Mites that infest and parasitize domestic animals cause disease and loss of production. Mites are small invertebrates, most of which are free living but some are parasitic. Mites are similar to ticks and both comprise the order Acari in the phylum Arthropoda. Mites are highly varied and their classification is complex; a simple grouping is used in this introductory article. Vernacular terms to describe diseases caused by mites include scab, mange, and scabies. Mites and ticks have substantially different biology from, and are classed separately from, insects (the class Insecta). Mites of domestic animals cause important types of skin disease, and some mites infest other organs. Diagnosis of mite infestations can be difficult because of the small size of most mites, but understanding how mites are adapted to feed within the structure of the skin is useful.
Life-cycles
The life-cycle of mites begins with eggs that are laid on the vertebrate animal host or within the nest or environment of the host. From the egg hatches a larva, characterized by having three pairs of legs. The larva feeds on the host and molts to a nymph—similar to the larva but has four pairs of legs—which then feeds and molts itself. This molt is either to the first of several more nymph stages, or to an adult. The adult is defined by sexual maturity and has four pairs of legs. These similar stages (or instars) are in a sequence known as an incomplete metamorphosis. The potential reproductive capacity of a female mite is low compared to ticks because the eggs are large relative to the small female. However, the survival of larvae laid on their hosts or in nests of their hosts is high, and the life-cycle is short, so mite populations can expand rapidly under favorable conditions. Ectoparasitic mites typically transfer by crawling between hosts in close contact (see also section on control). The unusual life-cycle of trombiculid mites is described in the section on blood-sucking mites.
Diseases
Outer skin Infestation of the outer skin is typically caused by psoroptic mites. Psoroptes ovis, for example, infests sheep and cattle. Psoroptes ovis infests the superficial layers of the skin among the dead cells of the stratum corneum. Irritation of the outer skin by the mite's mouthparts and saliva results in cutaneous hypersensitivity and inflammatory exudation of serum and fresh cells. The mites feed on this exudate. The skin loses its hair at the sites of infestation and large flakes of dried serum and cells accumulate. The mites cause intense pruritus (itching) and the host will groom compulsively and may become severely distressed. Depilation (hair loss) may be substantial. Psoroptes ovis infests sheep worldwide and can be a serious welfare and animal production problem, mainly for sheep, but also cattle. Psoroptes cuniculi infests rabbits, mainly on their outer ear. Chorioptes bovis infestations are found on cattle, sheep and horses but do not cause the severe reactions associated with Psoroptes mites. Other common psoroptic mites are in the genera Chorioptes and Otodectes. Otodectes cynotis infestations in the ears of dogs are a common problem.
Living layer of epidermis This is typically caused by sarcoptic mites. Sarcoptes scabiei is an example: it infests many species of mammals, including humans. Other common sarcoptic mites are in the genus Notoedres, and the genus Knemidokoptes (or Cnemidocoptes) which infest birds. Sarcoptic mites as adults are microscopic, nearly circular in outline, and their legs are short, adapted for burrowing. The females, after mating with males on the surface of their host's skin, burrow into the living layers of the epidermis (mainly the stratum spinosum). They make long tunnels horizontal to the surface of the skin. Eggs are laid in the tunnels and development of larvae and nymphs occurs in such tunnels. The feeding of the mites and their excretory products irritates and inflames the skin, causing intense pruritus. Dermal hypersensitivity reactions will develop in the host. Chronic infestations lead to thickening of the skin by overproduction of epidermal cells (acanthosis), resulting in a characteristic depilated and scaly appearance. Stress caused by the pruritus can be severe, and will result in lost productivity of most species of livestock animals. Camels are prone to severe infestation and wild animals such as foxes may die from sarcoptes infestation.
Hair follicle One genus of mites, Demodex, has adapted to infesting the hair follicles of its hosts. Most species of mammal, including humans, are readily infested with these minute mites, but typically the infestation level is very low. The individual mites remain external to the epidermis within the follicle, but appear to be within the skin because they are below the general outer surface of the host. The mite Demodex canis is a common cause of demodicosis in dogs. Demodex mites are microscopic, cigar-shaped and have very short legs. These mites seem to feed on epidermal cells. They can crawl out on the surface of the skin, aided by secretions from the skin's sebaceous glands. Puppies can become infected by close contact with their mother during suckling. Puppies are more susceptible to infestation at this age because their immune systems are immature. All dogs will become infested in this way but usually only low levels of infestation persist, without pruritus or other signs of clinical disease. Some dogs become heavily infested, likely because of an immune dysfunction. This results in severe inflammation of the epidermis with acanthosis. The skin may become so thickened that folds form, and bacterial infection of excessive sebaceous secretions (seborrhea) may occur, producing an offensive smell. Demodicosis in cattle can occur as dense localized infestations. These create pustular folliculitis and indurated plaques within the dermis. This diminishes the commercial value of the animal's hide.
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