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Sarcoptic mange

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Category: Schede
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  • Disciplina: Dermatologia
  • Specie: Cane e Gatto

Sarcoptic mange, also known as canine scabies, is a parasitic, contagious, dermatological disease of canidae caused by the mite Sarcoptes scabiei var. canis. This mite can also exceptionally infest cats which have come into contact with infested dogs or foxes. Sarcoptic mange affects dogs of all ages, with no gender or breed predilection. Puppies, stray dogs and dogs housed in kennels or shelters are at greater risk, because of the highly contagious properties of the disease, which is mainly transmitted through direct contact. As mites can survive off the host for only short periods, up to a maximum of 2-3 weeks in the presence of optimal environmental conditions (temperature and humidity), indirect transmission through environmental spread is rare.

 

MORPHOLOGY AND BIOLOGICAL LIFE CYCLE OF THE PARASITE

 

The Sarcoptes scabiei var. canis mite is 200-400 µm long, the body is oval-shaped, flattened ventrally and convex dorsally: male mites are smaller and have a rounder body (Fig. 1). The cuticle of the dorsal surface of the mite has numerous transversally oriented striae, as well as characteristic pointed triangular scales and spiny bristles. The head has a short, square rostrum. The limbs are extremely short; in both sexes the front limbs have non-jointed pretarsi which terminate with a sucker-like structure (pulvillus). The rear limbs are rudimentary and do not extend beyond the body of the parasite. In males the fourth pair of limbs ends with pulvilli while the third pair ends with long bristles; in females bristles are present on all the rear limbs while pulvilli are absent. The anal opening is located at the posterior end of the body.

The life cycle of Sarcoptes scabieitakes place entirely within the host (permanent parasitism). After breeding, the gravid mite digs a protective tunnel within the corneal layer of the epidermis, at a speed of around 5 mm per day, laying eggs along the way (1-3 per day for a reproductive life of around 2 months). The egg-adult cycle usually takes between 17 and 21 days. The mite feeds on epidermal debris and tissue fluids. 

 

CLINICAL SIGNS

Clinical signs usually appear after 1-3 weeks of incubation. Pruritus, which is initially mild, becomes intense (uncontrollable), such that sarcoptic mange is considered one of the itchiest of all dermatological diseases. The exact mechanism that causes pruritus is not completely clear, although it appears to be secondary to the development of a state of hypersensitivity.

Mites seem to prefer areas of the body which are scarcely covered with hair, such as the tips and margins of the pinnae, limbs, elbows and hocks, as well as the ventral part of the abdomen and chest (Figs 2, 3 and 4). In chronic forms the disease may affect the entire body.

The characteristic, basic lesion of sarcoptic mange is a papulo-crusted eruption, that is, an erythematous papule surmounted by a small, yellowish scab of serum or blood (Figs. 5 and 6). Another lesion which is frequently observed on the skin of dogs with sarcoptic mange is desquamative dermatitis, with whitish or yellowish scales of variable size (Fig. 7). Furthermore, as a consequence of the intense pruritus, it is common to observe lesions secondary to self-mutilation, such as excoriations, alopecia and crusts as well as bacterial complications (Fig. 8).

The characteristic, basic lesion of sarcoptic mange is a papulo-crusted eruption, that is, an erythematous papule surmounted by a small, yellowish scab of serum or blood (Figs. 5 and 6). Another lesion which is frequently observed on the skin of dogs with sarcoptic mange is desquamative dermatitis, with whitish or yellowish scales of variable size (Fig. 7). Furthermore, as a consequence of the intense pruritus, it is common to observe lesions secondary to self-mutilation, such as excoriations, alopecia and crusts as well as bacterial complications (Fig. 8).

 

DIAGNOSIS

The diagnosis of sarcoptic mange is made by identifying the parasite or its eggs under the microscope. As mites live in the more superficial layers of the epidermis, the technique of choice for the identification of Sarcoptes is superficial skin scraping. The number of mites which cause sarcoptic mange is usually relatively small, consequently the identification of these parasites under a microscope is not easy, and often several scrapes are necessary to allow their identification; furthermore, the detection of papules and scales on the skin may be made difficult by the presence of lesions caused by self-mutilation or by secondary complications (bacteria or yeasts) which are constantly present during the chronic stages of the disease.

The diagnosis of the “crusted” or “Norwegian” variant is extremely simple, as it is sufficient to pass a scalpel blade on the surface of the epidermis to collect a high number of parasites. A diagnostic sign which is considered suggestive of the disease by many authors is the presence of a scratch reflex shown by the dog with its hind limbs when the margins of the pinna are gently rubbed between the thumb and index fingers by the examiner. It has been estimated that around 75-90% of dogs affected by scabies manifest this reflex, although its may also be present in other dermatological diseases that cause lesions to the skin of the pinnae and to the external auditory canal.

An indirect diagnostic test is an enzyme-linked immunosorbent assay (ELISA), which identifies the presence of anti-Sarcoptes scabiei antibodies in the serum. The test has a sensitivity of 92% and a specificity of 96%; however, it has the limitation that it does not enable an early diagnosis of the disease, as seroconversion is observed only after 5 weeks; it should also be considered that serology may remain positive for months after the disease has been resolved. In view of the very low number of parasites on the skin of the dog, histopathological examination of skin biopsies is not an investigation to be recommended to make the diagnosis. Furthermore, histopathological preparations do not show alterations that are pathognomonic for the disease, usually showing epidermal hyperplasia and superficial perivascular dermatitis characterized by an inflammatory infiltration composed of lymphocytes, histiocytes and a variable number of eosinophilic granulocytes.

 

DIFFERENTIAL DIAGNOSIS

  • Other ectoparasitoses
  • Atopic dermatitis
  • Adverse reaction to food
  • Flea bite allergy dermatitis
  • Leishmaniasis

 

TREATMENT

The treatment of sarcoptic mange is based on the use of compounds with acaricidal activity, which can be used both topically and systemically. Currently, the compounds registered in Italy for the treatment of sarcoptic mange in the dog include amitraz (topical solution 0.025%, by sponging every 5-7 days),  selamectin (two spot-on treatments, 1 month apart, although many veterinarians prefer using the treatment every 2 weeks) and moxidectin, available in combination with imidacloprid (two spot-on treatments, 1 month apart). Alternative protocols, using compounds which are not registered for the treatment of scabies in the dog (off-label use), consist of injectable ivermectin 1% (0.2-0.4 mg/kg s.c. two or three times, at 7- to 14-day intervals, or  0.2-0.4 mg/kg per os three times, at 7-day intervals), milbemycin oxime  (tablets, 2 mg/kg per os three to five times, every 7 days, or 1 mg/kg per os eight times, every 2 days), doramectin (subcutaneous injections of 0.2 mg/kg every 7-14 days), fipronil (0.25% spray, three or four administrations every 7 days), pyriprole (two spot-on treatments, 1 month apart) and amitraz, available in association with metaflumizone (spot-on, two treatments 1 month apart or four treatments at 2-week intervals). The acaricidal treatment should always be extended to all the animals which have come into contact with infested ones.

 

 

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