| The gray flesh fly,
Wohlfahrtia
vigil
is responsible for cutaneous myiasis in North America, particularly in southern Canada and the northern part of the USA. The adult flies have been recorded from the New England states to Alaska, but most reports are from eastern sections of Canada and the neighboring northeastern parts of the USA. All reports of infestation are in the skin of healthy animals, particularly the unbroken skin of the young. |
| All 3 larval stages are maggot-like in their appearance and have posterior spiracles that are unique to the species. The first larval stage is 1.5 mm at hatching and grows to 3.5 mm at the time of its molt to the second stage. The third stage is 7.0-18.5 mm long. Its posterior end is narrow, and it is covered with many irregular rows of small spines that have dark points and are directed posteriorly. This larva is better adapted to maintain an attachment to living tissues. The oral
hooks are strongly developed. The posterior end of the larva has its spiracular plate located in a deep pit formed by the margins of the segment. The posterior spiracles have wide slits and a strong peritreme. |
| The gray flesh fly is larviparous—it deposits larvae instead of eggs on healthy, uninjured skin of suitable hosts, particularly young animals. Larvae penetrate the unbroken skin and form a boil-like (furuncular) swelling. Development to the infective third-larval stage is usually completed in 9-14 days. The parasites then drop to the ground and pupate, ~11-18 days, varying with the season of the year and the temperature. When cold weather approaches, the pupation period is
greatly prolonged. Under laboratory conditions, it has been observed to last 7 mo. Parasites survive the winter in pupal form. Adults emerge and mate after ~3-4 days. Female flies begin larviposition ~1 wk later, depositing 6-16 larvae at a time. Female flies live for 35-40 days; males seldom survive >3 wk. |
| Pathology: |
| Female
W
vigil
deposit active larvae near or directly on the host. Although larvae usually penetrate unbroken skin, in small animals, penetration may go deeper than the dermal tissue, even into the coelomic cavity. |
| The first indication that an animal is infected is exudation of serum and matting of the hair coat over the site of penetration. In light-skinned animals, a small inflammatory area is noticeable in the center or to one side of which a tiny hole is visible. These lesions may be palpated as they develop. On the third or fourth day, the larvae are 1.5-2 cm long and produce abscess-like lesions resembling those of
Hypoderma
spp
in cattle. These lesions vary in size, shape, position, and the number of larvae they contain. The hair coat often becomes parted over the summit of the lesions and reveals an opening 2-3 mm in diameter. The posterior aspect of the larva is visible in these openings, through which it breathes. Openings are generally circular and well-defined; however, if several larvae are present in a single lesion, the shape of the opening is quite variable. Small animals infected
with ≥5 larvae for several days become emaciated, and the skin becomes dry and loses its luster. |
| The penetration of the skin by the larvae, their development in the subcutaneous tissues, and secondary bacterial infection produce intense irritation and inflammation. Attempts by the animal to remove the larvae or relieve the irritation tend to aggravate the condition. Young animals may die from exhaustion. It has also been suggested that the larvae may produce toxic secretions.
W
vigil
has been isolated from the skin of young children, particularly infants. |
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| Diagnosis: |
| Adult gray flesh flies are nonparasitic and as a result will probably not be seen by owners or veterinarians. They are large grayish flies (~13 mm long), about twice the size of a house fly. The dorsal surface of the thorax is marked with 3 longitudinal bands, while the dorsal surface of the abdomen has 3 well-defined rows of oval black spots that are confluent with one another. |
| The identification of adult flies and their associated larval stages should be left to an entomologist. The presence of a dermal swelling with a central opening may lead to a tentative diagnosis of myiasis due to
W
vigil
. A definitive diagnosis can be made only after extraction and identification of a typical larva. Extensive descriptions and dichotomous keys for the 3 larval stages are available. A tentative diagnosis may often be made by a history of either residence in or travel to a geographic area endemic for
W
vigil
. |
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| Treatment and Control: |
| Larvae must be extracted from the skin. Applying heavy oil, liquid paraffin, pork fat, or petrolatum jelly to the opening of the lesions will occlude the airway of the larvae. Applying a small amount of chloroform or ether to the opening may be helpful before removing larvae with forceps. Lidocaine hydrochloride can also be injected into the furuncular lesion to facilitate extraction. Great care should be taken during the extraction process to avoid rupturing larvae in situ,
although anaphylaxis has not been reported. Antibiotics should be prescribed. |
| This parasite often infects young mink. A teaspoon of ronnel can be placed in the bedding of their nest box as a control measure; however, ronnel should not be used in the bedding of kits <3 days old. Protection can be provided by keeping flies out of cages using wire gauze. |
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