Canine scabies is a well-known contagious skin disease due to the development and reproduction of a specific mite in and on the skin, belonging to the family Sarcoptidae, Sarcoptes scabieivar. canis. It represents 2 to 4 % of dermatology cases. It is one of the major causes of marked pruritus in dogs.
AETIOLOGY AND PATHOGENESIS
Sarcoptes scabiei is a permanent parasite. The parasitic cycle is short, the female being prolific. Sarcoptes scabiei is a histophage: the females are mated by males on the surface of the skin and burrow tunnels in the superficial layers of the skin (moving 2 mm per day) to lay their eggs (2 to 3 eggs per day during 2 to 4 weeks). The larvae emerge from the eggs and change into protonymphs, tritonymphs and adults. The duration of the cycle is about 14 days in favourable conditions. Parasites feed on serum and epidermal debris, and more rarely from blood. Sarcoptes scabiei var. canis can survive up to 9 days, at a temperature between 15 and 25°C and a relative humidity between 25 and 85 %. They are capable of infesting a new host during a third to a half of their life span.
The mechanisms which trigger the clinical signs are not yet well elucidated. Mechanical reactions and irritations are likely to occur. Also, hypersensitivity reactions occur, particularly against exo-Ag released by the mites. Type I, III and IV reactions have been demonstrated or strongly suspected, at least in humans. In case of re-infestation the parasite is eliminated spontaneously in 2 to 3 months.
EPIDEMIOLOGY
The prevalence and incidence of canine scabies are not well known. In most parts of Europe, it has been considered as rare and under-diagnosed, whereas it could be more common in countries such as Sweden, where the absence of flea infestation (pulicosis) renders application of insecticidal therapy less frequent. In North America it could be the 7th most common cause of dermatitis in the dog. It seems to have a stable frequency in dogs, contrary to human scabies, which can show cyclic variations. Enzootic areas are known and more generally it is considered as being a rather common disease in kennels, shelters and breeding facilities. Contagion is constant although it is not exceptional to see spared dogs in a canine community. In our population of referral cases scabies represent 4% of the dogs referred if we consider confirmed diagnosis and 7% of the dogs if we add strongly suspected cases. It is for us an important differential diagnosis of pruritic dermatoses in dogs.
There does not seem to be breed predispositions for scabies. Perhaps Labradors, Cocker Spaniels and Doberman Pinschers could be considered to be predisposed in North America. No sexual predisposition has been reported. Age predisposition is controversial, with some studies recognizing none and others showing young animals more frequently affected. A third of our cases are less than one year of age and two thirds are less than 3 years of age. However it is difficult to conclude because risk factors are more numerous for young dogs: life in community, contacts more frequent, etc.
Scabies is a very contagious disease. It is thought that 50% of exposed dogs will develop the disease. However, isolated cases in numerous groups have been described. Transmission occurs by direct contact but also by infestation from the environment. In 40% of our cases contagion to another dog occurred. In a fourth of our cases a stay in a kennel occurred in weeks prior to presentation. It is not unrealistic to state that contagion is subtle in about three-fourths of the cases.
Contagion to man is also important to consider. In about a fourth of our cases, owners are affected with pruritic papules on the forearms, legs and more rarely chest and neck, which resolve after treatment of the animal. This prurigo strophulus is highly suggestive of the disease, although the direct transmission of the parasite cannot be demonstrated. The mites can survive (but not reproduce) in human skin for 15 to 20 days. Human infestation is thus self-limited because of this host specificity.
CLINICAL SIGNS
The incubation period is variable and depends on the animal and the number of parasites. It is likely to vary between 1 and 3 weeks. It is shorter when the number of parasites is high, in debilitated and/or young animals or in case of re-infection (probably because of a hypersensitivity reaction) whereas it is longer in well cared for adult dogs.
Pruritus is intense and constant in most of the cases. An increase of pruritus at night, which is "classical" in man and perhaps due to an increase of mite activity, has not been recognized in dogs.
Primary lesions are rare because secondary lesions due to self-trauma (scratching, biting, licking) appear early. Nevertheless diffuse erythema, papules and small papulocrusts can be found. The latter may represent the penetration site of the female mite and should be looked for carefully since skin scrapings may reveal the mite if performed in theses areas. Wheals of urticaria have been reported in experimental infections but they seem to be rare in the spontaneous disease.
Secondary lesions are non-specific: excoriations, crusts, lichenification and hyperpigmentation. Secondary dermatoses can also occur: a kerato-seborrhoeic disorder, alopecia, pyotraumatic dermatitis and/or otitis externa. A secondary bacterial folliculitis can occur in about 25% of the cases.
The localization of lesions can be relatively suggestive. At least at the beginning of the disease they appear on the ear pinnae, particularly on their margins, the lateral aspects of the elbows and the ventral aspects of the thorax and abdomen. Generalization can occur but even in these cases the dorso-lumbar area seems to be spared.
Unusual cases have been reported anecdotally, e.g., cases with lesions on one ear pinna or a few digits. This might be due to the regular use of anti-flea products which could inhibit the dissemination of the mites.
In some cases the clinical aspects of the disease can be confusing if the animal suffers also from another concomitant skin disease such as atopic dermatitis, flea allergy dermatitis, etc.
Lethargy and prostration are not common even in very chronic cases (with an evolution of more than one year).
DIAGNOSIS
Diagnosis of scabies is based first on history and clinical aspects (a contagious pruritic ventral papulo-crustous dermatosis). Contagion is sometimes subtle and the source of infection is not clearly identifiable in 50% of cases. A good response to systemic glucorticoid therapy occurs in about 40% of the cases and consequently does not allow the exclusion of scabies. In addition to the lesions and topography described above, the pinnal-pedal reflex should be looked for: when the ear margin (with or without visible lesions) is rubbed, particularly at the site where the pinna is folded, this triggers an attempt to scratch by the dog's hind leg. This zone may give a "sandy" feeling when palpated. In the French literature it is called "Henry's zone" after the name of a parasitologist who noticed in the twenties both its oft-infestation of mites and its sensitivity to stimulation. The pinnal-pedal reflex is only suggestive and not specific: it occurs in about 80% of scabies cases but also in about 20% of other pruritic dermatoses.
The confirmation of the diagnosis is based on the finding of mites, eggs and/or pellets in skin scrapings. Sarcoptes scabiei is easily recognizable with its stocky aspect (female: 350 to 500 μm and male: 220 to 250 μm in diameter), short rostrum, triangular dorsal scales, two anterior pairs of short legs bearing long un-joined stalks with suckers, two posterior pairs of atrophied legs carrying long bristles (with suckers on the 4th pair in the male) and terminal anus. The mite is not always easy to find and it is generally considered that scrapings are positive only in 30 to 50% of the cases. It is essential, however, to select the best areas to perform the scrapings. A magnifying lens with illumination is very useful to discover the papules and papulo-crusts to be scraped. Parasites should also be looked for in the ear margins, particularly in the "Henry's zone". When scrapings are positive, the number of visualized mites remains low (1 or 2). In human dermatology, the average number of adult mites in an individual with papulous scabies is 12. Both in man and the dog, there is no correlation between the number of mites which are discovered and the severity and extension of the dermatosis. In very rare cases, called Norwegian scabies, high numbers of mites can be found in thick crusts: 200/cm2 in man, 18000/g in pigs, 1000/cm2 in rabbits. An underlying immunodeficiency disorder is often present.
Cutaneous histopathology is rarely diagnostic because it is exceptional to visualize a mite in a skin biopsy. Generally, skin biopsies show a non-specific superficial peri-vascular hyperplastic dermatitis. Lymphocytes, histiocytes and eosinophils predominate, suggesting a cutaneous hypersensitivity reaction. The number of eosinophils is higher in chronic lesions. Sometimes, foyers of eosinophilic necrosis in the superficial epidermis may suggest the trail of the burrowing mite but this sign is not pathognomonic and is seen perhaps only in about 50% of the cases. Marked epidermal hyperplasia is controversial as a suggestive sign.
Scabies serology has been developed recently. It is an ELISA based technique, using a purified preparation of Sarcoptes scabiei var. vulpes. The test has a good sensitivity (92%) and specificity (96%), but seroconversion can take up to 5 weeks after inoculation and 1 to 3 weeks after the first symptoms so testing should not be done at a too early stage. Sera of dogs infested with other parasites (Cheyletiella yasguri, Demodex canis, Otodectes cynotis, Linognathus setosus), affected with flea allergy dermatitis, or atopic dogs that are sensitised to Dermatophagoides farinae and pteronyssinus give a negative reaction to this test. In contrast dogs with scabies can show positive reactions to these house dust mites, both in vivo (skin-tests) and in vitro (IgE and IgG assays). This clearly indicates that false positive reaction to house dust mites extracts may lead to diagnostic errors in dogs affected with scabies.
A therapeutic diagnosis is sometimes necessary: "if you suspect it, treat it" (GH Muller). But a good response may sometimes be difficult to interpret because other parasitic diseases can respond to acaricides/insecticides (cheyletiellosis, oto-acariasis, trombiculosis, pediculosis, flea allergy dermatitis).
Differential diagnosis of scabies should include all other causes of pruritus in the dog, including allergic skin diseases (atopic dermatitis, food allergy, flea allergy dermatitis), Malassezia dermatitis, bacterial folliculitis and bacterial over-growth, pemphigus foliaceus and sylvatic dermatophytosis (e.g., due to Microsporum persicolor or Trichophyton mentagrophytes). Some cases of food allergy and pemphigus foliaceus may be so intensely pruritic that they can truly mimic scabies.
THERAPY
Treatment of canine scabies can be done topically or systemically. Symptomatic (keratomodulating and/or antiseptic) shampoos and moisturizers are recommended before topical treatment or in association with systemic treatment.
Clipping is recommended before topical treatment, at least in severe cases. Lime sulfur is an effective compound used in North America but it is not available in Europe. It stains and has a bad odour. Dimpylate (that is diazinon), an organophosphate used at 0.1 to 0.25% as a sponge-on, is very effective when applied 2 to 3 times a week during 3 to 4 weeks. Organochlorines such as lindane have been abandoned for toxicity and ecological reasons. Amitraz at 0.025 to 0.05% as a sponge-on applied at least 3 times at weekly or biweekly intervals, is also effective. Amitraz is toxic for diabetic pets and diabetic owners, Chihuahuas, puppies less than 3 months of age and nursing bitches. Secondary effects include depression and bradycardia which can last a few hours. Fipronil at a concentration of 0.25% has been anecdotally reported as effective in canine scabies when used as a sponge-on at the dose of 3 to 6 ml/kg 2 to 3 times at weekly intervals. In the absence of larger studies; fipronil should be reserved for the treatment of sensitive/weak individuals i.e., mainly young puppies and pregnant bitches.
Systemic treatment is considered as the best approach to the management of scabies, both in individuals living alone and in groups of dogs. Macrocyclic lactones are used. Ivermectin is a molecule produced by fermentation of Streptomyces avermitilis and has insecticide, acaricide and anthelmintic properties. Recommended doses are 200 μg/kg 3 times at 7 to 10 days of interval or 250 to 400 μg/kg twice at 2 weeks of interval, administered subcutaneously (we use 400 μg/kg twice). A pour-on formulation is also usable and may be more convenient in large groups of dogs. The oral route is also usable at 200 to 400 μg/kg every week for 4 to 6 weeks. Ivermectin is contra-indicated in Collies, Bearded Collies, Shetland Sheepdogs, Old English Sheepdogs, Australian Shepherds and their crosses because of idiosyncratic reaction. The molecule can cross the haemato-meningial barrier and cause neurological disorders (depression, tremors, coma and death). This is due to a deletion mutation of the mdr 1 (multidrug resistance) gene. Collies, Shelties and Australian Shepherds homozygous for the deletion (35% of Collies) exhibit ivermectin-sensitivity due to the production of a severely truncated, non-functional P-glycoprotein, a component of the blood-brain barrier that functions as a drug efflux pump. Treatment of such intoxications includes the use of physostigmine or picrotoxine; diazepam should not be used (Hopper 2002). The use of ivermectin is off-label in the dog. Response to therapy is so rapid (usually up to 5 days) that a therapeutic trial with this molecule remains an interesting diagnostic tool in canine dermatology. In man, ivermectin is used at the dose of 200 mg/kg once in non-immuno-suppressed patients and twice in patients with AIDS.
Milbemycin oxime has been used per os at the dose of 1 to 2m/kg every other day or every week, respectively for 3-5 to 8 times. Another milbemycin, moxidectin is usable either orally or by sub-cutaneous injection.
The dose is 0.2 to 0.25 mg/kg per week for 2 to 6 weeks or 0.4 mg/kg twice 2 weeks apart. They seem to be effective but their use is also off-label. Milbemycins are produced by fermentation of Streptomyces hygroscopicus aureolacrimosus and also have insecticide, acaricide and anthelmintic properties. Their structure is close to the avermectin's one with only a different radical. Milbemycin oxime might better tolerated than avermectins, but moxidectin may cause side effects such as urticaria and angioedema (particularly facial) which can regress spontaneously in a few hours. Toxicity might be low and their use could be possible in breeds sensitive to avermectins.
Selamectin is the only systemic treatment licensed for the treatment of canine scabies. It is a novel avermectin which is presented as effective at the dose of 6 to 12 mg/kg applied twice at a one-month interval as a spot-on. Some dermatologists have seen failures with this regimen and recommend the use of this product thrice at biweekly intervals. Also, the slow response to this molecule is a concern when a therapeutic trial is envisaged and this product should perhaps be restricted to confirmed cases and/or to the treatment of animals in contact with affected dogs.
In fact, all in-contact animals should be treated as well as affected patients. The environment should be treated as mites can survive in it and remain infective. Products such as foggers and above all pump-sprays for environmental control of fleas are appropriate since they include usually an adulticide and an insect growth regulator.
In the hands of veterinary dermatologists, scabies is curable in 100% of the cases. Pruritus decreases by at least 75% after 15 days in two-thirds of the cases and is low or has disappeared after 30 days in all patients.
CONCLUSION
Canine scabies is not so rare and is far from having disappeared. It is common in the young without breed or sex predisposition. It is a contagious disease but the source may be difficult to discover. Pruritus is intense and glucorticoid-responsive in 40% of the cases. Lesions are not specific except typical papules, often localized on the pressure points and the ventral part of the body. Ear pinnae are also often but not always affected with keratoseborrhoeic lesions. The pinnal-pedal reflex is not always present but is suggestive. The number of mites found with skin scrapings is not correlated with the severity and extension of the dermatosis. An appropriate acaricidal treatment is effective in all cases.
Further reading (detailed references are available on request)
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