In veterinary dermatology, afoxolaner, an isoxazoline administered orally in dogs, has transformed the management of demodicosis, sarcoptic mange, and infestations by fleas and ticks in particular. This article provides a comprehensive review ranging from pharmacology to the latest publications, covering both Nexgard and Nexgard Spectra.

 

Chapter 1 — Pharmacology of Isoxazolines: Afoxolaner and the Afoxolaner-Milbemycin Oxime Combination

1.1 Pharmacological Class and Structural Data

Afoxolaner belongs to the isoxazoline family, a pharmacological class of systemic insecticides and acaricides developed over the past decade. This class also includes fluralaner, sarolaner, and lotilaner, each presenting distinct pharmacokinetic properties but a common mechanism of action on the nervous system of arthropods. Afoxolaner has a fluorinated structure conferring high lipophilicity and a strong affinity for GABA receptors in invertebrates (Shoop 2014). The molecule was initially developed by Merial laboratories, now integrated into Boehringer Ingelheim Animal Health. The positioning of afoxolaner within the class is defined by its monthly administration interval, intermediate between sarolaner (monthly) and fluralaner (tri-monthly), and by its exclusive formulation as a chewable tablet for dogs, unlike fluralaner which is also available in a topical or injectable formulation. Afoxolaner (NexGard) and fluralaner (Bravecto) simultaneously received their European marketing authorization in February 2014, becoming the first representatives of the isoxazoline class marketed in Europe, followed by sarolaner and lotilaner.

The arrival of isoxazolines has led to a very significant decrease in the prevalence of certain parasitic dermatoses such as demodicosis

The pharmaceutical form chosen for marketing is a beef-flavored chewable tablet, whose palatability was evaluated in several trials showing voluntary acceptance by the majority of dogs at first presentation (Letendre 2014). If the dog does not directly accept the tablet, it can be mixed with food without significant modification of bioavailability. The product is packaged in boxes containing blisters of chewable tablets for dogs, with a dosage adapted to the dog’s weight. NEXGARD (Boehringer Laboratory) is available in four presentations covering weight ranges of 2 to 4 kg, 4 to 10 kg, 10 to 25 kg, and 25 to 50 kg, delivering an afoxolaner dose of between 2.5 and 6.3 mg/kg of body weight.

NEXGARD SPECTRA (Boehringer Laboratory) combines afoxolaner with milbemycin oxime, a macrocyclic lactone, in a single chewable tablet. This combination extends the antiparasitic spectrum to digestive nematodes, notably Toxocara canis, Ancylostoma caninum, and Trichuris vulpis, as well as the prevention of dirofilariosis caused by Dirofilaria immitis. NEXGARD SPECTRA dosages are adjusted to deliver a minimum dose of 2.5 mg/kg of afoxolaner and 0.5 mg/kg of milbemycin oxime, across five presentations covering weight ranges of 1.35 to 3.5 kg, 3.5 to 7.5 kg, 7.5 to 15 kg, 15 to 30 kg, and 30 to 60 kg. The minimum weight for use of NEXGARD SPECTRA has been lowered to 1.35 kg. Accurate dosing according to weight is a critical element of prescribing and requires systematic weighing before each administration.

The advantage of chewable tablets over topical spot-on formulations lies in several clinically relevant points. Therapeutic compliance is improved by the simplicity of oral administration and the palatability of the product. Efficacy is not affected by bathing, swimming, or exposure to rain, unlike topical formulations whose cutaneous distribution may be compromised by contact with water (Letendre 2014). The risk of transfer of active substance to children or pets sharing the treated dog’s basket is eliminated. In daily clinical practice, these properties facilitate the management of infestations in multi-animal households and in dogs with significant outdoor activity.

1.2 Mechanism of Action

Afoxolaner acts as a non-competitive antagonist of ligand-gated chloride channels, targeting in particular channels gated by gamma-aminobutyric acid (GABA) and by glutamate in arthropods (Ozoe 2013). The blocking of these channels inhibits the pre- and post-synaptic transfer of chloride ions across the neuronal cell membranes of the ectoparasite, leading to uncontrolled hyperexcitation of the central nervous system, followed by paralysis and death of the insect or mite. Fleas and ticks must feed on the treated host to be exposed to afoxolaner circulating in the plasma; the action is therefore systemic and not repellent.

The toxicity selectivity between arthropods and mammals is based on structural differences in GABA receptors. Isoxazolines generally show significantly higher affinity for invertebrate GABA receptors (Weber 2016). However, recent data show that afoxolaner can partially inhibit certain mammalian receptor subtypes, justifying regulatory monitoring. Furthermore, glutamate-gated chloride channels, which constitute a major target for isoxazolines in arthropods, are absent from the nervous system of vertebrates, contributing to the high safety margin of this class (Merola 2012). In vitro experiments have confirmed that afoxolaner is not a substrate of P-glycoprotein (an efflux transporter encoded by the ABCB1 gene), which differentiates it from macrocyclic lactones on a central point regarding interactions at the level of the blood-brain barrier (Drag 2022).

Milbemycin oxime, the endectocide component of NEXGARD SPECTRA, acts through a complementary mechanism by activating glutamate-gated chloride channels (GluCl) present in nematodes and arthropods, causing flaccid paralysis and death of the parasite (Noack 2021). This antiparasitic synergy between the two molecules allows for extended spectral coverage, covering both ectoparasites (fleas, ticks, mites) via afoxolaner and endoparasites (gastrointestinal nematodes, Dirofilaria immitis) via milbemycin oxime.

1.3 Pharmacokinetics in Dogs

After oral administration in dogs, afoxolaner shows rapid and significant digestive absorption. Absolute bioavailability is 74% for NEXGARD and 88.3% for the combination formulation in NEXGARD SPECTRA (Letendre 2014, Letendre 2016). The mean maximum plasma concentration (Cmax) is 1655 +/- 332 ng/mL, reached within 2 to 4 hours (Tmax) after administration of a dose of 2.5 mg/kg. Plasma concentration-time curves follow a two-compartment model and show dose-proportional increases over the range of 1.0 to 40 mg/kg (Letendre 2014).

Tissue distribution is extensive, with a volume of distribution of 2.6 +/- 0.6 L/kg after intravenous administration. Plasma protein binding exceeds 99.9%, explaining the persistence of effective plasma concentrations throughout the entire monthly dosing interval. The terminal elimination half-life is approximately 15.5 +/- 7.8 days in most dogs, which constitutes the pharmacokinetic basis for monthly administration (Letendre 2014). However, this half-life may vary by breed: in Collies receiving a dose of 25 mg/kg, the half-life reached up to 47.7 days, without any observed consequence on safety.

The metabolism of afoxolaner is primarily hepatic, by hydroxylation, leading to a major metabolite identified in the plasma, urine, and bile of dogs. Elimination occurs mainly via the biliary route, with a secondary urinary fraction. Systemic clearance is 5.0 +/- 1.2 mL/h/kg. A strong correlation has been established between plasma afoxolaner concentration and antiparasitic efficacy, with EC90 values of 23 ng/mL for Ctenocephalides felis and greater than or equal to 100 ng/mL for Rhipicephalus sanguineus sensu lato and Dermacentor variabilis (Letendre 2014).

Regarding milbemycin oxime in NEXGARD SPECTRA, Tmax is 1 to 2 hours, oral bioavailability is 80.5% for form A3 and 65.1% for form A4, and the elimination half-life is 1.6 +/- 0.4 days for form A3 and 3.3 +/- 1.4 days for form A4 (Letendre 2016). The absence of significant pharmacokinetic interaction between the two molecules in the combined formulation has been demonstrated.

1.4 Dosage and Administration Conditions in Clinical Practice

The systemic exposure range validated by marketing authorization extends from 2.7 to 7.0 mg/kg of afoxolaner for single tablets (NEXGARD), while the NEXGARD SPECTRA combination delivers a targeted exposure of 2.50 to 5.36 mg/kg of afoxolaner. For NEXGARD SPECTRA, the target dose is 2.5 to 5 mg/kg of afoxolaner combined with 0.5 to 1 mg/kg of milbemycin oxime. The choice of tablet is determined by the corresponding weight range. The influence of food on bioavailability has been evaluated and, although concomitant administration with food may slightly increase absorption, the product can be administered with or without food without clinically significant impact on efficacy.

The European marketing authorization (MA) was granted in 2014 for NEXGARD and in 2015 for NEXGARD SPECTRA by the European Medicines Agency (EMA). The European marketing authorization number corresponds to the centralized procedure. The officially registered indications include treatment of infestations by fleas (Ctenocephalides felis and C. canis) and ticks (Dermacentor reticulatus, Ixodes ricinus, Ixodes hexagonus, Rhipicephalus sanguineus). Since July 2025, NEXGARD and NEXGARD SPECTRA have benefited from two additional official indications: prevention of canine babesiosis, defined as the reduction of the risk of transmission of Babesia canis by Dermacentor reticulatus, and reduction of the risk of transmission of Dipylidium caninum by fleas. For NEXGARD SPECTRA, indications also extend to prevention of dirofilariosis (Dirofilaria immitis) and treatment of gastrointestinal nematode infestations. The shelf life of the product and special storage precautions (temperature below 30 degrees Celsius) are specified in the summary of product characteristics. Special precautions for the disposal of unused medicines or waste materials derive from applicable regulations.

Chapter 2 — Generalized Demodicosis Caused by Demodex canis

2.1 Pathophysiology of Demodicosis: Immune Dysregulation, Breed Factors, and Genetic Predispositions

Demodicosis caused by Demodex canis results from excessive proliferation of the commensal mite Demodex canis within hair follicles and sebaceous glands, facilitated by a dysregulation of T-cell-mediated immunity specifically directed against this mite. The transition from asymptomatic commensal carriage, present in nearly all healthy dogs, to clinical demodicosis involves a deficit in the type 1 T helper lymphocyte response and local immune anergy. The IL-10/TGF-beta signaling pathway is overexpressed in generalized demodicosis lesions, contributing to the suppression of anti-parasitic effector mechanisms (Ferrer 2014). The increase in regulatory T lymphocytes (Treg) CD4+CD25+FoxP3+ in the perifollicular dermis of dogs with generalized demodicosis, compared to healthy controls, has been documented and suggests a central role of local immunosuppression in permissiveness to mite proliferation. Studies using real-time PCR have demonstrated that isoxazolines, at recommended therapeutic doses, do not alter Demodex populations in healthy dogs, confirming that efficacy is linked to the elimination of mites present in pathological excess rather than to eradication of commensal carriage (Silbermayr 2017). The distinction between juvenile demodicosis (onset before 18 months) and adult-onset demodicosis (onset after 4 years) remains clinically relevant: the adult form requires investigation for an underlying immunosuppressive cause (hypothyroidism, hypercortisolism, neoplasia, iatrogenic immunosuppression).

Certain breeds show a marked predisposition to generalized demodicosis. The Staffordshire Bull Terrier, American Staffordshire Terrier, Shar-Pei, and French Bulldog are among the most represented breeds in recent clinical studies (Lebon 2018). In the Shar-Pei, generalized demodicosis frequently manifests as extensive erosive lesions of the interdigital spaces and face, with recalcitrant deep bacterial superinfection. In the American Staffordshire Terrier, the clinical picture typically involves multifocal erythematous alopecia of the trunk and extremities, with generally rapid therapeutic response to isoxazolines (Lebon 2018). The French Bulldog preferentially develops facial and periocular lesions with adherent follicular casts. No specific susceptibility gene has been formally identified for canine demodicosis to date, unlike other genodermatoses such as ichthyosis in the Golden Retriever linked to the PNPLA1 mutation or canine atopic dermatitis associated with filaggrin defects.

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Juvenile demodicosis in a French Bulldog

2.2 Controlled Clinical Efficacy Data: Reduction in Demodex Counts and Parasitological Cure

The efficacy of afoxolaner in the treatment of generalized demodicosis has been demonstrated in several controlled studies. In a laboratory study comparing afoxolaner (NEXGARD, minimum dose of 2.5 mg/kg) to the imidacloprid/moxidectin combination (ADVOCATE ND, Bayer) in 16 dogs with generalized demodicosis, mite count reductions were 99.2%, 99.9%, and 100% at days 28, 56, and 84 respectively in the afoxolaner group, versus 89.8%, 85.2%, and 86.6% in the comparator group (p < 0.05 at each evaluation) (Beugnet 2016a). Improvement in skin lesions was significantly faster in the afoxolaner-treated group.

A European multicenter field study, conducted at veterinary dermatology referral centers in France, Italy, and Poland, enrolled 50 dogs (48 completing the study) with generalized demodicosis, treated orally with afoxolaner (NEXGARD or NEXGARD SPECTRA) three times at monthly intervals (Lebon 2018). The combined efficacy of the two formulations was 79.8% at D28, 93.5% at D56, and 95.8% at D84 in terms of mite count reduction. Severity of skin lesions and pruritus showed significant improvement (p < 0.0001) at D84. Both formulations were well tolerated with no treatment-related adverse events.

A single-blind, randomized, controlled study with a negative control group compared the efficacy of NEXGARD and NEXGARD SPECTRA in 24 dogs with generalized demodicosis (Rehbein 2018). Three monthly administrations of NEXGARD resulted in a 99.9% reduction in mite counts at D84, while NEXGARD SPECTRA achieved 100% efficacy at D84, with complete absence of live mites in skin scrapings from all dogs in this group. Hair regrowth, assessed by clinical scoring, was significantly greater in both treated groups compared to the control group (p < 0.0001).

2.3 Treatment Protocols: Administration Intervals, Cure Criteria, and Minimum Treatment Duration

The recommended treatment protocol for generalized demodicosis is based on monthly oral administration of afoxolaner (NEXGARD, 2.5 mg/kg or NEXGARD SPECTRA, 2.5 mg/kg afoxolaner and 0.5 mg/kg milbemycin oxime) for at least three consecutive months. Parasitological cure is defined as obtaining two series of negative deep skin scrapings, performed one month apart, at all previously positive lesional sites. Since the biological cycle of Demodex canis extends over a period of 18 to 24 days, a single negative scraping cannot be considered a reliable indicator of complete remission (Beugnet 2016a).

In practice, the majority of dogs require three to four months of treatment to achieve parasitological cure. Severe cases with associated deep pyoderma may require a longer duration. Concomitant antimicrobial treatment of bacterial superinfections is essential and should be guided by bacterial culture with antibiotic susceptibility testing in refractory forms. Monthly clinical monitoring, including examination of skin scrapings, photodocumentation of lesions, and evaluation of the lesion score, forms the basis of therapeutic follow-up.

Chapter 3 — Sarcoptic Mange Caused by Sarcoptes scabiei var. canis

3.1 Pathophysiology of Sarcoptiform Hypersensitivity and Diagnostic Challenges in General Practice

Sarcoptic mange caused by Sarcoptes scabiei var. canis is a highly contagious parasitic dermatitis characterized by intense pruritus, crusted papules, and extensive alopecia. The pathophysiology is based on a type I and type IV hypersensitivity reaction directed against mite antigens and their excretory products. Anti-Sarcoptes specific IgE induce early mast cell degranulation, while the delayed cellular component sustains chronic cutaneous inflammation, with eosinophilic and lymphocytic perivascular infiltrate. The oto-pedal reflex, although not pathognomonic, has a sensitivity of approximately 80% in typical presentations.

Diagnosis remains a challenge in general practice because demonstration of the mite by skin scrapings is obtained in fewer than 50% of infested cases, due to the low parasitic load sufficient to induce a clinically significant immune response. The scraping technique should favor areas of transition between healthy and lesional skin, particularly the edge of ear pinnae, elbows, and hocks, where the probability of finding mites is greatest. Examination of more than 90% of dogs presenting typical clinical signs of sarcoptic mange allowed identification of Sarcoptes scabiei as the etiological agent in a screening study of 106 dogs (Hampel 2018). Anti-Sarcoptes serology by ELISA is a complementary diagnostic tool, with reported sensitivity of 84 to 92% and specificity of 90 to 96% depending on the study, but the seroconversion delay of 2 to 5 weeks after infestation limits its usefulness in very recent presentations. Empirical anti-sarcoptic treatment frequently serves as an indirect diagnostic tool when scrapings remain negative.

Sarcoptic mange in an English Bulldog

3.2 Comparative Efficacy of Afoxolaner Alone and the Afoxolaner-Milbemycin Oxime Combination: Quantitative Data

The first field study validating the efficacy of afoxolaner against sarcoptic mange was published in 2016 (Beugnet 2016b). In this randomized controlled trial, 20 dogs naturally infested with Sarcoptes scabiei var. canis, confirmed by skin scrapings before inclusion, were divided into two equal groups; the treated group received NEXGARD (minimum dose of 2.5 mg/kg) at D0 and D28. Treated dogs showed mite counts significantly lower than untreated controls at D28 and D56 (p < 0.001), with a complete absence of mites in scrapings at both time points (100% efficacy). Lesion resolution was significantly better at D56: no treated dog showed pruritus (versus 7/10 in the control group), 1/9 treated dog retained crusting (versus 5/10 controls), and 8/9 treated dogs had recovered 90% of their coat in previously lesional areas (versus 0/10 controls).

A multicenter clinical field study conducted in Portugal and Germany evaluated the efficacy of NEXGARD and NEXGARD SPECTRA in 80 dogs naturally infested with Sarcoptes scabiei var. canis, of which 65 cases were evaluable at the end of the study (Hampel 2018). Dogs received two oral administrations one month apart according to the MA label instructions. The reduction in geometric mean counts of live mites was 98.9% and 99.7% for dogs treated with NEXGARD (n = 38) at one and two months respectively, and 99.6% and 100% for dogs treated with NEXGARD SPECTRA (n = 27) at the same time points (p < 0.001 for both treatments). Both treatments resulted in significant improvement in pruritus, papules and crusts, and alopecia at one and two months after treatment initiation (p = 0.0001). No treatment-related adverse effects were observed in dogs that completed the study.

3.3 Treatment Protocols, Management of In-Contact Animals, and Prevention of Re-infestation

The recommended protocol for sarcoptic mange caused by Sarcoptes scabiei var. canis consists of two oral administrations of NEXGARD or NEXGARD SPECTRA one month apart. The rapid acaricidal action of afoxolaner allows significant reduction of the parasitic load from the first month of treatment, with resolution of pruritus often notable within 7 to 14 days following the first administration. Animals in contact in the same household must be treated simultaneously to prevent re-infestation, as the contagiousness of Sarcoptes scabiei var. canis is high through direct contact. Environmental treatment is generally not necessary due to the low survival of the mite off the host (a few days at room temperature), but washing the dog’s bedding and basket is recommended. Sarcoptic mange is a potential zoonosis: human lesions, localized to areas of contact with the infested animal, resolve spontaneously after treatment of the definitive host.

Chapter 4 — Ear Mite Infestation Caused by Otodectes cynotis

4.1 Pathophysiology of Auricular Infestation and Associated Periauricular Skin Lesions

Otodectes cynotis is a highly contagious, non-burrowing surface mite that primarily colonizes the external ear canals of dogs and cats. Its complete life cycle, lasting approximately 21 days, occurs entirely on the host. The mechanical and immunological irritation caused by the presence of mites and their excretions induces a pruriginous ceruminous external otitis, with excessive production of dark brown cerumen. The prevalence of ear mite infestation varies across studied populations, but Otodectes cynotis is one of the most frequent etiological agents of external otitis in puppies and young dogs. In severe infestations, mites can be found outside the ear canal, on the head, back, and tail of the animal, causing pruriginous erythematous and papulo-crusted periauricular skin lesions. The local immune response involves a type I hypersensitivity component with mast cell degranulation, explaining the intensity of auricular pruritus that is sometimes disproportionate to the parasitic load. Bacterial and fungal superinfections, notably with Malassezia pachydermatis, frequently complicate the clinical picture and require specific therapeutic management in addition to acaricidal treatment.

4.2 Efficacy Data for NexGard Spectra in Canine Ear Mite Infestation: Results from Clinical Studies

The efficacy of afoxolaner against Otodectes cynotis has been evaluated in several experimental and field studies. A first controlled laboratory study demonstrated that a single oral administration of afoxolaner at the minimum dose of 2.5 mg/kg results in a 98.5% reduction (geometric means) and 99.4% reduction (arithmetic means) in live mite counts at D28, compared to the untreated control group (p < 0.05) (Carithers 2016). At D28, two out of eight dogs in the treated group still carried a residual number of mites (1 and 4 mites respectively), versus seven out of eight in the control group.

A field study conducted in Italy on 20 naturally infested dogs confirmed these results: two monthly oral administrations of afoxolaner (NEXGARD) achieved 100% efficacy based on live mite counts at D42 after auricular rinsing (Panarese 2021). Improvement in clinical scores (cerumen, auricular debris) was significant from D30. A recent study conducted in Greece on 32 naturally infested dogs compared a single administration of NEXGARD (n = 8), two monthly administrations of NEXGARD (n = 8), and two monthly administrations of NEXGARD SPECTRA (n = 8) against a control group (n = 8). All three treated groups showed a 99.9% reduction in mite counts at D45, with no reported adverse effects (Kostopoulou 2024).

4.3 Combined Therapeutic Strategy: Systemic Treatment and Local Management

Systemic oral treatment of ear mite infestation represents a major advantage over traditional topical auricular treatments, which require repeated instillations that are difficult to administer in painful or uncooperative animals. Afoxolaner, distributed systemically, reaches mites throughout their entire location, including outside the ear canal, limiting the risk of ectopic parasitic persistence. In cases of associated secondary external otitis with bacterial or Malassezia pachydermatis fungal superinfection documented in several studies, complementary local treatment with ear cleaning solution and appropriate topical antibiotic or antifungal product is recommended. Treatment of all animals in the household is imperative due to the high contagiousness of Otodectes cynotis.

Chapter 5 — Flea Control and Flea Allergy Dermatitis (FAD)

5.1 Pathophysiology of Hypersensitivity to Flea Saliva: Immunological Mechanisms

Flea allergy dermatitis (FAD) represents the most frequent allergic dermatosis in dogs in areas of flea endemicity. The pathophysiology is based on hypersensitivity to salivary allergens of Ctenocephalides felis, involving type I (immediate, IgE-dependent), type IV (delayed, T lymphocyte-mediated), and potentially type III (immune complex) mechanisms. The major salivary allergens of the flea, particularly high molecular weight proteins, trigger a cutaneous inflammatory cascade with recruitment of eosinophils, mast cells, and CD4+ T lymphocytes into the superficial dermis. The triggering threshold is extremely low in sensitized animals: a single flea bite may be sufficient to induce a pruritic flare lasting several days. The clinical lesions of FAD are typically concentrated on the lumbosacral region, base of the tail, caudal surface of the thighs, and ventral abdomen, forming a suggestive, though not pathognomonic, clinical picture. Self-induced alopecia, excoriations, and secondary lichenification are the clinical markers of chronicity. In certain predisposed breeds such as the German Shepherd, Labrador, and Golden Retriever, FAD frequently coexists with canine atopic dermatitis, complicating differential diagnosis and therapeutic management.

5.2 Lethal Kinetics Against Ctenocephalides spp. and Impact on Environmental Infestation Dynamics

Afoxolaner exhibits rapid lethal kinetics against Ctenocephalides felis. In a speed-of-action study, NEXGARD demonstrated onset of adult flea mortality as early as 30 minutes after initial administration (Beugnet 2015), with efficacy of 99.7 to 100% between 8 and 24 hours (Kunkle 2014). Curative efficacy against pre-existing infestations is 15.0% at 2 hours and 99.5% at 8 hours (Kunkle 2014), while another study shows induced mortality of 45.7% after 30 minutes of exposure (Beugnet 2015). Preventive efficacy against weekly re-infestations by Ctenocephalides felis was maintained at more than 99% at 24 hours after each re-infestation for 35 days (Hunter 2014). Overall preventive efficacy was maintained at more than 95% within 12 hours of each weekly infestation for at least 3 weeks, and within 24 hours for at least 5 weeks (Beugnet 2015).

The product kills fleas before egg laying, which interrupts the reproductive cycle and prevents contamination of the domestic environment. Since the flea is the intermediate host of the cestode Dipylidium caninum, the rapid elimination of fleas also reduces the risk of infestation by this tapeworm: since July 2025, NEXGARD and NEXGARD SPECTRA carry an official indication for reduction of the risk of Dipylidium caninum transmission by fleas. A field study conducted in Florida demonstrated that monthly treatment with afoxolaner in infested dogs results in a 96.7% reduction in flea counts in the domestic environment at D28-30 and 98.9% at D82-86 (Dryden 2016). Environmental treatment (regular vacuuming, insecticide treatment of surfaces) remains recommended in cases of massive infestation, but regular treatment of the animal constitutes the cornerstone of environmental control. All animals in the household, including cats (with a species-appropriate product), must be treated simultaneously to avoid cross-re-infestations. Immature flea stages (eggs, larvae, pupae) infest the dog’s bedding, basket, and regular sleeping areas.

5.3 Role of Afoxolaner in the Multimodal Therapeutic Strategy for FAD

Strict control of flea exposure constitutes the etiological treatment of FAD. Afoxolaner, through its rapid lethal kinetics (elimination of adult fleas before egg laying within 8 to 24 hours following the blood meal), reduces the salivary antigenic load to which the allergic dog is exposed. The FDA approved in 2023 the indication extension of afoxolaner for improvement of clinical signs of flea allergy dermatitis (FAD), following a 90-day multisite field study demonstrating resolution of clinical signs in dogs with flea bite dermatitis treated with afoxolaner at a dose of 2.5 mg/kg. In Europe, the integration of afoxolaner into the multimodal therapeutic arsenal for FAD has been officially validated by the EMA since the initial registration of the medication in 2014. The multimodal therapeutic strategy for FAD combines continuous monthly antiparasitic treatment with afoxolaner (continuous protection, including during winter in areas with temperate climates where fleas survive indoors), symptomatic treatment of pruritus during acute phases (short-course corticosteroids, oclacitinib, lokivetmab), and rigorous environmental management. Special precautions for use note that afoxolaner acts systemically after a bite: the risk of transmission of pathogens by fleas is not completely excluded by a non-repellent acaricide/insecticide.

Chapter 6 — Treatment of Tick Infestations

6.1 Dermacentor reticulatus: Geographic Distribution, Seasonality, and Protection Against Piroplasmosis

Dermacentor reticulatus is a three-host tick widely distributed in continental Europe, with documented geographic expansion over the past two decades, facilitated by climate changes and the movement of companion animals. Its seasonal activity shows two peaks, in autumn and spring, with possible winter activity in areas with mild climates. In France, Dermacentor reticulatus is present throughout metropolitan territory, with increased density in humid grassland areas and forest edges. Dermacentor reticulatus is the main vector of Babesia canis in Europe, the causative agent of canine piroplasmosis whose mortality rate reaches 10 to 20% in the absence of early treatment.

The acaricidal efficacy of afoxolaner against Dermacentor reticulatus has been demonstrated in several controlled studies. Oral administration of afoxolaner (NEXGARD) at a mean dose of 2.7 mg/kg eliminated pre-existing infestations with more than 99% efficacy at 48 hours post-treatment and controlled weekly re-infestations with more than 96% efficacy for 30 days against both European species tested (Dermacentor reticulatus and Ixodes ricinus) (Dumont 2014). A controlled experimental study demonstrated for the first time the ability of afoxolaner (NEXGARD) to block the transmission of Babesia canis by Dermacentor reticulatus ticks: none of the treated dogs developed infection after repeated infestations with carrier ticks, whereas all controls developed confirmed clinical babesiosis (Beugnet 2014). This was the first demonstration that an oral acaricide could prevent the transmission of a pathogen despite the necessity for the tick to attach and begin its blood meal before being killed. An analogous blockade of transmission was subsequently established for Babesia rossi carried by Haemaphysalis elliptica, in an African epidemiological context (Beugnet 2019).

More recently, the preventive efficacy of the afoxolaner-milbemycin oxime combination (NEXGARD SPECTRA) against transmission of Babesia canis by Dermacentor reticulatus was confirmed in a blinded, randomized, controlled experimental study (Tielemans 2025). After a single oral administration at D0 at the minimum recommended dose, none of the treated dogs showed clinical signs of babesiosis or PCR or serological positivity after two infestations (D1 and D28) with infected ticks, while all control dogs developed the infection. These data formed the basis of the official indication extension, effective since July 2025, of both NEXGARD and NEXGARD SPECTRA to the prevention of canine babesiosis (reduction of the risk of transmission of Babesia canis by Dermacentor reticulatus).

6.2 Ixodes ricinus: Major Vector Species in France and Reduction of the Risk of Lyme Disease and Anaplasmosis Transmission

Ixodes ricinus is the most frequently encountered tick in Europe, representing up to 59.5% of ticks collected from humans in certain surveillance studies (Ixodes ricinus is identified as the most frequently found species). Its activity extends from March to November, with bimodal spring and autumn peaks. Ixodes ricinus is a vector of Borrelia burgdorferi sensu lato (causative agent of Lyme disease), Anaplasma phagocytophilum, Babesia spp., and flaviviruses responsible for tick-borne encephalitis.

The acaricidal speed of action of afoxolaner against Ixodes ricinus was evaluated in a dedicated study: lethal effect is observed as early as 12 hours post-infestation, with complete efficacy at 24 hours maintained throughout the entire month following NEXGARD administration (Halos 2014). In this study conducted on 12 Beagles in a controlled blind design, weekly re-infestations with 40 female and 10 male Ixodes ricinus were evaluated at 12 and 24 hours post-infestation for 28 days. Acaricidal efficacy exceeded 90% from 12 hours and reached 100% at 24 hours at each weekly evaluation. This speed of destruction is clinically significant because transmission of Borrelia burgdorferi generally requires more than 24 hours of tick attachment, which confers high protection against Lyme disease in treated dogs. Two controlled laboratory studies confirmed the efficacy of afoxolaner in preventing Borrelia burgdorferi infections after infestation by vector Ixodes ticks at D28 post-treatment. Protection against transmission of Anaplasma phagocytophilum, which also requires a prolonged attachment period (24 to 48 hours), is expected but warrants specific confirmatory studies.

6.3 Ixodes hexagonus: The Hedgehog Tick and Companion Dog Tick

Ixodes hexagonus, commonly known as the “hedgehog tick,” is an endophilic tick frequently found on companion dogs in peri-urban areas, particularly in dogs with access to gardens frequented by hedgehogs. This species is a potential vector of Borrelia spp. and Anaplasma phagocytophilum. The European MA of NEXGARD and NEXGARD SPECTRA includes Ixodes hexagonus among the tick species covered, based on the demonstration of the acaricidal activity of afoxolaner against this species. Efficacy data show activity comparable to that observed against Ixodes ricinus, with elimination of ticks within 48 hours following infestation throughout the one-month protection period.

6.4 Rhipicephalus sanguineus: Mediterranean Distribution and Protection Against Babesia canis and Ehrlichia canis

Rhipicephalus sanguineus sensu lato, the brown dog tick, has a predominant distribution in Mediterranean and tropical regions. Its adaptation to domestic environments allows it to reproduce indoors, including in kennels and homes, with populations potentially reaching high densities in the absence of treatment. Unlike exophilic ticks such as Ixodes ricinus and Dermacentor reticulatus, Rhipicephalus sanguineus can complete several full biological cycles per year in subtropical regions, while in temperate Mediterranean regions, adult activity is concentrated in spring and summer, with emergence of immature stages in autumn. This epidemiological characteristic implies that anti-tick protection must be maintained continuously, including in the off-season, in areas of Rhipicephalus sanguineus endemicity. Rhipicephalus sanguineus is a vector of Ehrlichia canis (causative agent of canine monocytic ehrlichiosis), Babesia canis vogeli, Hepatozoon canis, and Bartonella spp. Transmission of Ehrlichia canis can begin within the first hours following tick attachment, which places particular importance on the speed of acaricidal action of the product used.

The efficacy of afoxolaner against Rhipicephalus sanguineus has been demonstrated with more than 93% efficacy at 48 hours post-infestation for 30 days in controlled laboratory studies. A comparative study evaluating the ability of afoxolaner (NEXGARD), fluralaner (BRAVECTO ND), and permethrin/imidacloprid (ADVANTIX ND) to block transmission of Ehrlichia canis by infected Rhipicephalus sanguineus ticks showed that afoxolaner protected 75% of treated dogs against seroconversion, compared to 100% for permethrin/imidacloprid which combines an acaricidal and repellent effect (Fourie 2016). This result highlights a clinical nuance: isoxazolines, acting systemically after tick attachment and initiation of the blood meal, cannot guarantee 100% protection against pathogens whose transmission begins within the first hours of attachment, such as Ehrlichia canis or certain rickettsiae.

6.5 Hyalomma marginatum: An Emerging Species in France and Issues Related to the Risk of Crimean-Congo Hemorrhagic Fever

Hyalomma marginatum is a large tick native to the Mediterranean basin and North Africa, whose geographic expansion northward into France has been documented since the 2010s, facilitated by climate warming and passive transport by migratory birds. This species is the primary vector of Crimean-Congo hemorrhagic fever (CCHF) virus, an arboviral disease with high zoonotic potential. Following controlled clinical studies demonstrating robust acaricidal efficacy, the European Medicines Agency (EMA) officially validated and integrated the species Hyalomma marginatum into the MA indications of afoxolaner.

6.6 Practical Therapeutic Strategy

The strategy for preventing tick infestations is based on an individual risk assessment, taking into account the geographic area of residence and travel of the dog, its lifestyle (outdoor access, hunting, hiking), and the local seasonality of vector species. Monthly treatment with afoxolaner provides continuous systemic acaricidal protection, with documented efficacy against the main tick species Dermacentor reticulatus, Ixodes ricinus, Ixodes hexagonus, and Rhipicephalus sanguineus, thus covering the most frequent tick infestations in companion dogs in Europe. The choice between NEXGARD and NEXGARD SPECTRA depends on the patient’s endoparasitic coverage needs: in areas endemic for Dirofilaria immitis (southern France, Italy, Spain, Portugal), the formulation combining milbemycin oxime offers the advantage of integrated ectoparasitic and endoparasitic prevention in a single monthly tablet. In areas where heartworm risk is negligible, NEXGARD alone provides equivalent ectoparasitic coverage.

Complementarity with a topical repellent acaricide, such as permethrin in spot-on or collar formulation, may be considered in high-risk exposure situations, particularly in areas endemic for piroplasmosis or ehrlichiosis, where prevention of tick attachment (repellent effect) provides an additional advantage compared to the exclusively acaricidal effect of isoxazolines. This combined approach, associating a systemic acaricide (afoxolaner) and a topical repellent (permethrin), is advocated by several European experts for dogs with high-risk exposure lifestyles (hunting dogs, herding dogs, dogs engaged in canicross or hiking in wooded areas). The recommendation for continuous, year-round treatment rather than seasonal treatment is supported by recent data showing winter activity of Ixodes ricinus and Dermacentor reticulatus ticks in certain European regions (Probst 2023). The ESCCAP (European Scientific Council Companion Animal Parasites) recommends continuous anti-tick protection for dogs at risk of exposure. Unnecessary use of antiparasitics or use that does not comply with the label instructions may increase the selective pressure for resistance and lead to decreased efficacy.

Chapter 7 — Safety Profile, Tolerance, and At-Risk Populations

7.1 Identified Adverse Effects and Pharmacovigilance Data

7.1.1 Gastrointestinal Effects and General Manifestations

The most frequently reported adverse effects with afoxolaner are gastrointestinal in nature, including vomiting and diarrhea, observed infrequently and generally transient, resolving spontaneously without treatment. Lethargy and anorexia have also been reported, with very low frequency. In safety margin studies in Beagle puppies aged 8 to 9 weeks treated at 1, 3, and 5 times the maximum exposure dose at 28-day then 14-day intervals (6 administrations total over 126 days), effects observed (vomiting, diarrhea, decreased appetite) occurred in both treated and control groups, suggesting uncertain attribution to treatment (Drag 2017). No clinically significant hematological, biochemical, or urinary abnormality was attributed to treatment in these safety margin studies. Body weight and food consumption remained comparable between treated and control groups. The acute oral LD50 of afoxolaner in rats is greater than 1000 mg/kg and the dermal LD50 greater than 2000 mg/kg, demonstrating a wide toxicological safety margin in mammals.

7.1.2 Neurological Adverse Effects

In September 2018, the US FDA issued an alert concerning the isoxazoline class (afoxolaner, fluralaner, sarolaner, lotilaner), reporting neurological effects in certain dogs, including muscle tremors, ataxia, and seizures. Seizures have been reported in dogs receiving isoxazolines, including in animals with no prior history of epilepsy. FDA pharmacovigilance data covering the period from January 2013 to September 2017 report 32,374 reportable adverse events for all isoxazolines combined, with a seizure rate of 6.9% of reports for afoxolaner. An absolute frequency categorized as very rare (less than one case per 10,000 treatments) per the EMA SPC (Donohoe 2020). These figures must be interpreted with caution as they represent spontaneous reports (recognized under-reporting) and not incidence rates in treated populations. Causality has not been established for each individual report.

7.1.3 Data from Regulatory Agencies (EMA, FDA) and Updated Post-MA Pharmacovigilance Summary

Both the European (EMA) and American (FDA) regulatory agencies have maintained a favorable benefit/risk assessment for isoxazolines, while requiring precautionary labeling on product inserts. The SPC (Summary of Product Characteristics) states that mild gastrointestinal adverse effects, pruritus, lethargy, anorexia, and neurological signs (seizures, ataxia, muscle tremors) have been reported very rarely (fewer than one animal per 10,000 treated). In the event of overdose, symptoms, emergency management, and antidotes should be considered: there is no specific antidote for afoxolaner and treatment of overdose is symptomatic and supportive. Special precautions for each target species are detailed in the SPC. The isoxazoline class as a whole is considered to present a favorable safety profile when used in accordance with MA recommendations.

7.2 At-Risk Breeds: ABCB1 Mutation (MDR1) and Clinical Implications

7.2.1 Genetic Basis of the ABCB1 Mutation and Affected Canine Breeds

The ABCB1 mutation (formerly MDR1), consisting of a 4-nucleotide deletion, results in a premature stop codon and production of a truncated, non-functional P-glycoprotein. P-glycoprotein, an efflux transporter expressed at the blood-brain barrier, in hepatocytes, enterocytes, and renal tubular cells, normally ensures the exclusion of potentially neurotoxic substrates from the central nervous system. The most frequently affected breeds carrying this mutation include the Collie (homozygous in 35 to 70% of individuals depending on the population), the Australian Shepherd (allele frequency of 20 to 50%), the White Swiss Shepherd, the Shetland Sheepdog, the Border Collie, and the Old English Sheepdog, as well as their crossbreeds.

7.2.2 Specific Safety Data in Homozygous Collies and Related Breeds

Afoxolaner is not a substrate of P-glycoprotein, as confirmed by in vitro efflux experiments (EMA data). This property distinguishes it from macrocyclic lactones (ivermectin, milbemycin oxime, moxidectin) which are recognized substrates of this transporter. The safety of afoxolaner in homozygous MDR1-deficient Collies was specifically evaluated in two studies (Drag 2022). A total of 30 adult homozygous MDR1(-/-) Collies were treated orally: 9 with placebo, 9 with afoxolaner alone (mean dose corresponding to 3.8 to 4.7 times the maximum recommended therapeutic dose), 6 with milbemycin oxime alone (4.7 times the maximum dose), and 6 with the afoxolaner-milbemycin oxime combination. No signs of neurotoxicity were observed in groups receiving afoxolaner alone. Use of Collies receiving up to 10 times the MA dose of afoxolaner without adverse effects has been reported.

However, for milbemycin oxime present in NEXGARD SPECTRA, the increased sensitivity of MDR1-deficient dogs to macrocyclic lactones warrants caution, even though the doses used in the formulation (0.5 to 1 mg/kg) are considered safe in MDR1-deficient dogs. The afoxolaner-milbemycin oxime combination at the tested dose did not cause neurological signs in homozygous Collies (Drag 2022).

7.2.3 Practical Recommendations

Genotypic screening for the ABCB1 mutation is recommended in at-risk breeds before any prescription of macrocyclic lactones at high doses. For use of NEXGARD (afoxolaner alone), no specific restriction is necessary in MDR1-deficient dogs, as afoxolaner is not a P-glycoprotein substrate. For NEXGARD SPECTRA (afoxolaner-milbemycin oxime combination), use at recommended doses is considered safe in MDR1-deficient dogs, but clinical monitoring post-administration is recommended during first use in these breeds.

7.3 Contraindications, Precautions for Use, and Drug Interactions

7.3.1 Absolute and Relative Contraindications Listed in the MA

No absolute contraindication is mentioned in the European SPC of NEXGARD or NEXGARD SPECTRA other than hypersensitivity to the active substances or excipients. Use in puppies under 8 weeks of age and/or in dogs below the minimum weight for use (2 kg for NEXGARD, 1.35 kg for NEXGARD SPECTRA) should be based on a benefit/risk assessment by the responsible veterinarian. Use is not recommended in dogs with a history of seizures or neurological disorders, in accordance with the class warning for isoxazolines.

7.3.2 Documented Pharmacokinetic and Pharmacodynamic Interactions

For NEXGARD SPECTRA, potential drug interactions of milbemycin oxime have been identified with ciclosporin, azole antifungals, and erythromycin, as these molecules may inhibit P-glycoprotein and increase cerebral exposure to milbemycin oxime in sensitive dogs.

7.3.3 Use in Puppies, Pregnant, and Lactating Females: Available Data

The safety of afoxolaner was evaluated in Beagle puppies from 8 weeks of age, at doses up to 5 times the maximum exposure dose, without evidence of treatment-related toxic effects (Drag 2017). Laboratory studies in rats and rabbits did not reveal teratogenic effects or adverse effects on the reproductive capacity of males and females. Following the update of their MA, NEXGARD and NEXGARD SPECTRA are now authorized for use in breeding females, pregnant and lactating females, which removes the previous restriction and facilitates antiparasitic management of female dogs throughout the reproductive cycle. Special precautions for the person administering the medication include washing hands after handling the product.

Conclusion

Afoxolaner, administered as an oral chewable tablet (NEXGARD and NEXGARD SPECTRA, Boehringer Ingelheim Laboratory), constitutes a leading therapeutic tool in canine veterinary dermatology. Its mechanism of action on ligand-gated chloride channels in arthropods, favorable pharmacokinetics (rapid absorption, prolonged half-life, high bioavailability), and documented safety profile in both healthy dogs and MDR1-deficient breeds make it a drug of choice for the treatment of flea and tick infestations, as well as for dermatologically relevant ectoparasitoses: generalized demodicosis caused by Demodex canis, sarcoptic mange caused by Sarcoptes scabiei var. canis, and ear mite infestation caused by Otodectes cynotis.

Controlled efficacy data demonstrate parasitic reduction rates exceeding 95% for all of these indications, with a speed of action compatible with the clinical requirements of pruritus management and prevention of vector-borne pathogen transmission. The combination with milbemycin oxime in NEXGARD SPECTRA provides unique monthly endectocide coverage including prevention of dirofilariosis caused by Dirofilaria immitis and treatment of gastrointestinal nematodes including Trichuris vulpis.

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