Parasitic Insects, Mites and Ticks: Genera of Medical and Veterinary Importance/Surface mites

Surface feeders: Scab-mites, Itch-mites, Fur-mites and Trombiculids
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Characters of a diverse group of surface-feeding mites
The legs are usually long, with claws at their ends and also may be equipped with terminal suckers. The first segment (coxa) may have a thickened extension (an apodeme) that joins it to the coxae of other legs. The trombiculid mites are only parasitic as larvae, which have three pairs of legs. Body profile is usually oval but circular in Psorergates. Psoroptic mites are relatively large but psorergatic mites are small. Mouthparts are well developed and protrude. Setae tend to be long and usually protrude from the body margin or from the ends of some pairs of legs.
 * Photograph shows a Psoroptes mite from an infestation on outer ear of a rabbit, causing ear-canker.


 * Diagram of feeding at skin represents a surface feeding mite such as Psoroptes within the dead layers of its host's skin (relative size of mites is exaggerated).

Glossary

 * Apodeme = An extension of the coxa along the body wall of mites; appears like a thick line or rod (5 on Otodectes).
 * Coxa = First segment of leg of acarines and insects, closest to the main body.
 * Pedicel = Technical term for the thin stalk that carriers a sucker on the end of legs of some mites (2 on Chorioptes).
 * Erythema = Inflammatory dilation of skin capillaries, tending to give the area a reddish color.
 * Scab = In this context: a large flake of dead outer skin and dried serum, but also a vernacular term for infestation of sheep with Psoroptes ovis causing these scabs. Compare with mange (see Burrowing mites).
 * Sucker = A cup shaped adhesive organ at the ends of some legs of some mites (2 on Psoroptes).
 * Stratum corneum = The non-living outer layer of the skin of vertebrate animals; liable to infestation by mites.
 * Stylostome = A feeding tube formed by salivary secretion of trombiculid mites, that penetrates skin of host (see diagram of trombiculid feeding).

Psoroptes (Psoroptidae)
Characters: male left, female right, both ventral. 1- Well developed legs protrude far from body margin. 2- Suckers are shaped like shallow cups; they are on long stalks (pedicels) on leg pairs 1, 2 and 3 of males, and legs 1, 2 and 4 of females; the stalks have rings. 3- Claws are present at ends of legs 1, 2 and 3 in males, and legs 1 and 2 in females. 4- Mouthparts are well developed and sharply pointed. 5- Adult mites are large enough to be visible to unaided vision. 6- Long setae protrude from leg pair 3 of both sexes. Also: dorsal surface has no spines, and profile of an egg shows in female at right.

Hosts: Sheep are parasitized by Psoroptes ovis, the Sheep scab-mite, and this species also can feed on cattle and horses. Rabbits are the main host of P. cuniculi; this species feeds in the outer ear and will also infest ears of sheep, cattle, horses and donkeys.

Signs and disease: Infestations of sheep start from back line and may spread as the mites progress into areas of fresh skin, leaving behind a depilated, raw and scabby skin surface. The mites remain external to and within the non-living stratum corneum where they induce an inflammatory exudate. Mites appear to feed on this exudate. Heavy infestations are conspicuous by the scabbing, depilation and severe pruritus. The itching can be so severe that it leads to distress and compulsive self-grooming by sheep. Sheep-scab is major cause of lost production. Transmission between sheep in close contact is by contagion; also these large mites can survive for days and be transmitted on fomites such as pieces of wool. Psoroptes cuniculi causes inflammatory otitis leading to ear canker.

Chorioptes (Psoroptidae)
Characters: male left, female right, both ventral. 1- Well developed leg pairs protrude far from body margin. 2- Suckers are shaped like deep cups; they are on short stalks (pedicels) and are present on all leg pairs of males but those on legs 4 are very small; on females the suckers are on leg pairs 1, 2 and 4; the stalks are not ringed. 3- Claws are present at ends of legs 1, 2 and 3 in males, and legs 1 and 2 in females. 4- Mouthparts are well developed and blunt. 5- Adult mites are moderately large. 6- Very long setae protrude from legs 3 of both sexes. Also: dorsal surface has no spines.

Hosts: Chorioptes bovis parasitizes cattle, sheep, goats and horses.

Signs and disease: Infestations usually start from the legs or base of tail, and may spread from there. The neck and head may be infested. The etiology of Chorioptes scab is similar to that relating to Psoroptes ovis but usually forms more localized scabbing and less severe disease and loss of production.

Otodectes (Psoroptidae)
Characters: male left, female right, both ventral. 1- Well developed legs protrude far from body margin (except for legs 4 of females). 2- Suckers are shaped like deep cups; they are all of equal size, on short stalks (pedicels) and are present on all leg pairs of males, and on legs 1 and 2 of females; the stalks have no rings. 3- Claws are present at ends of legs 1, 2 and 3 of males; and on legs 1 and 2 of females. 4- Mouthparts are well developed; blunt in males, sharper in females. 5- Apodemes of legs 1 and 2 are joined. 6- Adult mites are moderately large. 7- Very long setae protrude from legs 3 of both sexes. Also: dorsal surface has no spines.

Hosts: Dogs and cats are infested.

Signs and disease: Otodectes cynotis parasitizes the outer surface of the skin of its host, mainly confined to the outer ear. Allergic hypersensitivity leads to parasitic otitis. Heavy infestations in some individual hosts lead to thick waxy scabs and crusts forming on the inner surface of the ear pinna where the mites feed.

Psorergates (also known as Psorobia) (Psorergatidae)
Characters: female, dorsal (inset is ventral). 1- Body profile is circular, with all legs protruding well beyond body margin. 2- Legs are thick and equipped with small paired claws. 3- Mouthparts are well developed and protrude from body margin. 4- Second segment of legs has a forward projecting spine (seen ventrally), and third segment has hooked appearance. 5- Females have 2 pairs of long setae projecting from posterior of body; males have 1 pair. 6- These mites are small.

Hosts: Psorergates ovis are found on sheep and P. bos on cattle.

Signs and disease: These mites feed within the dead layer (stratum corneum) of skin. Infestations of sheep by P. ovis are irritating and pruritic; heavy infestations induce intense self-grooming that may reduce the wool crop.

Cheyletiella (Cheyletiellidae)
Characters: adult, dorsal. 1- Body profile shows distinctly the anterior gnathosoma and posterior idiosoma. 2- Palps are formed into very large claw-like extensions. 3- Chelicerae of mouthparts are formed into a pair of fine piercing parts. 4- Legs are well developed, project from body margin and end in claws formed like combs. 5- Setae are sparse but large and may have a finely feathered appearance. 6- Integument is partly striated. 7- A pair of large plain setae project from posterior body margin.

Hosts: Cheyletiella blakei (Cat fur-mite) infests the fur of cats, C. parasitivorax (Rabbit fur-mite) infests rabbits. These mites easily get onto other hosts, humans included, and temporarily infest them.

Signs and disease: Irritation, dermatitis, and dandruff occur; heavy infestations can sometimes build up causing erythematous pustules. These mites may also distress the owner's of the cat or rabbit.

Characters of trombiculid blood-feeding mites
These mites are only parasitic during their larval stage. Eggs laid on the ground hatch into larvae which wait on vegetation for hosts. The larvae attach by secreting saliva that forms a feeding tube (stylostome) in host's skin. The larvae engorge into bright red spheres before detaching. Then they molt into the next free living stages.
 * Diagram of feeding at skin represents a larval trombiculid mite. The mite produces a stylostome (feeding tube) through the epidermis of its host (relative scales not accurate).


 * Photograph shows a larval trombiculid mite, ventral view. As a larval acarine it has only three pairs of legs. The chelicerae and palps of the mouthparts are visible but this engorged specimen has detached from its host, leaving the stylostome feeding tube in the skin of its host.

Neotrombicula (also known as Trombicula) (Trombiculidae)
Characters: larva (unfed), dorsal. 1- Body profile is oval. 2- Mouthparts are well developed, with distinct chelicerae. 3- Legs (3 pairs only in larva) are well developed and protrude from body margin; they end in a pair of claws and an empodium. 4- Dorsal plate is distinct and bears several feathered setae and a pair of sensillae. 5- Body and legs have a relatively small number of large setae; some of them are smooth but most of them are feathered. 6- Unfed larvae are just visible to unaided vision.

Hosts: Larvae of trombiculid mites feed on many species of mammal: rodents, dogs, cats, rabbits, sheep, cattle, and humans. Some genera are adapted to feed on poultry birds. (Other genera of importance to domestic animals include: Eutrombicula, Leptotrombidium, Neoschoengastia. Vernacular names include: Harvest-mites, Berry-bugs, Scrub-itch mites, and Chiggers (but avoid confusion with names of Tunga fleas.)

Signs and symptoms. Irritation, and active self-grooming in response to intense pruritus are caused. The dermatitis is localized to the point where the mite fed then left behind its stylostome, which is antigenic. Engorging larvae become bright red and visible when clustering at typical sites such as the edge of ear pinna of dogs and cats. On humans the mites often crawl under clothing until the constriction of a belt or strap. There they attach and after detaching a distinct erythematous and intensely pruritic spot develops.

Disease: Continued exposure to numerous mites causes dermatitis and stress which can be a production problem in poultry rearing. Dogs and humans can suffer biting stress from these infestations. Trombiculid mites are notorious as vectors of Orientia tsutsugamushi bacteria to humans, causing scrub-typhus or Tsutsugamushi disease.

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