Successful Treatment of Demodex gatoi with 10% Imidacloprid/1% Moxidectin

Introduction

Demodex gatoi was first reported in the 1980s as a second, unnamed demodex species in felines and was officially described in 1999.¹ Of the three known feline Demodex species, D. gatoi seems to be the most commonly identified.² It is a short-bodied mite (90.6 to 108.3μm in length) that lives its entire life cycle in the stratum corneum.^1–3 D. gatoi possesses other characteristics that distinguish it from D. cati in that it most often causes pruritus, is transmissible to close contact cats, and is rarely associated with underlying disease.^2–4 Common clinical signs include focal or patchy alopecia, erythema, and self-induced excoriations.^2–4 Differential diagnoses should include atopy, flea bite hypersensitivity, food hypersensitivity, psychogenic alopecia, and hyperthyroidism.^3–5 The following case report details a household of cats with D. gatoi that presented with pruritus and localized dermatitis.

Case Report

A household of 13 cats was evaluated by the dermatology service for pruritus and multifocal alopecia that had been present in at least eight of the felines for approximately 6 to 7 mo. The referring veterinarian had recommended a dietary trial and had administered methylprednisolone injections to the more severely affected cats. Although the repository corticosteroid temporarily relieved pruritus, the cats continued to develop new skin lesions. A few of the cats were allowed outdoors, but most were strictly confined indoors. They were housed with four dogs and multiple birds. A monthly flea and heartworm preventative^b,c was administered to all cats and dogs in the house on a regular basis. All cats were domestic short hair or domestic medium hair. Their ages ranged from 5 to 13 yr old and they were in overall good health. There were 2 intact females, 1 spayed female, and 10 castrated males. All the cats were related in that they were either offspring or siblings of each other. No cats were adopted or brought in from an outside source for many yr. The dogs were all given to the owners by a family friend more than 2 yr prior to the onset of clinical signs in the cats.

The cats were evaluated on two different days in order to accommodate the owner. Six cats were examined the first day. Two of these were considered the most severe in terms of observed pruritus and resulting self-trauma. The remaining four were considered asymptomatic to mildly affected according to the owner. Focal areas of alopecia and erythema of the neck, lateral thorax, and abdomen were seen in both of the severely affected cats (Figure 1). Miliary dermatitis was also found around the neck of the mild-to-moderately affected cats. The remaining seven cats were examined 1 wk later. Only one of these was considered severe with marked areas of self-trauma and erythema on either side of the neck (Figure 2). The other six cats ranged from asymptomatic to mildly pruritic.

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Cytology, dermatophyte cultures, and skin scrapings were performed on all of the cats. Cytology was performed by impression technique and only minimal amounts of coccoid bacteria were seen. Because the bacteria were extracellular and the samples lacked evidence of inflammatory cells, these were considered bacterial overgrowth. All dermatophyte cultures were negative after 21 days. Skin scrapings were both deep and superficially obtained from affected sites as well as areas that appeared clinically normal. Skin scrapings revealed a short-bodied mite from the first two severely affected cats (Figure 3). Mites were not found on any scrapings from the remaining 11 cats, including the other severely affected cats. Based on size and morphology, these mites were identified as Demodex gatoi. Once this diagnosis was made, the owner was asked to provide fecal samples from the cats to determine if mites could be identified, and to help recognize possible asymptomatic carriers. Only five samples were received and these were negative for evidence of mites.

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Because of the difficult task of performing lime sulfur dips on 13 cats, an alternative form of therapy was considered. Each cat was treated with the entire contents of a single tube containing 10% imidacloprid/1% moxidectin^a applied to the skin of the back of the neck. The dose was determined by manufacturer recommendations based on body weight. Frequency of application was once a wk until clinical resolution was achieved in all affected cats. The owner was aware that this drug combination used once a wk was considered off-label usage, but they still provided consent for treatment. They were also warned of potential side effects, such as application-site irritation, lethargy and hypersalivation if ingested, as well as specific symptoms of moxidectin toxicity (depression, mydriasis, muscle tremors, and ataxia).⁶ Repeat skin scrapings were performed after 2, 4, and 6 wk of treatment to gauge efficacy and to monitor for asymptomatic carriers. Each subsequent skin scraping was negative. The owner reported a decrease in pruritus over the first 4 wk of treatment for all the cats, but the three most severely affected cats had developed new focal areas of alopecia. Cytologies of these three cats showed moderate-to-severe numbers of coccoid bacteria in the alopecic and erythematous lesions. All three cats were treated with cefovecin sodium (8mg/kg)^d subcutaneously once every 14 days for a total of three doses. At the end of this additional treatment, all 13 cats had received eight doses of 10% imidacloprid/1% moxidectin.^a The client reported marked decrease in pruritus in all affected cats and no formation of new lesions. An additional 2 wk of 10% imidacloprid/1% moxidectin^a was recommended, and treatment was completed after a total of ten doses. No cat experienced adverse events during the course of treatment. A follow-up phone call 3 mo after treatment revealed that the cats continued to do well, and no recurrence of pruritus was noted. The source of the infection was never found in these cats. Because several of the cats were allowed outdoors unsupervised, it is unknown if contact with other cats was the inciting cause.

Discussion

Currently there are three known types of demodex mites found in cats: Demodex cati, D. gatoi, and a newer, unnamed species.^2,7,8 Cats affected by D. cati can have localized or generalized disease as seen in dogs. Clinical signs can include alopecia, crusting, erythema, and variable pruritus. Similar to D. canis, this mite has a long body and is found in the hair follicle, which requires deep skin scraping for diagnosis.^2,5,7 The unnamed species is shorter than D. cati, but longer than D. gatoi.^2,7–8 It is unknown at this time if this mite is truly a separate species or a morphologic variation of one of the other two. Pruritus appears to be an inconsistent feature of this mite.⁸

Demodex gatoi exhibits clinical characteristics that are analogous to canine scabies mites instead of those of the typical demodex mite. D. gatoi resides solely in the stratum corneum, appears to be transmissible among cats, and results in pruritic dermatitis. It also has been suggested that cats may develop a hypersensitivity to the mite, similar to what is seen in animals affected by Sarcoptes.^2–3

In this household, only a small number of mites were found and D. gatoi was not identified in all clinically affected cats. This is similar to findings previously reported in six households with D. gatoi in Finland.³ The lesions of the cats in the current report were mainly erythematous patches of alopecia, especially on the neck and lateral abdomen. Miliary type papular lesions were also noted. As previously stated, pruritus varied among the cats from none to severe. The most severely affected cat had evidence of self-excoriation around the neck. All of these lesions are consistent with previously reported clinical signs associated with D. gatoi.^2–3,5

Given its presence in the superficial layers of the skin and the self-grooming tendencies of cats, D. gatoi can be difficult to find on skin scrapings.² To yield better results, it is recommended that skin scrapings be obtained from a large surface area, especially areas that are difficult for cats to reach. Because of the small size of D. gatoi, slides should be read at 10x objective. The microscope should also be adjusted to reduce the condenser, and the iris should be closed to provide better contrast in visualizing this translucent mite.^2–4 Fecal flotation may also be useful in diagnosing D. gatoi, especially in asymptomatic cats.⁹ Fecal samples from only 5 of the 13 cats in the current household were obtained, but no mites were found. For households with suspected D. gatoi, samples from all cats would be recommended for diagnosis and therapeutic monitoring. Lastly, a quantitative polymerase chain reaction test was recently developed for the diagnosis of demodex in cats. Samples were obtained through skin scrapings and tested for D. gatoi or D. cati DNA. All symptomatic cats with D. gatoi were positive, but the test failed to detect a positive, asymptomatic cat. To the author's knowledge, this test is not currently available, but it may be useful for diagnosis in the future.¹⁰

The treatment of choice for D. gatoi demodicosis is a lime sulfur dip once a wk for up to 6 wk.^2–3 Amitraz dips and oral ivermectin have also been used with variable success.^2–3,7,9 Both of these drugs are used in an extra-label manner for demodicosis. Regardless of treatment choice, all cats in contact with those affected should be treated.² Because of the large number of cats in this household, once a wk dips were not feasible for the owner. 10% imidacloprid/1% moxidectin^a was chosen for the ease of application and the expectation that the extra-label protocol would be tolerated based on data from manufacturer safety studies.⁶

10% imidacloprid/1% moxidectin^a is a topically applied product indicated for the treatment, prevention, and/or control of fleas, heartworms, intestinal parasites, and ear mites in cats.⁶ Imidacloprid is a neonicotinoid insecticide with activity against fleas, while moxidectin is a semi-synthetic macrocyclic lactone with activity against nematodes and acarids. Moxidectin binds to pre-synaptic glutamate gated ion channels in neuronal membranes of nematodes or muscular membranes of arthropods causing increased release of gamma-amino butyric acid. Gamma-amino butyric acid binds to chloride channels at post-synaptic membranes leading to an influx of chloride ions. This results in decreased nerve transmission and flaccid paralysis leading to death of the parasite. These glutamate channels are specific for invertebrates and are not expressed in mammalian hosts. Moxidectin generally does not cross the blood brain barrier, which decreases the likelihood of neurologic side effects. Once topically applied, imidacloprid spreads over the surface of the body and remains in the lipid layers. The moxidectin portion is absorbed systemically via the dermis and distributed throughout the body's tissues. Moxidectin reaches maximum serum concentration in cats in less than 2 days and the half-life is approximately 15 days.⁶ By applying the solution once a wk, the drug concentration levels would remain higher throughout the duration of treatment and may be an explanation for the resolution of D. gatoi mites in this case.

The combination imidacloprid and moxidectin has been used weekly in the treatment of mild-to-moderate cases of canine demodicosis.¹¹ To the author's knowledge, this is the first time it has been reported with success for D. gatoi. One reason for this may be the length of time needed to see complete resolution. The cats in this report required eight doses before resolution was seen. It is also important to note that three of the cats required antibiotic therapy to treat secondary bacterial infections that may have contributed to pruritus. It is reasonable to believe that severely affected cats may need antibiotic therapy to relieve clinical signs while they are being treated for the mite.

One flaw of this report is that the status of retroviruses was not known in these cats. The owner declined testing for unknown reasons. Feline demodicosis from D. cati has been associated with underlying diseases including feline leukemia virus, feline immunodeficiency virus, diabetes mellitus, and toxoplasmosis. Most cases of D. gatoi are not linked to immunosuppression, but the mite has been reported in a cat with feline immunodeficiency virus.⁵ It is possible that one cat with immunosuppression from an underlying disease process contracted the mite and then allowed it to spread amongst the other felines. Physically, all of the cats were in good health, but testing for systemic illnesses was not performed.

Conclusion

This is the first reported case of successful treatment of Demodex gatoi using 10% imidacloprid/ 1% moxidectin.^a The drug was used in a household of 13 felines in which 8 of them were symptomatic. All those affected responded to therapy after 8 wk, and after 1 yr there has been no recurrence of clinical signs. It has been previously noted that this drug and other spot-on products are not effective for D. gatoi. Anecdotal reports also suggest variable success, but it is unclear of the dosing frequency and the length of treatment in those cases.^2–3 Although the cats in this report responded to treatment, further studies are recommended to determine if the success can be duplicated.