Bots and Warbles

This fact sheet was updated on 10/29/2020. 

Other names: Deer nose bots, Cuterebra


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


Cuterebra and deer nose bots are of no public health significance, and the meat of infected animals is safe to eat as long as it is properly cooked. However, severe infestation with Cephenemyia has been the cause of death in at least one white-tailed deer.

Species Affected

Nasal bots are known to infect white-tailed deer, mule deer, elk, moose, caribou, and reindeer. 

Cuterebra primarily infect rodents, especially squirrels and chipmunks, and also rabbits, but these larvae can parasitize many mammalian hosts. Cuterebra have been found in deer, foxes, mink, cattle, pigs, mules, cats, dogs, humans, and even some exotic primates such as howler monkeys. Importantly, the few human cases of parasitism by Cuterebra that have been reported in the United States were acquired from the environment not from animal hosts.


There are at least 26 species of Cuterebra and 5 species of Cephenemyia found throughout North America.


Nasal bots

After mating, the female fly deposits a packet of eggs around the mouth or nose of a deer host. As the deer licks the packet of eggs, the larvae within are released and ingested. The larvae then migrate into the nasal cavity, where they attach in clusters and develop. Once fully developed the larvae are yellow-brown in color and approximately 25 to 36mm (1 to 1.4 inches) long. They are expelled from the nasal cavity and throat and finish their lifecycle in the soil.



Female Cuterebra deposit fertilized eggs in locations where they are more likely to encounter a host such as at an entrance to a den or along runways. The eggs attach to the host feet or pelage before they hatch into larvae in response to increased temperature and carbon dioxide. The larvae then enter the host through a natural opening, commonly through the mouth, nose, eyes or an open wound. The larvae can remain in the oral or nasal passageways for several days before migrating to specific locations where they create protected spaces under the skin, cut a respiratory pore through the skin and continue development into a maggot. Larval development ranges from 3 to 7 weeks depending on the species of fly and occurs in specific anatomical locations in different hosts. Developed Cuterebra larvae are dark brown and 20-42 mm (0.8-1.7 inches) long and 7-10 mm (0.3-0.4 inches) wide. As with nasal bots, the remainder of the lifecycle occurs in the soil after the larvae are expelled through the skin via the breathing hole.


Clinical Signs

Warble and nasal bot infestations occur most often during the summer when flies are most prevalent.


Nasal Bots

Deer that are parasitized by nasal bots may show no clinical signs. However, they may snort, shake and lower their heads, and may have nasal discharge. Rarely, heavily parasitized deer die of suffocation or die when larvae migrate into the brain or lungs. At necropsy, larvae can usually be found within the nasal passages. Different development stages may be present in an affected animal.



Once the Cuterebra larvae encyst (encloses itself into a protective capsule) usually in the legs or neck of the host, they can be seen or felt as swellings under the skin and the breathing hole may also be seen. As the larvae grows, the animal may scratch or lick at the area. Multiple larvae per animal are common. Some small mammals, such as chipmunks, may have difficulty walking because the large larvae interfere with normal movements making them more susceptible to predation. Secondary bacterial infections can also affect the open wound left behind by a Cuterebra warble, but these resolve rapidly.


Both Cuterebra and deer nose bot infections can be diagnosed by finding the larvae at characteristic locations within the host.


It is unnecessary and impractical to treat free-ranging wildlife for Cuterebra or deer nose bots.


Bots may affect individual animals, but do not seem to have a significant impact on wildlife populations in North America, so management is not necessary. Affected animals are considered safe for human consumption.

Suggested Reading