Cuterebriasis in Dogs and Cats

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. 

Figure 1. Adult Cuterebra botfly. Note the bumblebee-like appearance (scale in centimeters).

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 

Figure 2. Magnified view of the spiracular plate of a third-instar larva. The spiracular plate is located at the posterior end of the larva and can be used to distinguish Cuterebra from other bot species.

Figure 3. Various larval instars of Cuterebra botfly (scale in centimeters).

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. 

Figure 4. Map of the United States indicating states in which cases of cuterebriasis have been reported in cats (yellow), humans (gray), and cats and humans (yellow stripes). Cases of human and feline cuterebriasis have also been reported in Ontario, Canada (not pictured).

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

Figure 5. (A-C correspond to the rostral through caudal aspects of the lesion) Transverse plane T1-weighted post-contrast images of a cat’s head. There is a curvilinear region of contrast enhancement with a non–contrast-enhancing central region extending from the right parietal region toward the left and caudodorsally through the left dorsal parietal region of the brain (arrows). This cat was treated with ivermectin and discharged with an anti-inflammatory dose of prednisolone; no neurologic deficits were noted at a follow-up examination 15 months later.

Figure 5. (A-C correspond to the rostral through caudal aspects of the lesion) Transverse plane T1-weighted post-contrast images of a cat’s head. There is a curvilinear region of contrast enhancement with a non–contrast-enhancing central region extending from the right parietal region toward the left and caudodorsally through the left dorsal parietal region of the brain (arrows). This cat was treated with ivermectin and discharged with an anti-inflammatory dose of prednisolone; no neurologic deficits were noted at a follow-up examination 15 months later.

Figure 5. (A-C correspond to the rostral through caudal aspects of the lesion) Transverse plane T1-weighted post-contrast images of a cat’s head. There is a curvilinear region of contrast enhancement with a non–contrast-enhancing central region extending from the right parietal region toward the left and caudodorsally through the left dorsal parietal region of the brain (arrows). This cat was treated with ivermectin and discharged with an anti-inflammatory dose of prednisolone; no neurologic deficits were noted at a follow-up examination 15 months later.

Figure 6. (A-C correspond to ventral through dorsal aspects of the lesion) Dorsal plane post-contrast T1-weighted images of a cat’s head. Note the diffuse meningeal enhancement of the left cerebral hemisphere (arrowheads). There is also mild contrast enhancement of the caudal aspect of the left nasal cavity (asterisk).

Figure 6. (A-C correspond to ventral through dorsal aspects of the lesion) Dorsal plane post-contrast T1-weighted images of a cat’s head. Note the diffuse meningeal enhancement of the left cerebral hemisphere (arrowheads).

Figure 6. (A-C correspond to ventral through dorsal aspects of the lesion) Dorsal plane post-contrast T1-weighted images of a cat’s head. Note the diffuse meningeal enhancement of the left cerebral hemisphere (arrowheads).

Prevention

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.

Figure 7. Necropsy findings associated with the case pictured in Figure 6. In (A) and (B), note the yellow-tan discoloration of the dorsolateral surface of the left cerebral hemisphere of the left temporal lobe with associated mild to moderate collapse of the gray matter.

Figure 7. Necropsy findings associated with the case pictured in Figure 6. In (A) and (B), note the yellow-tan discoloration of the dorsolateral surface of the left cerebral hemisphere of the left temporal lobe with associated mild to moderate collapse of the gray matter.

Figure 7. Necropsy findings associated with the case pictured in Figure 6. (C) A degenerated, fibrin-covered arthropod larva (6 mm × 1.5 mm) was attached to the dura of the adjacent calvaria (D).

Figure 7. Necropsy findings associated with the case pictured in Figure 6. (C) A degenerated, fibrin-covered arthropod larva (6 mm × 1.5 mm) was attached to the dura of the adjacent calvaria (D).

Conclusion

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. 

Acknowledgments 

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.