Lindsay A. Starkey
DVM, PhD, DACVM (Parasitology)
Dr. Starkey earned her BS in Animal Science from the University of Arkansas and her DVM and PhD at Oklahoma State University, where her graduate research focused on vector-borne infections. She completed her residency training through the National Center for Veterinary Parasitology at Oklahoma State University. In 2016, Dr. Starkey joined the faculty at Auburn University, where she is involved in various research projects involving vector-borne and foodborne infections, diagnostic parasitology, and parasite consultation and outreach. Dr. Starkey is also heavily involved in teaching parasitology to first-, second-, and fourth-year veterinary students.Read Articles Written by Lindsay A. Starkey
There are 2 types of people in this world: those who are disgusted by bots and those who are fascinated by them. Regardless of personal feeling, most veterinarians are likely to encounter at least a few cases of botfly infestation (obligatory myiasis); therefore, this article provides a brief overview of this condition.
Etiology, Life Cycle, Seasonality, and Distribution
Botflies in the genus Cuterebra, which comprises more than 50 different species, are typically parasites of rodents and rabbits; however, they may find their way into pet cats and, less frequently, dogs.1-3 Additional hosts include horses, cows, pigs, reptiles, birds, zoo animals, other wildlife, and even humans.3 Cuterebra larvae are often referred to as “warbles” or “wolf worms.”1
Short-lived adult female Cuterebra species do not deposit their eggs directly on a host, as most other botfly species do (FIGURE 1). Rather, they oviposit near areas frequented by their natural hosts (e.g., nests, dens).3 The first-stage larva (first instar) hatches when it experiences an abrupt increase in temperature. It then clings to and traverses the host’s hair to make its way to a natural opening (e.g., mouth, nares), where it penetrates the mucosal tissues. The larva spends the next several weeks burrowing through connective tissue with the goal of finding an appropriate cutaneous site for further development.
Once situated, the larva creates an opening in the skin where it can exchange air through its posterior spiracles (FIGURE 2). Development through all 3 instars takes up to 2 to 3 months in the host (FIGURE 3); when complete, the third instar leaves the host to pupate in the environment.3 The lesion usually closes up and heals within a week or 2 in normal hosts.3
Most cases of cuterebriasis in North America are reported June through October in the eastern United States and Ontario, Canada (FIGURE 4).1,2,4 Depending on the season, adults may emerge from the pupae or the pupae may overwinter.
Clinical Signs, Diagnosis, and Treatment
The typical manifestation of a bot infestation is a seeping, cutaneous nodule (furuncular lesion) that may be mistaken for an abscess. These lesions are most often found near the head, along the neck, or extending caudally along the shoulders, thorax, or sides.1 Diagnosis of the cutaneous manifestation is usually straightforward after visualization of the opening in the skin or oral cavity with the posterior portion of the larva inside.
Treatment for this presentation is mechanical extraction with or without sedation or anesthesia. Adjunct therapies may include analgesics, antimicrobials, and/or steroids.2 Treatment to kill the larva in situ may incite additional inflammation. Although rare, an anaphylactic hypersensitivity-type reaction may occur.2 Treatment with steroids is recommended, and pretreatment with diphenhydramine before extraction may reduce this risk.2
Occasionally, bot larvae find their way to other organs and cause respiratory, ophthalmic, aural, or neural clinical signs.1 Systemic inflammatory response syndrome has been reported in dogs (particularly small-breed dogs) and cats, as has disseminated intravascular coagulopathy in dogs.2 Cases involving the eye may affect either the anterior or posterior chamber as well as the eye socket itself. Surgical removal is recommended; however, blindness may result in some cases due to extensive retinal or corneal damage.1
Larval migration through the nervous tissues may cause lethargy, ataxia, circling, blindness, head tilt, behavior change, seizures, and/or convulsions.1,5 Diagnosis of the neural form is more challenging as hematologic findings are inconsistent, thus requiring advanced imaging, such as magnetic resonance imaging (FIGURES 5 AND 6).2,5 A novel enzyme-linked immunosorbent assay for the detection of IgG has been shown experimentally to aid in the diagnosis of noncutaneous manifestations.6 Depending on the severity of clinical signs and the response to symptomatic therapy, euthanasia may be elected (FIGURE 7).1,5
Prevention centers on alteration of host behavior to preclude animals from venturing near dens or burrows.3 Broad-spectrum parasite preventive products with insecticidal activity may aid in killing first-instar larvae before entry or soon into their migration; however, data are lacking as experimental studies to confirm such activity have not been pursued or reported.2 Nonetheless, year-round, broad-spectrum parasiticides are recommended as a part of a comprehensive parasite prevention program to protect against heartworm disease as well as fleas, ticks, mites, and/or intestinal parasites.
Although Cuterebra infestations are usually more traumatic for the client than the animal, severe manifestations are possible. Therefore, it is worthwhile to educate clients on where botflies lay their eggs so that they can take measures (e.g., leash walking) to prevent infestation. Furthermore, in areas where clinical cases of cuterebriasis have been seen, educate clients about the potential clinical signs associated with noncutaneous manifestations, especially during the summer and fall, and encourage them to seek appropriate veterinary care should they suspect a problem in their pet.
Images courtesy of the Auburn University Department of Clinical Sciences and Department of Pathobiology. Special thanks to Dr. Jey Koehler, Dr. Rachel Moon, Tracey Land, and Jamie Butler for assistance in image gathering and captioning.
- Pezzi M, Bonacci T, Leis M, et al. Myiasis in domestic cats: a global review. Parasit Vectors. 2019;12(1):372.
- Rutland BE, Byl KM, Hydeskov HB, Miniter B, Johnson CA. Systemic manifestations of Cuterebra infection in dogs and cats: 42 cases (2000–2014). JAVMA. 2013;243(4):493-495.
- Slansky F. Insect/mammal associations: effects of cuterebrid bot fly parasites on their hosts. Annu Rev Entomol. 2007;52:17-36.
- Delshad E, Rubin AI, Almeida L, Niedt W. Cuterebra cutaneous myiasis: case report and world literature review. Intl J Dermatol.
- Rissi DR, Howerth EW. Pathology in practice. JAVMA.
- Davis LL, Puozzo A, Vitalo DC, et al. Enzyme-linked immunosorbent assay using a crude antigen extract to measure Cuterebra-specific immunoglobulin G in sera of cats with systemic infections. J Vet Diagn Invest. 2013;25(3):395-401.