Cuterebra species — rabbit bot

Adults of the oestid (fly) genus Cuterebra are free-living and non-feeding. The females lay their eggs around the burrows of rodents and rabbits and the larva that hatches from each egg must gain access to a mammalian host to continue development. Normally the larvae enter the host (such as dogs and cats) through a natural orifice (e.g. nose or wound) and migrate to their "preferred site" (often subcutaneous tissues). The larvae then make a hole in the skin, develop through several moults, then emerge and pupate in the environment. In dog and cats, growing larvae usually present as a subcutaneous swelling detected in late summer or early fall, but sometimes the larvae invade deeper tissues, including the brain. Treatment of the subcutaneous lesions is usually surgical.

Phylum: Arthropoda
Class: Insecta
Order: Diptera 

Sub-order: Brachycera

Family: Oestridae

Other oestrid flies of veterinary importance include Hypoderma (warbles in cattle), Gasterophilus (bots in horses) and Oestrus (bots in sheep).

Note: Our understanding of the taxonomy of parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on Deplazes et al. eds. Parasitology in Veterinary Medicine, Wageningen Academic Publishers, 2016t

Adult Cuberebra are large, with a shiny black or blue abdomen, and have vestigal mouthparts.  It is the larval stages that are important in veterinary medicine, and that are recovered from animals.  All the larval stages are covered with obvious, short black spines and have caudal spiracles.  The first and second stage larvae are whitish.  The fully developed third-stage larvae are up to approximately 45 mm in length, yellowish to dark brown to black, and covered with short, robust spines.

Adult Cuterebra are free living, and the larvae occur in a wide range of mammals, including occasionally people, in many parts of the world, including Canada.  In North America, the most commonly infected hosts are rabbits, squirrels, chipmunks, mice, cats, and dogs.

Adult female Cuterebra lay eggs around entrances to the burrows of small mammals, or along rabbit runs in spring.  If a suitable mammal, including dogs and cats, comes close, the resulting slight rise in temperature causes the first-stage larva within each egg to hatch instantaneously and moves into the animal’s haircoat.  From here they enter the body through natural openings or skin lacerations and continue to develop for 3-7 weeks, usually subcutaneously, but sometimes in the nose or tissues of the mouth or in deeper tissues, including the trachea, brain and eye.  The infection is most commonly detected in late summer and fall, when the larvae has developed to the third-stage.  In the normal lifecycle, the third-stage larvae emerge from the subcutaneous site, drop to the ground, pupate, overwinter in the environment, and complete their development to adult flies the following spring.

If the Cuterebra larva is subcutaneous, it often results in a painful swelling, usually with a breathing hole.The symptoms associated with deeper larvae depend on their location, are often serious, and these infections may (rarely) be fatal

Third stage larvae are large and may be visualized via the breathing hole.  There are no differential diagnosis for these bots in Canada, although there are other myiasis flies with much smaller larvae.  Time of year (late summer/early fall) is helpful.

For subcutaneous larvae, very careful surgical removal usually works well (crushing may trigger an anaphylactic response). It is important not to rupture the larva during removal, and to remember the possibility of secondary bacterial infection. Larvae in the mouth area and nose can often be removed relatively easily. Larvae in deeper tissues present a more serious problem.

Successful control of Cuterebra depends on preventing access by dogs and cats to areas where the female flies may have laid eggs. For rural pets this can be very difficult

Occasional human infections with larvae of Cuterebra – usually subcutaneous or nasal – have been reported in North America. It is believed that people become infected in the same way as other mammals.They cannot acquire the parasite directly from an infected dog or cat.