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Dermatobiahominis (Linnaeus, 1781)

(Figure 5-70)

ETYMOLOGY:Dermato = skin + obia = way of living and hominis for the choice of human hosts. Common names include the human botfly, tropical warble fly, beef worm, bekuru, bikuru, berne, borro, colmoyte, forcel, gusano macaco, gusano de monte, gusano de mosquito, gusano de zancudo, gusano peludo, kturn, kitudn, ikitugn, mberuaró, mirunta, moyocuil, muskietenworm, nuche, nunche, suglacuru, suylacuru, torsel, tórsalo, tupe, ura, and ver macacque

SYNONYMS:Oestrushominis, Oestrushumanus, Oestrusguildingii, Cuterebra cyaniventris, Cuterebranoxialis, Dermatobianoxialis, and Dermatobiacyaniventris (Guimaraes and Papavero, 1966)

HISTORY: Records of early exploration in Panama reveal that Dermatobiahominis has been known as a human parasite in Panama for almost a century and a half. If earlier records were available, they would probably show that this fly has been known by indigenous Americans for centuries (Dunn, 1934). The natives of Central and South America have long known animals to be infected with larvae of Dermatobia species. The ancient Mayans referred to this parasite as "saglacuru" and believed that it owed its existence to the bite of some kind of mosquito (Hoeppli, 1959). It is strange how close to the truth these ancient peoples were. The ancient Mayans extracted the larva of Dermatobia by covering its breathing pore with heavy oil (Ockenhouse etal., 1990). In the earliest report of feline infestation by Dermatobiahominis, Dunn (1934) reported that "three half-grown larvae were found in a vagrant cat."

GEOGRAPHIC DISTRIBUTION: This botfly occurs in Mexico, Central America and South America (File et al., 1985).

LOCATION IN HOST: The larval Dermatobia. hominis penetrates the skin of the definitive host producing a cutaneous furunculoid (boil-like) myiasis (Pallai etal., 1992).

IDENTIFICATION: The adult Dermatobiahominis is about 1.5 to 1.8 cm in length, approximately the size of a bumblebee. It has a blue-gray thorax, a metallic blue abdomen, and yellow orange legs (Pallai et al., 1992). The adult has no functional mouthparts and takes no nourishment (Rossi and Zucoloto, 1973). Food stored during the larval stage provides the adults with nourishment (Prasad and Beck, 1969).

The mature larva at two to three months of age is 1.8 to 2.4 cm in length (Fig. 5-70). It has a definite club shape and can be identified by rows of posteriorly directed spines on its anterior segments. The larval stage also possesses the caudal spiracles which protrude through the host's skin to the exterior to guarantee an adequate air supply (File etal., 1985).

The larva of Dermatobiahominis is narrow and tubular at its posterior extremity and somewhat flask-shaped anteriorly (Patton and Evans, 1929). The presence of the superficially positioned swelling with a central opening may lead to a tentative diagnosis of myiasis due to Dermatobiahominis. A definitive diagnosis can be made only after extraction and identification of the typical larva.

Diagnosis of infestation with Dermatobiahominis may often be made if the patient has a history of residence in or travel to an endemic area (Prasad and Beck, 1969, Rossi and Zucoloto, 1973, Iannini et al., 1975, Kleeman, 1983, Kenny and Baker, 1984, File et al., 1985, Pallai et al., 1992). However, there are other flies from different geographic areas that might produce a similar lesion . These should also be considered in a differential diagnosis. These myiasis-producing flies include: Cordylobiaanthropophaga, the African tumbu fly; Wohlfahrtiavigil, the grey flesh fly; Chrysomyiabezziana, the Oriental fly; Cochliomyiahominivorax, the primary screwworm; and Cuterebra species, the rabbit botflies. These obligatory myiasis-producing flies should be considered in the differential diagnosis in cats with an appropriate residence or travel history.

LIFE CYCLE: This dipteran fly has a most unusual life cycle. The adult fly inhabits the forests of Mexico, Central America and South America. Unlike many of the obligatory myiasis-producing dipterans (with the exception of Cuterebra species), the female fly does not deposit her eggs directly on the host. Instead, she captures another dipteran fly, usually a bloodsucker, or a tick and using a quick-drying adhesive, cements the eggs to one side of the carrier's body. The eggs are attached to the carrier in such a manner that when contact is made with the prospective definitive host, the anterior end of the egg is directed downward. An operculum forms on this end of the egg, through which the larva emerges. The larva penetrates the skin of the host to the subcutaneous tissues and produces a warble (swelling) at the point of contact. Dermatobiahominis does not meander through the subcutis. The tórsalo matures to its most advanced larval stage in the body of the host (Harwood and James, 1979). Development in the host requires 35 to 70 days. The larva then drops to the ground where it enters the soil for pupation. The entire life cycle takes 90 to 120 days (Harwood and James, 1979).

CLINICAL PRESENTATION AND PATHOGENESIS: Although often not reported, cats are host to infestation with this parasite (Silva Junior, et al., 1998). Clinically, the initial lesion is a small, often pruritic, nodule resembling a common insect bite. As the larva matures, the lesion enlarges around it to form an malodorous, purulent, furuncular lesion. It may be 1 to 2 cm in diameter and 0.5 to 1 cm in height. A serosanguinous fluid begins to exude from the lesion during the second week. Purulent discharge may result from excretions from the larva or from secondary bacterial infection. Each nodule contains a central pore that denotes the presence of the larva (Pallai etal., 1992). Close examination of the larva in situ may reveal the up-and-down respiratory movements of the larva (Kenney and Baker, 1984) or actual visualization of the larval spiracles. It is possible to palpate the larva within the nodule (Pallai etal., 1992).

TREATMENT: The goal of treatment is to remove the larva. In man, several methods have been reported for extraction of larval Dermatobia. hominis from the skin. These techniques should also apply to the feline definitive host. The simplest method (although somewhat time consuming) is the application of a viscous, occlusive substance (e.g., petroleum jelly) over the hole through which the larva breathes. This cuts off the larva's air supply and stimulates premature extrusion. Manual extraction of the dead larva may be necessary using this technique. Hot compresses may be used to make the lesion more pliable and reduce discomfort.

Topical application of 5% chloroform in olive oil to produce a sublethal hypoxia of Dermatobiahominis, followed by manual extraction has been used successfully in man.

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 through a linear incision using thumb forceps. The wound should be irrigated, debrided and packed open to provide adequate drainage. A disadvantage of surgical removal is that remains of the larval bodies may be accidentally left in the lesion. Careful inspection of the extracted larva and irrigation of the wound should reduce complications (Pallai etal., 1992).

EPIZOOTIOLOGY: Since this parasite is spread by both zoophilous and anthropophilous mosquitoes (and other bloodfeeding arthropods), it may be found in a wide variety of definitive hosts. Host selection is performed by "porter" species - mosquitoes and flies. Dermatobiahominis females oviposit on these arthropods and depend on them to carry their eggs to mammalian species.

HAZARDS TO OTHER ANIMALS: Because Dermatobiahominis is the human botfly and because it may be transmitted to a wide variety of domesticated and wild animals, it is considered to be a zooanthroponosis. In addition to cats, suitable hosts include cattle, swine, dogs, horses, mules, sheep, goats, monkeys and certain wild mammals. Birds (toucans and ant birds) are known to harbor it (Harwood and James, 1979). Dermatobiahominis is a serious pest of livestock in many parts of Latin America (Thomas, 1988).

HAZARDS TO HUMANS:Dermatobiahominis is also known as the human botfly. Its larval forms have been extracted from various parts of the human body, primarily the head, arms, back, abdomen, buttocks, genitalia, thighs and axilla (Prasad and Beck, 1969, Rossi and Zucoloto, 1973, Iannini et al., 1975, Kleeman, 1983, Kenny and Baker, 1984, File etal., 1985, Pallai etal., 1992).

CONTROL/PREVENTION: Perhaps the best method of controlling this parasite depends on controlling Dermatobiahominis in cattle, its major definitive host in Latin America. Ivermectin in both topical application and in slow-release bolus forms has been demonstrated to be effective in controlling this parasite in cattle (McMullin et al., 1989, Uribe et al., 1989). Mosquitoes and other blood feeding flies that can serve as phoretic hosts of this fly should be restricted from indoor environments


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Figure 5-70. Dermatobia hominis. Larva recovered from the arm of a human that had been working in South America.