Sarcoptic mange is caused by the Sarcoptes scabiei mite (Fig.1), a non-species-specific parasite that lives in tunnels burrowed into the epidermis. S. scabiei infests a wide variety of mammals, humans included. Depending on the specificity for the host, some authors divide the species into varieties, among which S. scabiei, var. cuniculi; cross-infestations, with different species of mites attacking different hosts are possible, however these generally result in atypical and transient cases of dermatitis.
Although considered a rare parasitic disease by literature, in Italy sarcoptic mange is in fact relatively common in pet rabbits. It is more commonly found in rabbits recently acquired in pet-shops, and in subjects that have come into contact with them.
LIFE CYCLE OF THE PARASITE
Sarcoptes scabieiis an oval shaped, sarcoptiform mite; male adults measure 200-250 microns and adult females 350-500 microns. Adults and nymphs have eight legs, larvae six; the morphology of larvae is similar to that of adults, except for the smaller size. The eggs are oval shaped and measure approximately 90-250 microns.
Females begin laying eggs 4-5 days after mating in tunnels burrowed into the stratum corneum of the epidermis. Females lay approximately 40-50 eggs in two months. The life-cycle is of about 10-26 days.
First-stage larvae remain in skin tunnels while adult males, non-fertilised females, second-stage larvae and nymphs live on the skin surface. The infection is mainly spread by the juvenile forms that live on the skin surface. The parasites feed on the extracellular fluid that transudates from the dermis into the stratum corneum.
S. scabieiis an obligate parasite; off the host the mite survives for a maximum of 2-3 weeks.
The damages caused by the parasite to the host are considered consequential to the mechanical trauma to the epidermis consequent to the digging of tunnels, the irritating action of its secretions and excretions, the allergic reaction with the release of cytokines and the production of toxic substances.
CLINICAL SIGNS
The infestation usually begins in areas with limited hair coverage: periungual skin (Fig. 2), snout (Figs. 3 and 4), ear margins (Fig. 5) and eyelids (Fig. 6), and then progressively throughout the body (Figs. 7-9). The lesions consist of white-yellowish crusts adherent to the skin, which easily bleed when removal is attempted (Fig. 10). The lesions have a peculiar and pathognomonic characteristic: with time the crusts tend to stratify and thicken, reaching a considerable thickness.
Because of the inflammation of the periungual tissues, the nails grow in a deformed way (Fig 11). When the under surface of a limb is severely affected, in view of the pain the rabbit is reluctant to move (Fig. 12). On the rabbit’s nose, the progressive stratification of the crusts which incorporate the hair can give rise to a prominent lesion (Fig. 13). The same mechanism may cause the formation of a "goatee-like" crust formation on the chin (Fig. 14) and to crust-like formations on the digits parallel to the nails (Fig. 15).
The disease is very itchy and causes the progressive deterioration of the rabbit’s general health status, decreased appetite, weight loss and death.
DIAGNOSIS
The initial presentation can be confused with fungal infections; in later stages the manifestations are quite characteristic and the diagnosis can be based on the clinical appearance. The diagnosis can eventually be confirmed by the microscopic examination of crusts and scrapings in KOH preparations or Vaseline. The parasites are not always easy to find; the presence of mite faecal material is sufficient to confirm the diagnosis (Fig. 16). In case of doubt, the diagnosis can be made ex-adiuvantibus; in the presence of sarcoptic mange with therapy an improvement is expected within a few days.
THERAPY
Avermectins are very effective in the treatment of mange. Ivermectin or doramectin (0.2-0.4 mg/kg q10d for 3 times) may be used, or selamectin (6-18 mg/kg q14-21d for 2-3 times). In rabbits, Fipronil is contraindicated.
With treatment a rapid improvement of clinical conditions is observed. Nail deformation can be permanent; should nails grow in abnormal directions a periodic trimming may be necessary. Scab removal is contraindicated as crusts fall spontaneously within a few days after initiation of therapy.
Sarcoptic mange is an extremely contagious parasitic disease: all cohabiting subjects must be treated simultaneously.
ZOONOTIC CONSIDERATIONS
Although being a zoonosis, the transmission from rabbit to humans is rare and transient. In humans, the infestation is characterised by extremely itching papules mainly located on the hands, arms and abdomen. In immunosuppressed subjects the infestation can be severe, with the formation of extensive crusts.