Description and Distribution
Warbles are the larval stage of the botfly, which characteristically infect rodents and rabbits. They are found under the skin, usually around the legs and neck. They have also been reported from deer, cattle, cats, dogs, hogs, mules, mink, foxes, and man. The adult botflies are large (20 mm or more in length) and dark blue or black with fine, dense hairs on the face, genae and thorax.
Only one genus, Cuterebra, is currently recognized from North America. Twenty-six species are known to occur in the U.S. and Canada. They are also found in Mexico and the neotropical regions. The taxonomy is poorly defined, and existing keys are for the most part inadequate for separation of the species.
In Michigan, warbles have been found in cottontail rabbit, eastern chipmunk, eastern gray squirrel, eastern fox squirrel, thirteen-lined ground squirrel, house mouse, and white-footed mouse. Undoubtedly, other mammals are infected as well. The parasites are usually not seen by the rabbit hunter since most of the warbles have dropped off before hunting season.
Transmission and Development
Under natural conditions, mating of Cuterebra takes place in the host's immediate habitat. In the spring and summer, male flies characteristically take up watchful positions on terminal stems of vegetation. Receptive females approach the site and mate with males in flight. Gravid females then deposit eggs, usually along runways or at entrances to the host animal's burrow. The eggs hatch into larvae in response to a sudden increase in temperature, and possibly to the moisture emitted by the host. The larvae enter the host by way of a natural body opening, commonly the nose or mouth, or a minute abrasion of the skin. They frequently remain in oral and nasal passages several days before migrating to preferred locations under the skin, where they develop into warbles. In the dusky-footed wood rat, C. latifrons will select the neck region and upper chest. Cuterebra horripilum tends to seek out the throat region in cottontail rabbits, and C. fontinella commonly selects the abdominal or caudal region in the deer mouse. The larvae encyst within the skin of the host and complete their development. Larval development within the host may last from 19 to 38 days in small rodents and from 55 to 60 days in jackrabbits. After leaving the host, pupation takes place in loose soil, debris or forest duff. The pupation period may be as long as 7 to 11 months or as short as 28 days, depending on temperature. Adults will mate within a few days after emergence and they seldom live more than 14 days.
Newly hatched larvae measure from 2 to 4 mm long and are grayish-white. As the larvae mature, this color changes gradually to reddish-brown, and finally to dark brown just prior to emergence from the host. Fully grown larvae will measure from 20 to 42 mm in length and 7 to 10 mm in width.
Clinical Signs and Pathology
The early stages of Cuterebra infection are rarely evident from external inspection, and they do not become noticeable until growth of the warbles can be detected by touch. In smaller mammals, such as chipmunks, the larvae often produce an obvious awkwardness in locomotion, which may render them more susceptible to predation.
The observed effects of cuterebrid parasites in mammals is still conjectural and varies considerably with the host species involved and the intensity and incidence of parasitism. In some instances, secondary bacterial infection may have a greater detrimental effect on the host than the primary attack by the Cuterebra warbles. It has been observed that, after larvae dropped from chipmunk hosts, the resulting wounds, with few exceptions, became purulent and the host's activity was markedly reduced.
Recognition of parasitism involves recovery of larvae from host tissues or subdermal cysts. Determination of the species, particularly in mixed infections, is augmented by rearing the developed larvae to adult forms.
Treatment and Control
In wild mammal populations, control is usually impractical. Decreases in host populations ordinarily result in a lower subsequent incidence of the parasites. Treatment of infected captive animals consists of mechanical removal of larvae from the cysts. The opening should be enlarged by an incision and the parasite squeezed out. The wound should then be rinsed with an antiseptic solution and a topical antibiotic administered.
There is little doubt that the warble can have a debilitating and possibly fatal effect on some animals, particularly young ones. It has been suggested that warbles may actually depress cottontail populations during years when the incidence of parasitism is high and infections are heavy. This parasite is of no public health significance and properly cooked meat from infected animals is safe to eat.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.