Infestation by rodent or rabbit bot fly larvae is most common in late summer or early autumn and can affect dogs, cats, ferrets, and rabbits (wild or pet). A fistulous swelling that can ooze purulent exudate is typical. Definitive diagnosis is based on discovery and identification of a larva (bot). Treatment is manual extraction of the parasitic bot. The lesion is cleaned and allowed to heal by granulation.
Cuterebra larvae (bots) are opportunistic parasites of dogs, cats, pet rabbits, and ferrets. Infestation is caused by the rodent or rabbit bot fly, Cuterebra spp (order Diptera, family Cuterebridae).
The life cycles of bot flies are usually host and site specific. However, rabbit Cuterebra are less host-specific and are usually associated with dog and cat infestations. Rarely, cats and dogs can be infested by Hypoderma spp or Dermatobia hominis. Ferrets housed outside can be infested by Hypoderma or Cuterebra spp. Pet rabbits housed outside can be infested by Cuterebra buccata.
Etiology of Cuterebra Infestation in Small Animals
Adult Cuterebra flies are large and beelike and do not feed or bite. Females deposit eggs around the openings of animal nests or burrows, along runways of the normal hosts, or on stones or vegetation in these areas. A female fly can deposit 5–15 eggs per site and > 2,000 eggs in her lifetime.
Animals become infested with Cuterebra larvae as they pass through contaminated areas; the eggs hatch in response to heat from a nearby host. In the target host, the larvae enter the body through the mouth or nares during grooming or, less commonly, through open wounds.
Cuterebral larval size varies by stage:
First-stage instars are slender, transparent, and 1–1.5 mm long.
Second-stage instars are gray to white/cream and 5–15 mm long (see bot image).
Third-stage larvae are 3–4.5 cm long and darkly colored, and they have spicules.
Courtesy of Dr. Karen A. Moriello.
After penetrating their host, bot fly larvae migrate to various species-specific subcutaneous locations on the body, where they develop and communicate with the air through a breathing pore typically visible on the skin surface.
After approximately 30 days, the larvae exit the skin, fall to the soil, and pupate. The duration of pupation varies depending on environmental factors and winter diapause (suspended development).
Clinical Findings of Cuterebra Infestation in Small Animals
Well-demarcated, soft, fistulous, subcutaneous swelling in late summer or early autumn
Depression, lethargy
Neurological abnormalities (eg, seizures, blindness, abnormal mentation)
Hyperthermia or hypothermia
Sneezing, upper respiratory infections
Cuterebra lesions are most common in summer and autumn, when the larvae enlarge and produce a fistulous subcutaneous swelling approximately 1 cm in diameter (see lesion image).
Courtesy of Dr. Karen A. Moriello.
Dogs, cats, and ferrets are abnormal hosts for the Cuterebra parasite; aberrant migrations can involve the head, brain, nasal passages, pharynx, and eyelids.
Free-roaming cats are more likely than indoor cats to develop Cuterebra lesions.
In the skin, typical lesions of cuterebriasis occur around the head, neck, and trunk. The first sign is often matting of the hair that the cat may be aggressively grooming (see bot site image). Inspection reveals a subcutaneous mass beneath the breathing pore.
Courtesy of Dr. Karen A. Moriello.
Pain at the site of a Cuterebra lesion is variable and usually associated with secondary infections. Purulent material can exude from the lesion; the most common differential diagnosis is an abscess or foreign body.
Clinical signs are often associated with the CNS and typically occur between July and September. Cats can exhibit depression, lethargy, or seizures; upper respiratory infections; or abnormal body temperatures (either hyperthermia or hypothermia). These clinical signs can be the result of inflammation or parasite migration.
One key historical finding in cats with neurological signs caused by Cuterebra infestations is an acute episode of violent sneezing weeks to months before clinical presentation. Common neurological findings include blindness, abnormal mentation, and signs of unilateral prosencephalic disease (eg, blindness, vestibular signs). Idiopathic vestibular signs in cats could be due to aberrant parasitic migration.
Yorkshire Terriers may be at increased risk of Cuterebra infestation and can have a marked systemic inflammatory response, DIC, or both.
Diagnosis of Cuterebra Infestation in Small Animals
Physical examination findings
Advanced imaging
Definitive diagnosis of Cuterebra infestation is based on the discovery and identification of a larva. The presence of a bot in a subcutaneous swelling with a central opening (ie, breathing pore) is diagnostic.
Second-instar Cuterebra larvae are 5–15 mm long and are gray to white/cream in color. Third-instar larvae are dark, thick, and heavily spined; they are the stage most commonly found by veterinarians. Morphological identification of genus and species is difficult; laboratory identification is usually not needed for treatment.
In cats, advanced imaging (eg, CT or MRI) can help identify Cuterebra larvae, especially in cases with neurological or respiratory signs in the absence of furuncular myiasis.
Treatment of Cuterebra Infestation in Small Animals
Manual removal of larva
Wound cleaning and supportive care
Education of owner about disease and risk factors
Suspect Cuterebra lesions should be explored by carefully enlarging and probing the breathing pore or fistula with mosquito forceps. It is not unusual for the parasite to retreat into the opened pore, making it difficult to grasp. Covering the breathing pore with white petroleum jelly for 10−15 minutes before grasping the parasite can make it easier to remove.
The lesion should not be squeezed, because doing so can rupture the Cuterebra larva and lead to a chronic foreign body reaction and secondary infection. There are anecdotal reports of larval rupture causing anaphylaxis.
Pearls & Pitfalls
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If possible, the larva should be removed in one piece (see bot removal image); recurrent abscesses at the site of previous Cuterebra infestation suggest residual infection or larval remnants.
Courtesy of Dr. Karen Moriello.
The area should be thoroughly flushed with sterile saline solution (0.9% NaCl), debrided (if necessary), and allowed to heal by granulation. Healing can be slow, taking weeks to months.
Ivermectin has been described as a treatment for cats with CNS cuterebriasis. Diphenhydramine (4 mg/kg, IM) and dexamethasone (0.1 mg/kg, IV) are administered 1–2 hours before ivermectin (0.1–0.3 mg/kg, SC, once). In the US, ivermectin is not approved for this use in cats, so this use is extralabel.
Isoxazolines (sarolaner in dogs and fluralaner in cats) have been reported as effective for the treatment of myiasis (1, 2, 3).
Educating the owner about the Cuterebra parasite's life cycle and risk factors to pets can help avoid recurrent episodes.
Key Points
Opportunistic Cuterebra infestation of outdoor animals is most common in late summer and early autumn.
Fluctuant subcutaneous soft tissue swellings with a fistula are typical and should be explored for parasitic larvae.
Manual removal of larvae involves enlarging the opening/breathing pore and removing the parasite using forceps; the wound is cleaned, flushed to remove debris, and allowed to heal by granulation.
For More Information
James FMK, Poma R. Neurological manifestations of feline cuterebriasis. Can Vet J. 2010;51:213-215.
Starkey LA. Cuterebriasis in dogs and cats. Today's Vet Pract. September/October 2021.
Companion Animal Parasite Council (CAPC), Cuterebriasis.
Also see pet owner content regarding Cuterebra infestation in dogs and cats.
References
Oliveira PC, Almeida GPS, Cardoso JD, et al. Efficacy of sarolaner on the treatment of myiasis caused by Cochliomyia hominivorax (Diptera: Calliphoridae) in dogs. Vet Parasitol. 2019;276:108966. doi:10.1016/j.vetpar.2019.108966
Ribeiro Campos D, Pereira de Assis RC, de Oliveira Chaves JK, et al. Furuncular myiasis caused by Dermatobia hominis in five cats and efficacy of topical fluralaner for its treatment. Vet Dermatol. 2021;32(5):438-e117. doi:10.1111/vde.12998
Andriotti PA, Souza CP, Oliveira PC, Melo RC, Verocai GG, Fernandes JI. Effectiveness of sarolaner in the clinical management of furuncular myiasis in dogs naturally infested with Dermatobia hominis (Diptera: Cuterebridae). Parasit Vectors. 2021;14(1):401. doi:10.1186/s13071-021-04910-3
