Arthropods as Parasites

Sarcoptes scabiei. (Itch mite)

S. scabiei (0.5 mm in length)is cosmopolitan in distribution. It parasitizes both domestic animals and man, causing a disease known as scabies in human, mange in animals.

The adult mites enter the skin, digging sinuous burrows in the upper layers of the epidermis.

Eggs deposited in the burrows hatch after three or four days into larvae, which excavate new burrows and mature in about four days.

Preferred sites of infestation are the interdigital and popliteal folds, the groin and the inframammary folds. Many other parts of the body may also be involved.

The activities and secretions of the mites cause intense itching of the affected areas. Very small vesicles may be seen on the skin surface. Scratching may result in bleeding and scab formation, frequently followed by secondary bacterial infection.

Transmission of the mites is accomplished by direct contact with infected persons or with their clothing or bedding.

Diagnosis of scabies can be made is by finding the parasites or their eggs.

Demodex spp.

The mite Demodex folliculorum hominis (class Arachnida, order Acarina), lives around hair follicles or in the secretory ducts of sebaceous (fat) glands connected to the hair of humans. Varying in size from 0,1 mm to 0,4 mm long, its live in pores and hair follicles, typically on scalp, cheeks, nose, eyebrows and often in the roots of eyelashes.

In humans, the infestation is known as demodicosis and occurs world-wide.

Glanulomatous reactions and inflammation in acute and chronic forms may occur

Demodicosis may also occur.

The skin irritation of the infected animals is sometimes very extensive and results in loss of hairs and severe skin rashes.

Pediculus humanus.

Lice are dorsoventrally flattened insects.

Males are about 2 mm., females 3 mm., long. The whole life cycle occurs on the human host.

Two subspecies, or biological races, are commonly recognized:

· Pediculus humanus capitis (head louse) infests the scalp. The female lice deposit their eggs on the hairs, where they are firmly attached. The eggs or nits are quite small white, and may be seen with the naked eye. About 10 days after deposition they hatch into nymphs, which are structurally quite similar to the adults and mature in about two weeks. Both larvae and adults feed on blood obtained by their piercing mouth parts and a pumping device located in the pharyngeal region.

· The body louse, P. humanus humanus lives on the protected parts of the body. Its life history is similar to that of the head louse, which it resembles in appearance.

Head or body lice are transferred from one person to the next by direct contact or by contact with clothing, hats, or hairs from "lousy" individuals. Body lice are important vectors of disease, transmitting epidemic typhus, trench fever, and the louse-borne variety of relapsing fever

Phthirus pubis (pubic or crab louse) is found on the hairs of the genital region but may occur elsewhere on the body.

Infestation with lice is usually referred to as pediculosis. Some people are very sensitive to the bites, which produce macular swellings and a great deal of pruritus. In people who have had lice for long periods the skin may become thickened and show spots of hyperpigmentation, the maculae caeruleae.

Myiasis

Myiasis, or infestation with fly larvae, is commonly seen in domestic and wild mammals all over the world

Many different species of flies produce myiasis.

Infection with fly larvae may occur when flies deposit eggs or first-stage larvae on the body or its apertures.

An open wound or lesion, for example, may attract certain flies which will then deposit their eggs or larvae in the area of injury.

Larvae of certain other species invade the unbroken skin.

Some will gain access to the body via the nose or ears. If eggs are deposited on the lips, within the mouth or on food, they may be swallowed and develop in the stomach or intestine, giving rise to gastric or intestinal myiasis.

Specific Myiasis

Flies which require a host for the development of their larval stages belong to a large number of different species which habitually parasitize animals.

Various botflies (ОВОДЫ), including the sheep botfly, Oestrus ovis.

The fly deposits living larvae in the nostrils or on the eyes of sheep and goats; and in man they may develop in the conjunctival sac, producing ophthalmomyiasis.

Species of Hypoderma, the cattle botflies or ox warbles, are also worldwide in distribution. Eggs are deposited on the skin and hatch into larvae, which after having penetrated the skin migrate throughout the body. Eventually they come to lie beneath the skin, producing a local swelling.

Horses also have botflies, which belong to the genus Gasterophilus. The fly deposits eggs on the hair, skin or lips. After hatching, the larvae either penetrate the skin or are swallowed.

Semi-specific Myiasis

Among the flies which cause what is known as semi-specific myiasis, there are some species which normally lay their eggs in decaying animal or vegetable matter. Prominent representatives of this group are species of Phaenicia (Lucilia), the green-bottle flies; Callitroga, or blue-bottle flies; and black-bottles, belonging to the genus Phormia (мясные мухи). All of these are worldwide in distribution, frequenting areas of human habitation. They occasionally will lay their eggs on open sores of animals or man, especially if the sores are necrotic and malodorous. Large numbers of these flies congregate near slaughterhouses and other unclean places.

Another group which causes intestinal and other types of myiasis in man is the flesh flies, or sarcophagids. Species of Sarcophaga are world-wide in distribution. They normally breed in carrion or other decomposing matter and may deposit their eggs on various foods, such as meat or fruit. Thus, ingestion of contaminated food may be a source of infection.

Accidental Myiasis

Flies producing accidental myiasis have no requirement or even preference for development in a host. However, eggs may be deposited accidentally on oral or genitourinary openings and the larvae gain entrance to the intestinal or genitourinary tract. Musca domestica, the housefly; Fannia, or latrine flies; certain species of flesh flies; green- or blue-bottle flies; and many others may produce accidental myiasis.

Nosocomial Myiasis will often cause obstruction of the nasal passages, severe irritation and in some cases facial oedema and fever. Equally dangerous is aural myiasis, in which the patient may complain of crawling sensations and buzzing noises. A foul-smelling, purulent discharge may be present. If located in the middle ear, the larvae may find their way into the brain.

Myiasis in hospitalized patients occurs with some frequency. Bedridden patients with open wounds or sores may become infected, if flies are about. It is therefore important to keep the rooms of such patients free of flies.

Ophthalmomyiasis is not uncommon and is accompanied by severe irritation and pain.

A definitive diagnosis of myiasis can be made only upon finding the larvae, which should be removed and kept for identification. The clinical aspects of myiasis will vary with the regions affected, with the species of fly involved and with the numbers of maggots present. The possibility of maggot infection or infestation is frequently overlooked. The symptoms of intestinal myiasis are not specific, and neither are those of urinary myiasis. Cutaneous myiasis should be suspected in a patient with painful, indolent ulcers or furuncle-like sores of long standing. In addition, certain botfly larvae may give rise to dermal lesions similar to those of creeping eruption

Tunga penetrans (Chigoe Flea) parasitizes many different kinds of warm-blooded animals, including man. It is found in tropical and subtropical regions of America as well as in Africa and the Far East, where it has been introduced from the New World. Chigoe fleas are relatively small, measuring no more than I mm in length. While the males and virgin females behave in the usual manner of fleas, the fertilized female has the distressing habit of burrowing into the skin, preferably under the toenails or between the toes, where she feeds on blood. As the eggs develop, the embedded female becomes progressively larger until the abdomen assumes a nearly spherical shape. At this time, a part of the abdomen is seen to protrude slightly beyond the skin surface. Mature eggs, sometimes as many as one hundred, are expelled to the outside and continue their development in the soil. The female eventually drops out of the host.

Infection with chigoe fleas is extremely irritating and usually produces inflammation and ulceration; the latter is primarily due to the development of secondary infection. Diagnosis is made by detecting the dark portion of the flea's abdomen on the skin surface. Surgical removal of the flea is indicated.

Date: 2014-12-21; view: 866