has witnessed a paradigmatic change with the emergence of S. schenckii transmitted by cats. This new epidemiological modality rapidly established itself, progressively supplanting S. globosa as the principal etiological agent in numerous Asian regions. Countries such as Australia, Thailand, and Malaysia have documented this epidemiological transition, with a marked increase in cases of zoonotic sporotrichosis of feline origin.
This modification of Asian epidemiological structure illustrates a fundamental principle of infectious disease epidemiology: competitive superiority conferred by more efficient transmission modes. Feline transmission, allowing rapid and efficient dissemination between mammalian hosts without requiring an environmental intermediate, confers considerable adaptive advantage. This mechanism explains the capacity of S. schenckii and S. brasiliensis to rapidly supplant other Sporothrix species in zones where feline transmission becomes established.
Latin America as a whole is experiencing progressive geographic expansion of S. brasiliensis from its Brazilian focus of origin. The expansion map reveals concentric diffusion from southeastern Brazil toward adjacent regions, then toward neighboring countries. Argentina, Paraguay, Uruguay, and several Central American nations have reported isolated cases or small foci of S. brasiliensis sporotrichosis, often associated with cross-border movements of domestic animals. This continuing geographic progression raises legitimate concerns about potential establishment of pan-Latin American endemicity.
Climatic and environmental factors
Global climate warming constitutes a potentially aggravating factor in sporotrichosis geographic dispersal. Thermally dimorphic fungi present specific environmental requirements for their saprophytic growth and reproduction. Rising average temperatures, particularly in temperate zones previously bordering permissive regions, could extend the environmental distribution area of Sporothrix. Additionally, modifications in precipitation regimes, with intense precipitation episodes followed by prolonged humid periods, create optimal conditions for fungal proliferation in decomposing organic matter.
Extreme climatic events, including floods and hurricanes, can also contribute to pathogen dissemination. These phenomena disturb contaminated soils, dispersing spores and mycelial fragments over vast areas. Post-catastrophe reconstructions, involving manipulation of potentially contaminated materials, expose human and animal populations to increased infectious risks. Several sporotrichosis outbreaks have been historically associated with natural disasters, although systematic documentation of these associations remains incomplete.
Clinical aspects of feline sporotrichosis
Spectrum of clinical manifestations
Cats represent the most affected animals by sporotrichosis, with cutaneous ulcers constituting the primary observed clinical sign. Feline sporotrichosis presents a wide spectrum of clinical manifestations, ranging from isolated cutaneous lesions to severe disseminated systemic forms potentially fatal. The most frequent clinical presentation involves multiple cutaneous lesions with mucosal involvement, particularly affecting nasal mucosa. However, in certain cases, cutaneous lesions may be absent, considerably complicating diagnosis.
Other mucosal sites such as conjunctiva, oral cavity, and genital areas can also be affected. Lymphatic ganglion hypertrophy is frequently observed, while lymphangitis occurs less commonly. Systemic involvement and respiratory symptoms are frequent in cats, often leading to severe cases difficult to treat and potentially fatal. Interestingly, systemic involvement severity in cats does not seem associated with immunodeficiency caused by co-infection with feline retroviruses (FIV and FeLV).
Feline sporotrichosis lesions present characteristic appearance: brilliant or lustrous ulcers, deep, with thickened and raised borders, often accompanied by sero-sanguineous or purulent exudates. Topographic distribution favors the head and extremities, particularly forelimbs. This preferential localization likely reflects anatomical sites most exposed during natural feline behaviors – fights, hunting, environmental exploration.
Nasal involvement carries particular prognostic significance. Cats presenting nasal mucosa lesions frequently develop respiratory signs including sneezing, mucopurulent or sero-hemorrhagic discharge, and nasal obstruction. This latter complication seriously compromises olfactory capacity, leading to severe secondary anorexia. Inability to perceive food aromas suppresses appetite, creating a vicious cycle where malnutrition aggravates immunodepression and compromises therapeutic response. This particularity makes treatment of these cases substantially more difficult and prolonged.
Clinical classification and prognostic implications
Classification of clinical forms is critically important as these aspects determine therapeutic approach and prognosis. When a cat or dog presents only a single cutaneous lesion, this presentation is qualified as localized cutaneous form, associated with better prognosis and reduced treatment duration. Disseminated cutaneous form is characterized by multiple cutaneous lesions, frequently with nasal mucosal involvement. In this type of condition, prognosis is more guarded and treatment requires prolonged duration.
A distinct classification, lymphocutaneous form, is more frequently observed in canines and humans. In this presentation, the lymphatic system manifests ganglion hypertrophy and may present a characteristic beaded pattern, following lymphatic vessel drainage pathways from the initial inoculation site. Systemic form encompasses numerous cases with different presentations including conjunctival, bone, renal, hepatic, or other extra-cutaneous involvement. Lesions typically present as brilliant and deep ulcers with thickened borders, accompanied by sero-sanguineous or purulent exudates.
Prognostic classification fundamentally distinguishes three severity categories. Localized cutaneous forms, representing the least severe cases, generally respond favorably to moderate-duration systemic antifungal treatment. Disseminated cutaneous forms, intermediate category, require prolonged therapeutic protocols with close monitoring. Forms with respiratory involvement or systemic dissemination constitute the most severe presentations, associated with high therapeutic failure rates and substantial mortality even under optimal treatment.
Individual prognostic evaluation must also consider the animal’s general condition. Well-nourished cats living exclusively indoors without comorbidities present favorable prospects even in severe cases. Conversely, stray or semi-stray cats, malnourished, carrying intercurrent diseases, manifest considerably diminished therapeutic response rates. Therapeutic compliance, largely dependent on owner cooperation and patient accessibility, also constitutes a major prognostic determinant.
Pathophysiological mechanisms and feline susceptibility factors
Feline behaviors and transmission dynamics
Many aspects of the emerging mycosis caused by Sporothrix brasiliensis, presenting severe clinical forms in both immunocompetent and immunodepressed hosts, remain poorly understood. Fundamental questions include high cat susceptibility to this fungal species, S. brasiliensis virulence likely linked to its recent introduction in urban feline populations, and mechanisms underlying feline sporotrichosis emergence.
Cat behaviors and lifestyle may partially explain their role as primary sporotrichosis transmitters. Activities such as roaming, living in peridomestic areas, surface scratching, soil defecation, mating, and territorial conflicts (resulting in bites and scratches) facilitate fungal dissemination to other susceptible hosts. Furthermore, cats are common pets with close human contact and constitute primary rat predators.
Studies by Lutz and Splendore demonstrated that rats can acquire sporotrichosis through ingestion, suggesting a possible transmission route by which cats could have been infected by consuming infected rodents, thus allowing the fungus to adapt to specific conditions of feline saliva. This trophic transmission hypothesis proposes a plausible ecological mechanism for initial introduction of Sporothrix in urban feline populations. Rodents, occupying burrows in intimate contact with contaminated soil, would constitute intermediate reservoirs, concentrating the pathogen before transmission to feline predators.
Ecological adaptation and salivary environment
The feline saliva environment, with pH 7.5 to 8.0 and body temperature 37.7 to 39.1°C, resembles conditions found in decomposing plant material, the environment on which Sporothrix species depend for their growth. These conditions, including high temperature and humidity during decomposition and fermentation, can induce metabolic modifications and oxidative stress in the fungus, triggering morphological transformation favoring invasive yeast growth. This host change from plants to animals illustrates complex ecological and evolutionary adaptation meriting thorough investigation.
The remarkable convergence between physico-chemical parameters of the feline oral cavity and those of the decomposing plant microenvironment suggests fortuitous adaptation allowing fungal colonization and multiplication. Slightly alkaline pH, high body temperature of cats (higher than most mammals), and constant humidity create a favorable environment for maintaining Sporothrix in pathogenic yeast phase. This constellation of factors could explain why cats, unlike other animal species, develop exceptionally high fungal loads in their lesions.
Feline saliva also contains various substances potentially nutritive for fungi, including glucose, amino acids, and peptides. The relative absence of effective antifungal components, compared to other species, could contribute to feline permissiveness toward Sporothrix. Complementary studies exploring feline salivary composition and its interactions with fungal metabolism remain necessary to fully elucidate these mechanisms.
Fungal load and transmission
The extraordinarily high fungal load observed in feline sporotrichosis lesions constitutes a distinctive and epidemiologically determining characteristic. Cytological examinations of feline lesions frequently reveal abundant yeast-like structures, both intracellular (within macrophages and neutrophils) and extracellular. This massive proliferation contrasts remarkably with the situation in canines and other species, where fungal organisms prove considerably rarer in lesional specimens.
This quantitative disparity largely explains the ease of cytological diagnosis in cats compared to other species. It also underlies the exceptional transmissive efficacy of infected cats: each scratch, bite, or contact with exudates potentially introduces a massive fungal inoculum, maximizing infection probability in the new host. Quantitative studies have estimated that feline lesions could contain up to 10^6 to 10^7 fungal organisms per gram of tissue, concentration several orders of magnitude higher than observed in canines.
Immunological mechanisms underlying this feline tolerance to such high parasitic loads remain enigmatic. Contrary to expectations, feline sporotrichosis infection is not systematically associated with preexisting immunodepression. Immunocompetent cats frequently develop severe forms with massive fungal loads, suggesting specific deficiency in antifungal defense mechanisms rather than global immunodepression. Co-infections by feline retroviruses (FIV, FeLV), while concerning, do not seem correlated with sporotrichosis clinical severity, refuting the hypothesis of direct causality between retroviral immunodepression and sporotrichosis susceptibility.
The only co-infection demonstrating significant negative impact on clinical evolution concerns respiratory viral infections. Cats suffering simultaneously from sporotrichosis and feline respiratory complex (herpesvirus, calicivirus) present substantially aggravated prognoses. Viral respiratory involvement exacerbates anorexia induced by sporotrichosis nasal obstruction, creating deleterious pathological synergy seriously compromising recovery chances.
Laboratory diagnosis of feline sporotrichosis
Cytopathological and histopathological approaches
In feline species, cytopathology and histopathology prove extremely useful for diagnosis. Nevertheless, laboratory fungal culture of tissue samples and exudates for fungal isolation remains the gold standard and definitive diagnostic method for diagnosing human and feline sporotrichosis. This method demonstrates high sensitivity in both cases, particularly when the sample consists of pus from lesions. However, a negative culture result does not exclude diagnosis.
Cytopathological staining techniques such as Gram, rapid Panoptic, Wright, Giemsa, or Rosenfeld prove particularly sensitive in animals, especially cats. Cytopathological examination of exudates and cutaneous lesions frequently reveals high fungal load, allowing observation of Sporothrix yeast-like cells. These cells appear rounded, oval, or cigar-shaped and are surrounded by a transparent capsular halo, similar to those observed in Cryptococcus spp. and Histoplasma spp. These structures may be located inside macrophages, neutrophils, multinucleated giant cells, or be freely dispersed. In feline sporotrichosis cases, presence of asteroid bodies remains uncommon.
The rapid Panoptic method, a Romanowsky-type staining technique similar to Diff-Quik, has become widely used in veterinary clinics due to its practicality, financial accessibility, and great utility. This diagnostic method presents sensitivity varying from 52.6% to 95% in cats compared to the reference method of fungal culture. However, for non-ulcerated or minimally exudative lesions, this method’s sensitivity may be negatively impacted by high-dose antifungal treatments.
In recent years, laboratory diagnosis of feline sporotrichosis typically begins with cytological examination by lesion impression on glass slides, followed by fungal isolation by culture. Notably, an older diagnostic approach, cell block cytology, has demonstrated impressive 97.5% sensitivity for identifying feline sporotrichosis during epidemics. This technique, although more complex than conventional cytology, offers the advantage of preserved tissue architecture facilitating fungal organism identification and inflammatory response evaluation.
Practical performance of feline lesional cytology is accomplished through various methods depending on lesion nature. For crusty ulcers, gentle removal of hematic crust exposes underlying exudate, allowing direct impression on glass slide. Closed nodular lesions require fine needle aspiration, a technique particularly useful in canines where fungal load proves lower. Hypertrophied lymph nodes also represent diagnostic sampling sites, particularly in lymphocutaneous forms where fungal detection in primary lesions may prove difficult.
Morphological identification of Sporothrix yeasts relies on several distinctive characteristics. The pathognomonic “cigar-shaped” form, although not constant, constitutes the most specific diagnostic criterion. These elongated, fusiform structures, measuring approximately 2 to 6 μm in length, are easily distinguished from round yeasts of Histoplasma capsulatum (smaller) and Cryptococcus spp. (thicker capsule). Morphological variability nevertheless represents a challenge, with some organisms presenting strictly oval forms difficult to differentiate from other pathogenic yeasts.
Inflammatory context provides additional diagnostic clues. Sporotrichosis typically induces mixed pyogranulomatous reaction, associating neutrophils, macrophages, multinucleated giant cells, and lymphocytes. This mixed inflammation, although nonspecific, proves compatible with diagnosis when associated with appropriate yeast-like structures. The relative absence of asteroid bodies in cats contrasts with their frequent presence in human and canine forms, likely reflecting differences in host immunological response.
Cultural confirmation and molecular identification
Confirmation of fungal disease diagnosis is obtained by pathogen isolation in Sabouraud dextrose agar culture media supplemented with cycloheximide (25°C and 37°C), Mycozel® agar medium (37°C), or brain-heart infusion agar (37°C). Ideally, cultures should be performed in duplicate, with one sample incubated at 25°C to observe mycelial growth and another at 37°C to observe yeast growth associated with parasitism. Both samples should be incubated for 14 days to ensure more accurate diagnosis.
Characteristics of isolates incubated at 25°C include initial cream coloration progressively darkening to blackish tones due to melanin production. Microscopically, fine septate hyphae and conidia can be observed. Isolates cultured at 37°C are characterized by creamy appearance, with elongated or oval structures visible under microscopy. For a culture to be considered negative, it should be maintained approximately one month.
In endemic areas, combination of clinical and cytological diagnosis proves highly effective. Depending on lesion location, various materials can be collected to isolate the fungus. Using a swab, samples derived from nasal and lesional secretions and exudates respectively can be collected. Small dermal or mucosal fragments obtained by biopsy, aspirated purulent material, or even blood-tinged content can also be sent for cytology, histopathology, and culture.
In the 21st century, identification of Sporothrix species has become crucial, with PCR serving as the cornerstone of molecular diagnosis. This technique allows pathogenic DNA detection from clinical samples using multiplex assays, achieving impressive sensitivity detecting as few as three copies of the target. Current molecular diagnostic techniques also include LAMP (Loop-mediated isothermal amplification), which appears more specific and sensitive than conventional PCR, while being less expensive.
The importance of specific identification within the Sporothrix complex cannot be overestimated. Therapeutic, prognostic, and epidemiological implications differ substantially according to causative species. S. brasiliensis, with its increased virulence and propensity for severe disseminated forms, requires particular vigilance and possibly intensified therapeutic protocols. Identification of this species in previously free regions also signals potential endemic establishment risk, justifying proactive public health measures.
Serological techniques represent valuable complementary diagnostic and surveillance tools. Anti-Sporothrix antibody assays allow both diagnosis in cases where culture proves negative and monitoring therapeutic response. Decreasing antibody titers under treatment generally correlate with clinical improvement, providing objective marker of therapeutic efficacy. This approach proves particularly useful in disseminated or refractory cases where clinical evaluation alone may prove misleading.
Histopathological diagnosis and differential diagnosis
Histopathological examination of cutaneous or tissue biopsies provides complementary diagnostic and prognostic information. Standard stains (hematoxylin-eosin) reveal general inflammatory architecture, typically pyogranulomatous dermatitis or panniculitis. Special fungal stains, particularly PAS (Periodic Acid-Schiff) and Grocott-Gomori methenamine silver, facilitate identification of fungal structures that may be rare or difficult to distinguish on conventional stains.
Histologically observable fungal load varies considerably according to host species. Feline biopsies usually demonstrate abundant organisms, often visible even on standard stains. Conversely, canine specimens may present rare or absent organisms, necessitating minute examination under special staining. This quantitative disparity reflects lesional fungal load differences between species, confirming cytological observations.
Histopathological differential diagnosis includes various entities producing cutaneous or subcutaneous pyogranulomatous inflammations. Atypical mycobacterioses, cryptococcosis, histoplasmosis, blastomycosis, and certain bacterial infections (nocardiosis, actinomycosis) can mimic sporotrichosis histological appearance. Distinction relies on precise identification of infectious organisms, their morphological characteristics, and special staining profiles. In equivocal cases, microbiological culture and molecular analyses become indispensable.
An intriguing histopathological phenomenon occasionally observed concerns appearance of pseudoneoplastic cellular modifications in severe chronic sporotrichosis cases. Intense chronic inflammation can induce atypical epithelial hyperplasia mimicking squamous cell carcinoma on histological examination. Documented cases in canines have led to erroneous neoplasia diagnoses, with inappropriate oncological therapeutic orientations. Recognition of this phenomenon and systematic search for fungal organisms in any chronic inflammatory lesion with epithelial hyperplasia prove essential to avoid these diagnostic errors.
Therapeutic strategies for feline sporotrichosis
Oral antifungal options and standard protocols
Oral antifungal options available for treating sporotrichosis in cats remain limited. Itraconazole has demonstrated strong in vitro activity against S. brasiliensis strains isolated from cats. However, caution is necessary when correlating in vitro antifungal susceptibility results with in vivo therapeutic results. Itraconazole (100 mg/cat/24h) combined with potassium iodide (2.5 to 5.0 mg/kg/24h) constitute the most frequently used treatments for feline sporotrichosis, with itraconazole remaining the drug of choice.
Potassium iodide also represents an important option for treating cases refractory to itraconazole, particularly for cats presenting nasal mucosal lesions and/or respiratory signs. Its efficacy as monotherapy has been well documented in numerous studies, historically constituting the first effective sporotrichosis treatment before the advent of modern azoles.
For cats weighing 3 kilograms or more, the recommended itraconazole dose is 100 milligrams per cat per day. For cats weighing between 1 and 3 kg, the dose will be 50 milligrams per cat per day, while for cats under 1 kg, dosage is established at 25 mg per kilogram per day. Commercial itraconazole should be preferred over prepared formulations. Potassium iodide, at a dose of 2.5 to 5.0 mg per kilogram once daily, will be prepared extemporaneously. If the animal does not respond to treatment, increasing potassium iodide dose to 10-20 mg per kilogram per day may be considered, although this therapeutic escalation is frequently associated with increased side effects.
Administration of itraconazole with food proves essential to optimize its absorption. A practical strategy consists of removing the cat’s food overnight, then early in the day, mixing itraconazole (the capsule can be opened) with a small amount of wet cat food, before providing the complete meal later. This approach minimizes risk of owner exposure to scratches or bites during direct medication administration, thus reducing zoonotic potential.
Cure criteria and therapeutic duration
The cure criterion for feline sporotrichosis remains clinical, requiring complete resolution of all signs. Treatment should be continued for an additional month after clinical cure for localized cutaneous forms. For cats presenting lesions (cutaneous and/or mucosal) in the nasal region and/or respiratory symptoms, treatment should be prolonged by two months after clinical cure to reduce recurrence risk. Clinical cure can be achieved regardless of initial clinical presentation or co-infection with FIV and/or FeLV. Nevertheless, recurrences after clinical cure may occur, suggesting potential for lesional reactivation even after treatment conclusion.
Median treatment duration extends between four and six months, a considerable period imposing substantial financial and logistical constraints on owners. This therapeutic prolongation constitutes one of the main causes of treatment abandonment, with individual health and public health consequences this implies. Cats whose treatment is prematurely interrupted remain potential sources of zoonotic contamination, perpetuating the epidemiological cycle.
Cats presenting elevated transaminase levels may benefit from hepatoprotective therapy, such as oral silymarin (30 mg/kg, once daily) or S-adenosylmethionine (SAMe) (20 mg/kg, once daily). Therapeutic compliance represents a major challenge given median treatment duration extending between four and six months. This temporal and financial constraint sometimes leads to treatment abandonment by owners or, in the most unfavorable cases, abandonment of the animal itself.
Therapeutic resistance and alternatives
Despite its efficacy, an increasing number of itraconazole-resistant strains has been reported over time. It is important to note that generic itraconazole constitutes a viable alternative to the reference drug, but prepared itraconazole formulations are not bioequivalent and are not recommended for therapeutic use. These observations underscore the necessity for careful selection and monitoring of antifungal therapies in feline sporotrichosis cases. Comparative studies have demonstrated that human-use itraconazole proves superior to veterinary formulations, whether commercial or prepared. Oral liquid formulations have proven less effective than capsules in the Brazilian context.
Therapeutic failures result from several factors: inappropriate veterinary conduct with insufficient dosages, non-compliance with required long treatment duration, high financial cost leading to treatment abandonment, and variable quality of available itraconazole formulations. Patient clinical condition also influences prognosis: cats living exclusively indoors, well-nourished and without comorbidities present better cure prospects compared to stray cats not receiving daily medication.
For refractory or recurrent cases presenting a single residual cutaneous lesion, combination of itraconazole capsules with intralesional amphotericin B (5 milligrams per cat, once weekly for five weeks) may be considered. Cryotherapy and photodynamic therapy also constitute promising adjuvant approaches. For animals presenting multiple cutaneous lesions and/or mucosal lesions with persistent respiratory signs, treatment will combine itraconazole with high-dose subcutaneous amphotericin B.
Emergence of itraconazole-resistant strains represents a growing veterinary public health concern. Case series documenting complete therapeutic failures despite appropriate protocols suggest existence of authentic resistance mechanisms in certain S. brasiliensis isolates. In vitro antifungal susceptibility analyses confirm high minimum inhibitory concentrations in these problematic strains. Factors contributing to resistance emergence primarily include therapeutic abandonments with prolonged exposure to sub-therapeutic doses, creating selection pressure favoring resistant strains.
Intriguing morphological observations have been reported in lesions of cats refractory to treatment. Cytological examinations reveal Sporothrix yeasts presenting increased polymorphism, with enlarged structures and thickened capsules. This morphological modification, although not formally established as a resistance marker, empirically associates with unfavorable prognoses and therapeutic failures. The hypothesis of structural adaptations conferring antifungal resistance merits thorough investigation.
Adjuvant therapies and innovative approaches
Photobiomodulation (low-level laser therapy) and photodynamic therapy represent promising adjuvant therapeutic modalities for refractory sporotrichosis. Photobiomodulation uses specific wavelengths of laser light to stimulate tissue healing processes and modulate inflammatory response. Brazilian studies have documented accelerated lesional resolution when this modality is combined with systemic itraconazole, particularly for persistent isolated cutaneous lesions.
Photodynamic therapy involves topical application of photosensitizing compounds followed by exposure to specific activation light, generating reactive oxygen species exerting direct antifungal effects. This approach, although requiring specialized equipment and technical expertise, offers the advantage of powerful local antifungal effect without systemic toxicity. Its primary utility lies in treating refractory residual lesions after months of systemic antifungal therapy.
Cryosurgery represents another adjuvant modality applicable to localized cutaneous lesions. Application of liquid nitrogen or other cryogenic agents induces tissue destruction through intracellular crystallization, physically eliminating infected tissues. This approach can accelerate resolution of persistent nodules, although it generally requires several sessions and must be combined with systemic antifungal treatment to prevent dissemination.
Amphotericin B, reference polyene antifungal for severe systemic mycoses, finds selective applications in refractory sporotrichosis. Intralesional administration allows high tissue concentrations with minimal systemic toxicity, an approach particularly useful for persistent single lesions. High-dose subcutaneous amphotericin B represents a last resort option for refractory disseminated forms, although its use requires close monitoring of renal function and electrolytes.
Particularities of canine sporotrichosis
Clinical presentation and differential diagnosis
In canines, clinical signs present similarities with those observed in cats, although lymphocutaneous form is more frequently encountered. Dogs manifest more pain, fever, and systemic signs compared to cats. Typical presentation includes a first lesion often localized to the nasal level, followed by lymphatic ganglion hypertrophy and characteristic beaded pattern. It should be noted that dogs present residual scarring after clinical cure, unlike cats in which fur regrowth generally masks sequelae.
Fungal load in canine lesions proves significantly lower than observed in cats, making cytological detection more difficult. For this reason, sampling from hypertrophied lymph nodes rather than ulcerative lesions improves chances of fungal detection. Histopathology typically reveals pyogranulomatous inflammation with reduced quantity of yeast-like structures compared to feline samples.
Differential diagnoses include autoimmune diseases and leishmaniasis, particularly when lesions affect nasal mucosa. In some documented cases, chronic inflammation associated with sporotrichosis has induced cellular modifications mimicking carcinoma-type neoplasia during histopathological examination, highlighting this condition’s diagnostic complexity. Dogs have been erroneously oriented toward oncological protocols based on these pseudoneoplastic modifications, underlining the importance of systematic fungal search in any atypical chronic inflammatory lesion.
Adapted therapeutic modalities
Treatment of canine sporotrichosis follows similar principles to cats, with some dosage adjustments. Itraconazole dose for dogs is established between 10 and 15 mg per kilogram per day. Cure criteria remain identical, necessitating treatment continuation for one to two months after complete clinical resolution according to initial involvement severity. Association with potassium iodide also proves beneficial in refractory or disseminated cases.
Lymphocutaneous form, more frequent in canines, generally presents favorable prognosis under appropriate treatment. Characteristic ascending progression along lymphatic vessels generally allows orderly and predictable resolution under antifungal therapy, with lesions regressing in reverse order of their appearance. Severe disseminated forms nevertheless remain problematic, requiring prolonged and aggressive therapeutic protocols.
Zoonotic transmission and public health implications
Documented cases of feline-human transmission
Zoonotic transmission of sporotrichosis represents a major public health concern. Documented cases demonstrate that scratches, bites, and even simple contact with exudates from infected cats constitute efficient transmission routes. More unusual situations, such as transmission through licking in absence of apparent cutaneous trauma, have also been reported. In some cases, apparently asymptomatic cats have been able to transmit the disease, suggesting fungal carriage in claws and oral cavity even in absence of visible lesions.
Brazilian epidemiological studies have established that 70% of infected cat owners have contracted the disease from their animals. Immediate isolation of diagnosed cats therefore constitutes the first indispensable preventive measure. Veterinarians, technicians, and owners represent populations most exposed to zoonotic transmission risk, justifying rigorous protection measures when handling suspected or confirmed animals.
Human lesions typically present classic lymphocutaneous form, with initial nodule at inoculation site followed by ascending lymphangitis with beaded ganglion hypertrophy. Treatment in humans also requires itraconazole over prolonged periods, possibly extending over eight to nine months. General practitioners may confuse these lesions with furunculosis or other common dermatological conditions, hence the importance of thorough questioning seeking exposure to sick cats.
Fomite transmission and environmental contamination
Recent research has demonstrated Sporothrix fungal persistence on various inanimate surfaces: 10 days on wood, 12 days on tile, 16 days on textiles, and up to 25 days on stainless steel. These data raise major concerns regarding veterinary clinic contamination and the need for rigorous disinfection protocols. Fomite transmission, although less frequent than direct transmission, constitutes a potential route requiring particular attention, notably when handling surgical instruments. Cases of iatrogenic transmission have been documented, including after orchidectomy procedures despite apparently adequate material sterilization.
Current challenges and future perspectives
Global public health issues
Several factors contribute to geographic expansion of cat-transmitted sporotrichosis. Climate change, with increasing temperatures and precipitation, may favor fungal proliferation in the environment. The growing number of pets traveling internationally, without adequate sanitary barriers, facilitates introduction of pathogenic strains into previously free regions. Recent documented cases in the United Kingdom, Europe, and United States following importation of Brazilian cats illustrate this problem.
Emergence of antifungal resistance represents a major public health concern. Documented cases of S. brasiliensis infections presenting high minimum inhibitory concentrations for itraconazole result primarily from therapeutic abandonment and treatment conduct errors, particularly non-compliance with required prolonged duration. This situation justifies research for new antifungal molecules. Over the last decade, discovery and drug repositioning efforts have identified several molecules presenting antifungal potential, notably hydrazone derivatives demonstrating significant in vitro and in vivo activity.
Preventive strategies and epidemiological control
Control of feline sporotrichosis requires a multifactorial approach integrating several intervention levels. At individual level, maintaining cats in exclusively indoor environments constitutes the most effective preventive measure to limit their exposure to environmental fungus and infected congeners. Stray feline population sterilization programs contribute to reducing risky behaviors such as territorial fights and roaming.
Thorough clinical examination of animals before any international movement, ideally including cytological examinations of any suspicious lesion, would prevent introduction of pathogenic strains into free regions. Establishment of epidemiological surveillance systems with mandatory case notification would facilitate precise incidence quantification and early detection of emerging epidemics. Collaboration between veterinarians, physicians, mycologists, and epidemiologists proves indispensable for an effective “One Health” approach to this complex zoonosis.
Diagnostic and therapeutic innovations
Recent advances in molecular diagnosis, particularly LAMP technique, offer promising prospects for rapid, sensitive, and economically accessible sporotrichosis diagnosis. Precise species-level identification becomes crucial for epidemiological surveillance and detection of S. brasiliensis introduction into new geographic areas. Serological tests allowing diagnosis and therapeutic response monitoring constitute valuable complementary tools, particularly in cases where fungal culture proves negative.
Therapeutically, photobiomodulation and photodynamic therapy represent promising adjuvant approaches, particularly for refractory lesions. These modalities could accelerate clinical resolution and potentially reduce total systemic antifungal treatment duration. Intralesional amphotericin B offers an option for persistent isolated cutaneous lesions, although its systemic use requires close monitoring due to potential toxicity.
Conclusion
Sporotrichosis caused by Sporothrix brasiliensis represents an emerging zoonotic mycosis of growing importance in veterinary medicine and public health. The central role of domestic cats in this condition’s epidemiology marks a paradigmatic change from the classic environmental transmission pattern. S. brasiliensis‘ increased virulence compared to other Sporothrix complex species, combined with its propensity to generate epidemics among feline populations with considerable zoonotic potential, justifies particular vigilance from veterinary professionals.
Early diagnosis, based on a combined approach integrating history, clinical examination, cytology, and fungal culture, allows rapid institution of appropriate treatment. Itraconazole remains the treatment of choice, with prolonged therapeutic durations essential to prevent recurrences. Recognition of therapeutic failure predictive factors and adaptation of protocols accordingly optimize cure chances.
Prevention of zoonotic transmission requires owner education regarding risks associated with infected cats and appropriate isolation measures. Veterinarians, on the front line against this pathology, must maintain high awareness of its zoonotic potential and rigorously apply protection measures when handling suspected animals. Progressive geographic expansion of S. brasiliensis from its Brazilian focus to other continents underscores the urgency of establishing international surveillance systems and effective sanitary barriers to prevent its establishment in new regions.
Future research must elucidate precise mechanisms of S. brasiliensis adaptation to feline hosts, identify new therapeutic targets to counter emerging antifungal resistance, and evaluate effectiveness of stray feline population control strategies in endemic areas. The “One Health” approach integrating human health, animal health, and environmental health emerges as the necessary conceptual framework to effectively manage this complex zoonosis with multidimensional implications.
Sporotrichosis: Epidemiological and Clinical Approach. C Souza. NAVDF 2025
Clinical Brazilian Perspective. F Clare. NAVDF 2025