Classification: Taxonomic ranks under review (cf. Encyclopedic Reference of Parasitology, 2001, Springer-Verlag)

Metazoa (Animalia) (multicellular eukaryotes, animals)
Arthropoda (arthropods, segmented body, exoskeleton, jointed appendages)
Chelicerata (2 body parts, 8 legs, first mouthparts chelicerae, no antennae, wingless)
Arachnida (abdomen without appendages)
Acari (ticks and mites, ectoparasites)
Astigmata (Sarcoptiformes) (without tracheal system, respire through tegument)

Family: Sarcoptidae
Mites are small wingless arachnids with two body parts, eight legs and no antennae. Astigmatid mites are weakly sclerotized, lack stigmata and tracheae and respiration occurs directly through the tegument. They include many species of medical and veterinary significance and cause skin conditions known as mange, scab and scabies. Sarcoptid mites burrow in the skin of their hosts and lack claws but have suckers at the ends of their legs. They undergo incomplete metamorphosis whereby eggs hatch larvae which transform to nymphs and then adults. All feeding stages are parasitic and infestations are transmitted directly between hosts by contact.

Sarcoptes scabei [this species causes scabies in humans]

Parasite morphology: Mites form four developmental stages: eggs, larvae, nymphs and adults. The eggs are oval and large compared to the size of the adult mites (about half their length). Emergent larvae have three pairs of legs but undergo metamorphosis to form nymphs then adults which have four pairs (pairs 3 and 4 do not project beyond the body margin). These developmental stages are variable in size but successively become larger. Adult female mites are the largest (0.3-0.6mm long) while adult males are smaller (up to 3mm long). They have circular bodies which are flattened ventrally and covered with fine transverse striations. They have two body parts, the anterior gnathosoma bearing specialized feeding structures including palps and chelicerae, and the posterior idiosoma bearing the legs and elongate sensory setae.

Host range: Sarcoptes scabiei (scabies/sarcoptic mange mites) are minute skin parasites of homiotherms throughout the world. Different subspecies are found on different mammals and are responsible for causing mange in animals and scabies in humans. Although the cross-transmission potential of many subspecies has not been established, zoonotic transmission is thought to occur although such infestations do not appear to become permanently established on humans.

Site of infection: Sarcoptid mites are ectoparasitic and live on the skin of their hosts where the females burrow to lay their eggs. Infestations can occur anywhere on the body, but are more common in areas where the skin is thin and wrinkled, such as between the fingers, toes and genitals of humans, and the ears, muzzle and face of animals.

Pathogenesis: Infestation by mites is known as acariasis and can produce severe dermatitis. Nymphs and males do not burrow, but females form long tortuous tunnels in the horny layer of skin, depositing eggs and faeces, causing intense itching and rashes. Burrows may be 2-3cm in length and may be excavated at up to 5 mm per day. Mites reproduce on their hosts so infestations can become progressively worse without further exposure. Common signs are papular eruptions with erythema, pruritis and alopecia. The scabies itch takes 6-8 weeks to appear after the patient becomes sensitized, and is characteristically noctural and aggravated by warmth. As infestations progress, the skin becomes thickened and crusted with exudates. Septic pustules due to secondary infections are common in severe infestations, particularly when hosts scratch causing traumatic damage. A rash is sometimes evident around the waist, buttocks, wrists or ankles due to cell-mediated immune reactions. In immunocompromised patients, extensive thickening and crusting of the skin may occur.

Mode of transmission: Mites spend most of their lives in intimate contact with their hosts, so transmission between hosts is mainly by direct physical contact. Female mites lay 1-3 eggs per day and they mature within 4 days. Emergent larvae moult 2-3 days later to form nymphs which then moult several times over several days before forming adults. Larvae and nymphs move out of the burrows and find food and shelter in hair follicles. Adults feed and mate on the surface, the males die and the fertilized females start burrowing. The entire life cycle is usually completed within 3 weeks but can take as little as 12 days under the right conditions. Epidemics of scabies occur in human populations in 20-30 year cycles or in times or famine or war. However, mites are common in poor communities with inadequate washing facilities.

Differential diagnosis: Confirmation of suspect infestations is generally done by microscopic examination of skin scrapings or by wiping black ink over affected areas to reveal burrows.

Treatment and control: Scabies responds to whole body treatment from the neck down with acaricides, such as malathion, gamma benzene hexachloride, benzyl benzoate or crotamiton for infants. Topical steroids should not be used on humans and whole families should be treated. Infestations in animals can be treated with topical or systemic acaricides, including organochlorines (trichlorphon), bromocyclen, organophosphates (amitraz, phosmet) and the macrocyclic lactones (moxidectin, ivermectin, selamectin). Many formulations contain surfactants which serve to soften crusts and remove skin scales. Hair can also be clipped from affected areas and the skin cleaned with anti-seborrhoeic shampoos prior to acaricide treatment. Lime-sulphur dips have also been used at 10 day intervals for treating dogs and cats. Treatments should be repeated weekly for several weeks to ensure newly emergent mites are killed. Infested animals should be isolated or treatments extended to animals in close proximity. Corticosteroids can also be used in severely distressed animals to reduce pruritis and prevent further excoriation.