Bot fly is a term referring to any member of the Oestridae family of flies. To reproduce these flies rely on myiasis, the infestation of host skin for larvae nutrition. Myiasis caused by the bot fly is a self-limiting process as the bot will mature and expel itself in 6 weeks after the initial infestation. However, the bot fly infestation can be uncomfortable for patients and he/she may desire removal.
The most common species of flies that cause myiasis in humans are Dermatobia hominis(human botfly) and Cordylobia species. Dermatobia hominis is native to the tropics of Central America and South America. Coydylobia species (tumbu fly) occur in African tropics. There are about 40 Cuterebra species (Rodent bot) endemic to North America, but they rarely infect humans. In the United States and Canada, there have fewer less than 65 cases of myiasis in the past 70 years attributed to endemic bot fly species.
Bot fly infections can affect anyone regardless of sex, age, and genetic makeup. The vast majority of US cases are from travelers returning from Central and South America. There is no good data available for worldwide disease burden. Cases likely go underreported due to the self-limiting nature and availability of effective home remedies. There is some evidence that myiasis cases are on the rise worldwide. The increase in cases is potentially explainable by climate change. If true, the increase in cases is likely to accelerate.
Unique from all other flies causing myiasis, the female human botfly does not directly deposit eggs onto the host. Botflies use a phoretic or a vector of another species used for travel. The female botfly's most common phoretic is a mosquito, but ticks and other flies are used as well. The female botfly will capture a mosquito and attach up to 24 eggs on its abdomen. When the carrying mosquito lands on the host's skin, the abrupt rise in temperature will cause the botfly eggs to hatch. Larvae then enter the host's skin through hair follicles or defects created by the phoretic. The larvae will feed on host soft tissue for five to 10 weeks, digging deeper into the host's skin. At this point, a dome-shaped cavity has been created consisting of a fibrous capsule as the host responds to the infestation. There is some evidence that the botfly larvae evade the host immune system by suppressing antibody-mediated immunity. There is typically only one larva per cavity. At the end of the five to 10 week period, the larvae will eject itself from the cavity to continue its maturation to adulthood.
The most crucial aspect in the diagnosis of bot fly myiasis is travel history. Travel history should include specific locations visited in Central and South America as well as African tropics. Clinicians should ask about activities done while traveling, which may increase the risk of exposure, such as hiking and other outdoor activities. The examiner should ask about specific encounters with mosquitoes, other flying insects, and ticks. Complaints from patients are typically only related to the region of the infestation.
Patients describe a slow-growing nodule with swelling, usually noticed a week after an initial insect bite. The skin lesion can be described as pruritic, mildly painful with the occasional sensation of movement under their skin. Patients are usually afebrile and without lymphadenopathy. On physical exam, there will be an erythematous 1 to 3cm papule with a central opening. There may be a serosanguinous expression from the central opening after applying pressure to the papule.
Ultrasound can be a useful tool in making a diagnosis of bot fly myiasis. Ultrasound will show a hyperechoic superficial mass casting a shadow on to deeper structures. Complete blood count can show elevated leukocytes and elevated eosinophils. Erythrocyte sedimentation rate and C-reactive protein are typically within normal limits.
Most cases will not require surgical management. The clinician should seek a surgical evaluation if larvae are too large or difficult to remove; there is the involvement of the eye and/or orbit or scalp infections of pediatric patients. Non-surgical methods for removing the bot fly larvae include obstructing the central poor to deprive the larva of oxygen, removing the larva after injection of lidocaine, and using a 5mm punch excision. While these are all acceptable means of extraction, hydraulic expulsion via injection of lidocaine is the preferred initial step in the removal of bot fly larva. This method ensures that the bot fly larva is not excised in the process of removal, which can lead to a secondary infection. Due to the presence of spicules anchoring the larva in the canal, direct removal of a bot fly larva is not the recommended approach, as there is an increased risk of tearing the larva.
Effective home remedies found in the literature include the application of petroleum jelly, paraffin oil, pork fat products, or any other thick jelly to suffocate the larva. These home remedies are not preferable due to difficulty removing dead larva due to the presence of spines on their abdomen. Snake venom extractors have also been used effectively to remove larva.
Expectant management is also an appropriate means of management if the patient does not desire removal.
Since bot fly myiasis is rare in the United States, it can commonly be mistaken for more common skin lesions. Patients may present with a prescription from another clinician without improvement of the lesion. Differential diagnoses should include sebaceous cyst, cellulitis, folliculitis, abscess, foreign body, and atopic dermatitis. The furuncular lesion of Dermatobia hominis looks similar to an infectious furuncle as they both have erythematous changes to the skin with a purulent appearing center opening.
An infectious disease consultation may be able to help narrow down the differential diagnosis by ruling out specific infectious etiologies. General surgery should be consulted for cases of large larvae that are not removable after initial attempts and for at-home efforts in which retained larva segments are present. Ophthalmology should be consulted for cases with ocular involvement. Neurosurgery should consult on all pediatric scalp infestations and any adult scalp infestation with radiologic evidence for possible cerebral myiasis.
Travelers to endemic regions should receive education about how botfly larvae spread and how a botfly infestation looks. All travelers to Central American, South America, and Africa should bring protective clothing, insect repellent, and sleep with mosquito nets to avoid contact with bot fly larvae carriers.
Patient education about myiasis and traveler's health is also available from the CDC's website.
Lack of improvement of a furnicular lesion on appropriate antibiotic coverage should prompt further evaluation into non-infectious etiologies such as botfly myiasis.
Botfly myiasis frequently poses a diagnostic dilemma. Botfly myiasis is often misdiagnosed as a more common cause of furuncular skin lesions. Patients will often receive antibiotics from a primary care physician or an emergency medicine physician without improvement. When a patient comes back to the primary care office or emergency room, it is essential to seek consultation from specialists to make an appropriate diagnosis. Infectious disease physicians should be consulted to rule out more common infectious etiologies. General surgeons should consult on these cases to discuss surgical removal of the botfly larva. [Level 5]
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