Definition
A parasitic disease of the skin caused by the sarcoptic mange mite and often characterized by marked pruritis and papules.
Occurrence
Sarcoptic mange occurs in all age groups but is inapparent in neonatal pigs until signs and lesions have had time to develop. It is likely the disease occurs in all countries where pigs are raised in large numbers. Sarcoptic mange is a common disease and represents the most important ectoparasitic disease of swine. Sarcoptic mange occurs in many other species but the mite Sarcoptes scabiei var suis is specific only for swine.
Historical information
Sarcoptic mange was first reported about 60 years ago. Little attention was paid to early reports; it was widely believed that scratching was natural and normal behavior for pigs. Also, economic loss caused by the disease was not widely appreciated. Intensification of production after World War II, along with greater attention to economics of production, focused attention on the avoidable losses caused by sarcoptic mange. Development of better and more convenient acaricides has greatly improved control.
Etiology
The mite Sarcoptes scabiei var suis causes most mange in swine. The mite is about 0.5 mm in length, gray to white, and just visible to the naked eye when on a black background. All stages of the mite (ova, larvae, nymphs, adults) develop in the epidermis. The life cycle is completed in about 10-l5 days. Mites and ova survive away from the host for only short periods of time. Depending on temperature and humidity, this could be as short as one hour, or as long as 15 days.
Demodectic mange, caused by the mite Demodex phylloides, is unrelated to sarcoptic mange and relatively unimportant in swine. It is mentioned here because it occasionally causes dermatitis over the snout, inside the thighs and over the flanks. Occasionally it involves the whole body and leads to excessive trimming or condemnation at slaughter. Nodules caused by D. phylloides have been mistaken for swine pox.
Epidemiology
Mange in a herd usually develops after introduction of asymptomatic swine carriers.Sarcoptes scabiei var suis is rarely, if ever, transmitted from other species. If swine are moved immediately into quarters recently vacated by infested pigs, they are likely to become infested.
Spread is by direct body contact, often the result of pigs crowding together for warmth or shade. Mites from the dam often invade neonatal piglets within a few hours of birth.
Pathogenesis
Mites spread among pigs in close contact and invade the skin. They burrow through the epidermis where eggs are laid. The mites are a strong irritant to the skin and initiate severe scratching and rubbing. In about two to three weeks, an allergic hypersensitivity develops in many pigs causing them to rub and scratch even more vigorously, further traumatizing the skin.
Mites tend to first colonize the skin of the inner surface of the ear. From this site, they spread out over the body, tail and legs. Ear lesions first appear as small encrustations. These enlarge to form plaque-like lesions. Constant rubbing and scratching irritates the skin at many sites on the body and promotes hyperkeratosis and proliferation of connective tissue in the dermis. These lead to thickening, wrinkling and folding of the skin. Hyperkeratotic, gray to white patches often form on the irritated skin. Reddened macules and papules that develop on the skin of young growing pigs are thought to be related to hypersensitivity.
Clinical signs
Frequent rubbing and scratching are the usual signs. These signs often are more marked when the skin is first warmed by the sun, presumably the result of increased activity of mites. Signs of rubbing are often apparent on equipment or facilities which have been polished by rubbing pigs. Decreased rate of growth, inefficient use of feed and low sow productivity follow. Although morbidity is high, mortality from mange alone is unusual.
Lesions
Young pigs that have developed hypersensitivity to mange have reddened macules or papules, especially over the rump, flanks and abdomen. Chronic mange often affects relatively few older animals in the herd and is less pruritic. Lesions vary in extent and advanced cases can involve the skin over much of the body. The posterior aspect of the legs is often affected as those areas are easily scratched. In some cases, lesions may be mostly confined to the inner surface of the ears where hyperkeratotic areas often develop a brownish to red discoloration.
Diagnosis
Signs and lesions are suggestive of mange. Marked pruritis often helps differentiate mange from other diseases of the skin. Diagnosis can be confirmed by identifying the mites in skin scrapings or in exudate from the external auditory canal. Scrapings from the inner surface of the ear usually reveal more mites than do other sites. Scrapings are usually wetted with 10% potassium hydroxide, cover-slipped, and examined under a microscope using a low power objective. Another simple technique is to place the scraping on a piece of black paper for a few minutes. Then carefully blow off the superficial debris and examine the site on the paper for the small, light colored mites. Diseases that must be differentiated from mange include parakeratosis, dermatomycosis, exudative epidermitis (“greasy pig disease”),pityriasis rosea, and photosensitization or sunburn.
Inspection of carcasses at slaughter may be useful, both in diagnosis and in evaluating the efficacy of a mange control program. An enzyme-linked immunosorbent assay (ELISA) test for serologic diagnosis of mange has been developed but is not widely available.
Control
Control programs should focus first on the breeding herd which, if infected, inevitably transmits mites to the piglets. These programs usually involve treatment of dams with external acaracide sprays or with an injectable avermectin a few days prior to entering the farrowing facility. Many farms using this control strategy find it necessary to also treat growing pigs at eight to ten weeks of age.
Eradication of mange is achievable in most production settings. Initiating an eradication program usually starts with culling of all severely infested breeding stock. Immediately following this, two consecutive doses of an injectable avermectin, 14 days apart, are administered to every pig on the farm (adults, neonates, and growing pigs). Removal of bedding material, if present, as well as application of an effective pesticide to environmental surfaces is often recommended at the same time injections are given. While eradication can be an expensive task in the short term, it is reliably efficacious and requires only standard biosecurity procedures to keep the mite from returning to the farm.
The avermectin class of parasiticides controls many external and internal parasites. One should carefully follow the directions that come with any product as some require a lengthy withholding period following their use. See the table, Anthelmintics and Parasiticides.