Wohlfahrtia vigil Walker
ETYMOLOGY:Wolhfartia for Dr. Wohlfart and vigil = keeping vigil about the host
HISTORY: Walker (1920, 1922, 1931) reported the first and subsequent cases of Wohlfahrtiavigil in children; Brady (1923) and Chown (1924) reported additional cases in infants. Johanssen (1926) reported this parasite in rabbits near Ithaca, New York, USA while Kingscote (1931) reported the disease in a silver fox puppy. The first documentation of this parasite in domestic cats was in a report by Kingscote (1935) who recorded it in four cats; he also recorded the parasite in mink, man, dogs, ferrets, rabbits and foxes. Wohlfahrtiavigil produces an extremely rare cutaneous myiasis in cats. Only one case report of Wohlfahrtiavigil in cats, a case from the Veterinary Teaching Hospital at Colorado State University could be documented using the Veterinary Medical Data Base (Purdue University, West Lafayette, Indiana, USA),.
GEOGRAPHIC DISTRIBUTION: This parasite is responsible for cutaneous myiasis in North America, particularly in southern Canada and the northern part of the United States (James, 1947). The adult flies have been recorded from the New England states to Alaska (Walker, 1931), but most records of myiasis produced by their larvae are from eastern sections of Canada and the neighboring northeastern parts of the United States. The area in which most cases have been recorded extends roughly from the 43rd parallel of latitude to the 50th and from the 74th to the 104th parallels of longitude. The most southern record is from Erie, Pennsylvania; the northern from Winnipeg, Manitoba; the eastern from Montreal, Quebec; and the western from the Dakotas (Kingscote, 1935). A case of Wohlfahrtia has been reported in a cat from the Veterinary Teaching Hospital, Colorado State University (Personal communication, Nancy Hampel, DVM, 1994).
LOCATION IN HOST: Many members of the family Sarcophaginae, the flesh flies, deposit their eggs or larvae in carrion, others in purulent wounds or sores, and others in feces. Wohlfahrtiavigil is the only species for which all reports of infection are in the skin of healthy animals (Walker, 1931). Infection follows the deposition of fly larvae on the unbroken skin of young animals. The lesions in cats have been predominantly located on the head, often between the eyes (Kingscote, 1935).
IDENTIFICATION: Adult Wohlfahrtiavigil are non-parasitic and as a result will probably not be observed by the client or the veterinarian. They are large, grayish flies (approximately 13 mm in length), about twice the size of the house fly, Muscadomestica. The dorsal surface of the thorax is marked with three longitudinal bands, while the dorsal surface of the abdomen exhibits three, well-defined rows of oval black spots which are confluent with one another (Kingscote, 1935).
The first larval stage will probably go unnoticed due to its small size, from 1.5 mm at its hatching to 3.5 mm at the time of its molt to the second stage. The third stage is from 7.0 to 16.5 mm in length. The second stage fills in the size gaps. All three stages are "maggot-like" in their appearance and exhibit cephalopharyngeal sclerites and posterior spiracles that are unique to the species (Walker, 1931). The third stage larva is a large larva. Its posterior end is narrow and its integument is covered with many irregular rows of small, dark pointed, posteriorly directed spines which are larger than those of the members of the genus Sarcophaga. This larva is better adapted to maintain an attachment to living tissues. The oral hooks are strongly developed and the cephalopharyngeal skeleton is like that of the larva of Sarcophaga. 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 slits that are wider and a peritreme that is stronger and wider than those of Sarcophaga species (Kingscote, 1935, Walker, 1937).
The second and third stage larval Wohlfartiavigil are the stages that will be usually observed in the skin of an infested cat by either the owner or the veterinarian. These stages can probably be best identified by an entomologist. Extensive descriptions and dichotomous keys for these larval stages are available (Johannsen, 1921, Walker, 1937, James and Gasser, 1947).
LIFE CYCLE: This flesh fly is larviparous; it deposits larvae instead of eggs on healthy and uninjured skin of suitable hosts (Eschle and DeFoliart, 1965). The larva penetrates the unbroken skin and forms a boil-like (furuncular) swelling through which the posterior end of the larva may be seen. Here the larva develops to the third larval stage. The larval development is usually completed in nine to fourteen days (Kingscote, 1935). The parasite then leaves the host tissue, drops to the ground and pupates. The pupation period usually lasts from eleven to eighteen days; this variation corresponds with 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 seven months. The parasite survives the winter in the pupal form. The adults emerge from pupae and about three or four days later mate. About a week later, the female flies commence larviposition, depositing six to sixteen larvae at a time. The female flies live from thirty-five to forty days. The males seldom survive more than three weeks (Kingscote, 1935). Details of the life cycle of Wohlfartiavigil have been defined in the laboratory (Ford, 1932, Ford, 1936, Eschle and DeFoliart, 1965).
//CLINICAL PRESENTATION AND PATHOGENESIS: The female Wohlfartiavigil deposits active larvae in the neighborhood of a suitable host or directly on the host itself. The larva penetrates the unbroken skin and forms a boil-like (furuncular) swelling through which the posterior end of the larva may be seen. 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 an exudation of serum and matting of haircoat 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 opening is at times visible. As the lesions develop, they may be palpated. On the third or fourth day, the larvae are 1.5 to 2.0 cm in length and produce an abscess-like lesion resembling a warble of Hypoderma species in cattle. These lesions vary in size, shape, position, and the number of larvae they may contain. The haircoat often becomes parted over the summit of the lesions and reveals an opening generally 2.0 to 3.0 mm in diameter. The posterior aspect of the larvae are presented to these openings through which they breathe. These openings are circular and well-defined. If several larvae are present in a single lesion, the shape of the aperture is quite variable. Small animals infected with five or more larvae for several days become emaciated and the skin becomes dry and loses its lustre.
The penetration of the skin by the larvae, their development in the subcutaneous tissues and the secondary bacterial infection produce intense irritation and inflammation of the tissues. Attempts by the cat to remove the larvae or relieve the irritation tend to aggravate the condition. There is fever, loss of appetite and rest, and progressive emaciation. Young animals may die from exhaustion. It has also been suggested that the larvae may produce toxic secretions (Kingscote, 1935).
DIAGNOSIS: The presence of a dermal swelling with a central opening may lead to a tentative diagnosis of myiasis due to Wohlfahrtiavigil. A definitive diagnosis can be made only after extraction and identification of a typical larva. Extensive descriptions and dichotomous keys for the three larval stages are available (Johannsen, 1921, Walker, 1937, James and Gassner, 1947).
A tentative diagnosis may often be made by a history of either residence in or travel to a geographic area endemic for Wohlfartia. vigil. There are other flies from different areas throughout the world which might produce a lesion similar to Wohlfartiavigil. These should also be considered in a differential diagnosis. These myiasis-producing flies include: Cordylobiaanthropophaga, the African Tumbu fly; Dermatobiahominis, the South American tórsalo fly; Chrysomyiabezziana, the Oriental fly; Cochliomyiahominivorax, the primary screwworm, and Cuterebra species, the rabbit botfly. The obligatory myiasis-producing flies should be considered in the differential diagnosis in cats with an appropriate residence or travel history.
TREATMENT: The larva must be extracted from the skin of the feline host. As with all flies that localize in subcutaneous tissues, treatment is based on the requirement of the larvae for communication with the outside environment. Air exchange is accomplished via the larva's posterior spiracles. The airway may be occluded using heavy oil, liquid paraffin, pork fat or petroleum jelly.
Surgical intervention involves the injection of lidocaine hydrochloride into the furuncular lesion. This anesthetizes both the cat and the larva, allowing the larva to be manually extracted using thumb forceps. Antibiotics should be prescribed.
Great care should be taken during the extraction process to avoid rupturing the larva in situ, although no mention is made in the literature of anaphylaxis as occurs when larvae of Cuterebra species are ruptured during the extraction process. In suspect cases, the extracted larva should be submitted to an entomologist for definitive diagnosis.
EPIZOOTIOLOGY: There is evidence that adult rabbits and rodents may be implicated in maintaining populations of the fly other than in the whelping season of the carnivore host. It appears that the combination most conducive to a severe outbreak of myiasis in mink is warm temperatures in May and June following a winter with good snow cover and a decline in the rabbit population (Eschle and DeFoliart, 1965). Children and young mink, ferrets, dogs, cats, foxes and rabbits may become infected from the beginning of June until the end of September. Adult animals are seldom affected (Kingscote, 1935).
HAZARDS TO OTHER ANIMALS:Wohlfahrtiavigil is a pest of fox and mink ranches in parts of the United States and Canada. Young, newborn animals are attacked, usually fatally. Dogs, rodents and rabbits may be attacked (Kingscote, 1935,). Craine and Boonstra (1986) described myiasis by Wohlfartiavigil in nestling Microtuspennsylvanicus. Although birds, mice, and other small mammals may be suitable hosts for Wohlfahrtia, it has been observed that the cottontail rabbit, Sylvilagusfloridanusmearnsii, is an especially suitable host and perhaps the major host (Johannsen, 1926, Eschle and DeFoliart, 1965).
HAZARDS TO HUMANS:Wohlfahrtiavigil has been recovered from the skin of young children, particularly infants. The tender skin of young children may prove to be a route of easy penetrati`
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Chown G. 1924. Report of a case of cutaneous myiasis in an infant. Wohlfahrtiavigil (Walker) Infect 14:967-968.
Craine ITM, Boonstra R. 1986. Myiasis by Wohlfahrtiavigil in nestling Microtus pennsylvanicus. J Wild Dis 22:587-589
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