Bots and Warbles

Other Names: Deer nose bots, Cuterebra

Cause

Bot fly and warble fly maggots (larvae) parasitize mammalian hosts to complete their development into adult flies. There are many different species of bots and warbles that parasitize animals throughout the world. Nasal bots within the genus Cephenemyia are known commonly as deer nose bots and they parasitize the nasal passages of deer in North America. Bots from the Cuterebra genus also occur in North America, and mainly infect rodents and rabbits.

Significance

Cuterebra and deer nose bots are of no public health significance, and the meat of infected animals is safe to eat if it is properly cooked.

Species Affected

Cuterebra primarily infect rodents and rabbits, but these larvae can parasitize many mammalian hosts. Cuterebra have been found in deer, foxes, mink, cattle, pigs, mules, cats, dogs, and humans. Nose bots are known to infect white-tailed deer, mule deer, elk, moose, and caribou.

Distribution

Several species of Cuterebra and Cephenemyia are found throughout North America.

Transmission

After mating, female nasal bot flies deposit live larvae around the mouth and nose of deer hosts. The larvae migrate into the nasal cavity, where they attach in clusters and develop.

Female Cuterebra deposit fertilized eggs in locations where they are more likely to encounter a host (for example at the entrance of a den). The eggs hatch into larvae in response to increased temperature and carbon dioxide, caused by a nearby host . The larvae enter the host through the mouth, nose, eyes, or an open wound. The larvae may remain in the oral or nasal passageways for several days before migrating to specific locations where they create protected spaces under the skin and continue development.

Clinical Signs

Deer that are parasitized by nasal bots often show no clinical signs. However, they may snort or lower their heads, and may have nasal discharge. Rarely, heavily parasitized deer die of suffocation, and rarely infected deer die when larvae migrate into the brain or lungs. At necropsy, larvae can be found within the nasal passages. Deer nose bot larvae are 25-36 mm (1-1.4 in) long; they are white at first but become yellow-brown later in development. Different stages may be present in an affected animal.

Early signs of Cuterebra parasitism are rare. Once the larvae encyst, they can be seen or felt as swellings under the skin and the breathing hole may also be seen. Some small mammals will have difficulty walking because the large larvae interfere with normal movements. Developed Cuterebra larvae are dark brown and 20-42 mm (0.8-1.7 in) long and 7-10 mm (0.3-0.4 in) wide.

Diagnosis

Both Cuterebra and deer nose bot infections can be diagnosed by finding the larvae within the host. The exact species can be determined by allowing the larvae to develop into adult flies.

Treatment

It is unnecessary and impractical to treat free-ranging wildlife for Cuterebra or deer nose bots. Captive animals can be treated for Cuterebra infections by removing the larvae and cleaning the wounds. Some medications are effective against bots and warbles.

Information for hunters who harvested game with warbles

Rabbits, squirrels, and other game mammal species host warbles. Warbles are more common in warmer climates, especially during the summer and can sometimes be confused with fibromas and other lumps (see Fibromas). When a botfly lays its eggs on a mammal's hide, the larvae hatch and burrow under the skin. When a hunter skins a harvested animal, the warbles appear as large, raised bumps on the surface of the meat. While warbles are gross and unsightly, they can easily be removed with a knife. Even if they're not removed, the meat is perfectly edible. Warbles don't spread diseases to humans, and cooking kills them. There is no need to discard a harvested animal with warbles.

Management and Prevention

Bots and warbles may affect individual animals, but do not seem to have a significant impact on wildlife populations in North America, so management is probably not necessary.

Content Editor

Gray Squirrel with warbles. Photo credit: Steve Bower, Shutterstock