An Introduction to Medical Entomology
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There are many species in this Superfamily, the nymphs and adults of most being free-living. They are either vegetarian or predacious. Trachea are present and spiracles are located on or near the bases of the chelicerae. Mouthparts are raptorial or they are of the piercing and sucking kind. Their bodies are not heavily sclerotized and chitinous plates are usually absent.
Although there are a number of families, only one, the Trombidiidae, is of medical importance because the larvae are parasitic on humans and animals. Other families do have parasitic species, which are of minor importance.
The subfamily Trombiculinae includes the "harvest" and "chigger mites." They are conspicuous by their brilliant coloration. The widely known chigger, Entrombicula alfreddugesii (Oudemanus), is a pest of humans in North America. Typically in Trombidoidea, the larvae of this species are very tiny (e.g., E. batatas) and adhere to blades of grass in wild areas, from which they can transfer in large numbers to persons walking by. Then they become larger as they feed on body fluids, and eventually they drop to the ground to reproduce. The itching and subsequent scratching around the feeding sites can result in severe infections.
OTHER IMPORTANT SPECIES
Eutrombicula batatas (L.) is the "patatta mite" of South and Central America and the Caribbean. The life cycle differs slightly in the tropical environment. Trombicula autumnalis (Shaw) attacks animals and humans in Europe, where severe skin inflammation can result. In the Far East Trombicula akamushi (Brumpt) is the cause of "Japanese River Fever." Trombicula deiiensis Walch of the East Indies attacks animals and humans. Trombicula fletcheri W. & H. attacks humans in New Guinea. Many other unidentified species of Trombicula and other genera attack humans in the South Pacific.
Trombiculid mites have a complex life cycle and different terms have been applied to the developmental stages, but the terminology used by Service (2008) is applied as follows: Adults of this group are not parasitic but rather inhabit the soil where they feed on other arthropods. During warm weather a female mite may lay up to five eggs daily on organic material located on the soil surface, in field grasses, etc. "Deutorum" larvae with six legs emerge but initially do not leave the egg shell (the "Deutovum"). Activity begins about a week later when the mites swarm all over the soil and grasses. They try to attach to mammals and birds as well as to people with which they come into contact. They gather around soft and moist areas of a host.
The larvae then penetrate into the skin, injecting saliva that destroys cells. They feed on lymphal fluid instead of blood. The continued release of saliva then results a nasty skin reaction. Some species spend a whole month on a host, but the vectors of Scrub Typhus remain on a host for only about a week. When fully fed the larvae exit the host and drop to the ground where they bury into the soil or under leaf litter, etc. There they change into a "Protonymph," which moults within week and gives rise to a "Deutonymph" with eight legs. The deutonymphs like the adults feed for a couple of weeks on arthropods in the soil. Feeding stops and the nymphs change into a "tritonymph" that moults after about two weeks giving rise to the adult stage. The total life cycle generally takes up to two months, but sometimes 8-10 months are required.
Because nymphs and adults feed on other arthropods they require habitats where there are sufficient arthropods present to sustain them. Service (2008) noted that ideal habitats are often produced when vegetation is cleared for agriculture or wood products.
The mites can cause severe itching, which often leads to infections in humans. But some species are vectors of disease. Tstsugamushi Disease caused by a virus was first found in Japan where it is also known as "Japanese River Fever," but it is now widespread in Asia and Australasia The virus, Rickettsia orientalis, is transmitted by the bite of the red mite, Trombicula akamushi, and a local rodent serves as a reservoir of the virus. Incubation in humans is 7-14 days and mortality often follows, especially in older people.
Severe cases of infestation should always require the attention of a physician, but as with other groups of pestiferous mites avoidance of infested areas and the use of available repellants is advisable. Control of breeding sites in the environment may also be applied to reduce mite infestations. These sites exist as islands in the vegetation where mites can be reduced by burning or insecticide application.
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