Scabies (sarcoptic mange) is a highly contagious, pruritic skin disease in animals and humans caused by the obligate ectoparasitic mite Sarcoptes scabiei. The term scabies is derived from the Latin word scabere, meaning ‘to scratch’. S. scabiei is an arthropod taxonomically belongs to class Arachnida and family Sarcoptidae. Adult mites are roughly circular in shape, without a distinctive head, but have spine-like projections on the dorsal surface and four pairs of short legs (Figure 1). Females are almost twice as large as males. The female measures about 0.3 to 0.5 mm long by 0.3 mm wide, and the male is around 0.25 mm long by 0.2 mm wide. S. scabiei mites infest warm-blooded animals and tend to be host specific, with little transmission to other animal species or humans. Human scabies is caused by S. scabiei var. hominis. Other sarcoptic mites that can be found on animals, and occasionally infest humans include S. scabiei var. canis (dogs), S. scabiei var. bovis (cattle), S. scabiei var. caprae (goats), S. scabiei var. equi (horses), and S. scabiei var. suis (pigs).
Life cycle
The life cycle of Sarcoptes mites occurs in the superficial (keratinaceous) layer of the animal's epidermis. Female mites create tiny burrows into the skin in which they lay eggs over a period of 2–3 weeks. Eggs hatch in about 3 days and the immature stages exit the burrow, mature, mate, and move on to make other burrows. The entire life cycle from the egg through one larval and two nymphal stages to the adult may be completed in 14–21 days (Arlian and VyszenskiMoher, 1988). Sarcoptic mange is readily transmitted between animal hosts by direct contact; infestation by indirect contact is less frequent but occurs. Mites can live off the host for 2–3 weeks in temperate humid conditions and so there is the potential for infestation to be transmitted through shared bedding or grooming tools.
Clinical signs
Intense pruritus/itching, a major characteristic manifestation, is probably due to hypersensitivity to mites, mite eggs, and waste products (Figure 2). Primary lesions consist of papular eruptions that, due to selftrauma, develop into thick crusts. This allows secondary bacterial and yeast infections to occur. The initial lesions tend to occur on body parts with less hair such as the ventral abdomen, chest, elbows, hocks (Figure 3), ear margin, axillary and inguinal regions. If these are left untreated, lesions rapidly coalesce and become generalised. Dogs with chronic, generalised disease may develop seborrhea, lichenification and crust build-up, peripheral lymphadenopathy, emaciation (Pin et al, 2006). Sarcoptes mites seem to cause low-grade or sub-clinical infestations in Red fox population, indicating that both mite and fox might have co-evolved to adapt to each other over a long period of time (Davidson et al, 2008).
Diagnosis
Diagnosis is based on clinical presentation, history of pruritus, risk of exposure, and involvement of other mammalian hosts, including humans. A characteristic clinical sign, although non-specific, is the pinnal-pedal reflex (Mueller et al, 2001). If the affected pinna is gently manipulated this readily elicits a strong scratch reflex. This reflex can be tested for in nurse clinics where pets are presented for flea checks and the assumption is that dermatitis is associated with flea infestation. This can then act as an early diagnostic indicator that concurrent sarcoptic mange is present.
Laboratory diagnosis of sarcoptic mange can be accomplished by deep skin scrapings at a depth where some bleeding occurs in order to find the mites. This can be carried out in house or scrapings can be sent to an external lab. In house diagnosis can be confidently carried out by veterinary nurses and is useful as results are obtained very quickly. Gloves must be worn to take scrapings due to zoonotic risk and should be taken from at least 3 different sites at the margins of active lesions. Debris and thickening of the skin make scrapings less sensitive at the centre of lesions or if clinically affected areas are more chronic in nature. Scrapings may be taken with, and mounted in liquid paraffin. This has the advantage of the parasites being alive so movement can be looked for but may result in substantial skin debris obscuring the slide. This can be avoided by dry scrapings being treated with 10% potassium hydroxide (KOH) before light microscopic examination, but this kills the parasite. The presence of a single mite is enough for diagnosis. However, mites are often absent, even with multiple skin scrapings, making a definitive diagnosis difficult. Concentration and flotation of multiple scrapings may increase chances of finding the mites, eggs, or mite faeces. A serologic enzyme-linked immunosorbent assay (ELISA) is available and may be useful (Curtis, 2001; Lower et al, 2001), but cross reaction with other closely related acarids, such as ear mites, can still occur. If mites are not found, but the history and clinical presentation are suggestive of sarcoptic mange, trial therapy is warranted.
Feline scabies
Little is known about feline scabies compared with canine sarcoptic mange. A different mite, Notoedres cati is the causative agent of a disease similar to scabies in cats. Notoedric mange may infest humans or other animals, and has been reported worldwide. The feline strain of Sarcoptes scabiei can cause sarcoptic mange in cats (Bornstein et al, 2004; Mullen and O'Conner, 2009; Hardy et al, 2012; Malik, 2012; Huang and Lien, 2013), and should be considered in the differential diagnosis of cats presenting with crusting lesions on the pinnae and nose, and crusty pododer-matitis, especially when there is contact with foxes or dogs, and when owners have itchy papular lesions. However its differentiation from Notoedres cati is not essential for treatment.
Treatment and control
Due to the highly contagious nature of sarcoptic mange, affected dogs should be isolated from other animals while undergoing treatment. All in-contact animals, in multi-dog households and kennels, should be treated. Treatment of the environment is required, especially kennel runs, and housing often needs to be temporarily vacated and an environmental acaracide possibly employed.
Treatment with selamectin (Stronghold®, Zoetis) or moxidectin (Advocate®, Bayer Animal Health) spoton with a repeat treatment after 4 weeks is often sufficient to treat clinical cases. Advocate® has recently acquired a license specifically for the treatment of notoedric mange in cats. Treatment every 2 weeks in dogs may be required in severe or very chronic cases, but care should be taken in macrocyclic lactone sensitive breeds such as collies. Also, the heartworm (Dirofilaria immitis infection) status of any dog having travelled abroad should be evaluated before treatment with any macrocyclic lactone. This is due to the lack of efficacy of macrocyclic lactones against adult heartworms and the resultant risk of anaphylaxis caused by repeated waves of microfilaria in the blood-stream being killed. Although off license, fipronil spray may also be used as an adjunct to treatment in severe cases with variable results. Although milbemycin oxime and ivermectin have been used to treat sarcoptic mange off licence, the high efficacy of relatively safe licensed products means their use cannot now be justified. Amitraz wash (Aludex®, MSD) is licensed for treatment of sarcoptic mange but has largely been superceded in practice by spot-on applications due to ease of application, comparable efficacy and reduced risk of adverse skin reaction. The use of antiseborroheic and anti-fungal shampoos is invaluable in aiding the control of secondary complications. Before topical treatment, hair should be clipped and crusts and dirt removed. Secondary pyoderma will require treatment with a suitable antibiotic for 4–6 weeks. Which antiseborroheic treatment is chosen will depend on the severity of signs and secondary infections present, but a wide range is available.
Caught early, treatment of sarcoptic mange carries an excellent prognosis due to the availability of highly efficacious products but these products will only be effective if applied correctly and this is where the role of the veterinary nurse is vital in educating the client about correct administration and frequency of application. Correct application of treatment products can be demonstrated and/or applied in house. This is especially true where shampoos and preparation of the skin is required. It is important that zoonotic risks are explained to clients and correct precautions such as gloves and good hygiene are emphasised both in terms of handling the pet and also administering treatments. If patients are admitted for treatment it is important that barrier hygiene is maintained with protective gloves, aprons and good hand hygiene. Surfaces that the pet has contacted should be thoroughly disinfected after use and kennel areas rested until completely dry.
Public health impacts
Human scabies is a contagious, parasitic dermatosis, known as sarcoptic itch, with worldwide distribution. It affects at least 300 million individuals annually (Kumaresan and Sathiakumar, 2010). The mites can infest humans of any age, race and social rank. Risk factors for acquiring infestation in humans include poor hygiene, poverty, overcrowding, malnutrition and sexual promiscuity. Differential diagnosis includes atopic dermatitis, papular urticaria, eczema on hands and impetigo. Diagnosis in humans is based on clinical examination findings and locating a burrow, mites, and/or their products using non-invasive approaches, such as magnifying lens or videodermatoscopy (Lacarrubba et al, 2001; Prins et al, 2004; Micali et al, 2011).
If veterinary professionals are asked about human cases of scabies, the potential for transmission from dogs should be highlighted if the owner has been in contact with potentially affected animals, but also that human to human transmission is a much more likely source in generalised human scabies and that medical advice should be sought.
Conclusion
The ectoparasitic mite Sarcoptes scabiei is the causative agent of ‘scabies‘, a zoonotic parasitic dermatosis. Scabies is a worldwide disease with a major public health impact, affecting more than 100 animal species and 300 million individuals annually. Early identification of the disease will enable successful treatment of the affected patient; reduce spread of infestation and the requirement for mass treatment, thus limiting the potential for mites to develop drug resistance. The mites are generally susceptible to the currently available chemotherapeutic products that when promptly and properly applied are associated with better prognosis.