A global traveler puts the diagnostic skills of clinicians half a world away to the test.
Ms. W, a healthy 25-year-old American living in Afghanistan, presented with three subcutaneous lumps on her scalp. The lumps, which had been slowly enlarging for a week, exhibited a central hole or scab. Ms. W reported that they were intermittently pruritic and painful and had a “tickling” sensation associated with them. On the day of presentation, she also noted some lymphadenopathy in her neck. During history-taking, Ms. W said she had returned from Belize just two weeks ago. She was not using any medications for the lesions.
MAKING THE DIAGNOSIS
Each nodule on the patient’s scalp was approximately 1 cm in diameter with a central punctum. The lesions were of indeterminate etiology, but given the patient’s travel history, conditions such as tungiasis, cutaneous larva migrans, cutaneous leishmaniasis, and pediculosis were considered. The nodules could also have been epidermoid cysts or an indication of delusions of parasitosis.1
The most telling clues included the crawling sensation within the lesions, the central punctum, and of prime importance, the travel history. In addition, Ms. W had done her research, asking on arrival if anyone knew how to treat botflies.
None of the clinicians in the clinic had encountered myiasis before, but some quick research on medical Web sites demonstrated that Ms. W’s supposition was indeed plausible.
We occluded the furuncles with bacitracin ointment to try to detect larvae in the puncta. Soon small bubbles of air were seen, and a small yellowish object appeared at one opening.
After numbing the lesion with lidocaine, we made a small cruciate incision over the center. Lateral pressure was applied, and when the yellow object appeared at the top of the furuncle, we grasped it with forceps and pulled gently.
We compared our specimen to photographs and descriptions of botfly larvae in the literature. Other fly species cause myiasis in humans, but the consistent similarity between our specimen and those in the literature, along with Ms. W’s recent travel to Belize, made the diagnosis.
Using the same procedure, we extracted a smaller larva (Figure 1) from the second furuncle; no larva emerged from the third. Four days later, the lesions were healing, and Ms. W’s symptoms had resolved.
Myiasis is an infestation by the larvae or maggots of dipterous (two-winged) flies. Though more commonly seen in the tropics, e.g., Central America, South America, and Africa, dozens of cases of furuncular myiasis have been reported in North America since the condition was first described in 1920.2 In North America, myiasis is typically seen in travelers returning from endemic areas. One study found it to be the fourth-most common travel-associated skin disease.1
Dermatobia hominis, the human botfly, is an obligate parasite endemic to Mexico, South America, and Central America. The female catches another insect, such as a mosquito, and attaches up to 30 eggs to the insect’s abdomen. When the mosquito bites a host, the ambient temperature increases and causes the eggs to hatch. The larvae (Figure 2) drop onto the host and enter the skin.1
Each larva then forms a subcutaneous burrow around itself and feeds on the host. As the larva grows, the lesion increases in size and develops the appearance of a furuncle with a central punctum that allows the larva to breathe. Concentric rows of backward-projecting dark spines help prevent the larva from being dislodged from the tube.2 After several weeks, the larva exits the skin and drops to the ground, where it will pupate for several more weeks before an adult fly emerges.
Dermatobia hominis myiasis typically presents as a cutaneous nodule with a central punctum that may exude serosanguineous or purulent fluid. Patients may experience pruritus or a sensation of movement within the lesion, as well as localized pain, regional lymphadenopathy, fever, and malaise. Affected areas typically include the scalp, face, and extremities. The lesions are often presumed to have a bacterial origin, and the diagnosis is made only after they fail to respond to empiric antibiotics.
ELIMINATING THE LARVA
Treatment is focused on removal of the larva, e.g., by occlusion of the central punctum, surgical excision, manual squeezing, and use of avermectins.1,3 Occlusion of the punctum attempts to induce anoxia and force the larva to exit the lesion, making extraction easier. Substances used include paraffin oil, petroleum jelly, bacon, chewing gum, beeswax, butter, fingernail polish, makeup cream, and adhesive tape.1,4 Occlusion frequently needs to be maintained for as long as 24 hours and is not always successful. It is also possible for the botfly larva to suffocate but remain imbedded.2
As we did with Ms. W, removal is often accomplished under local anesthesia with a simple cruciate incision and extraction of the larva by gentle traction.1 Surgical excision of the entire lesion, while more invasive, may reduce the chance of incomplete removal of the larva and subsequent complications.2 Topical and systemic avermectins have also been used to kill the larvae, primarily in animals, which tend to have larger numbers of lesions.
Prevention is also important. Travelers can decrease the risk of parasite infestation by applying insect repellent, setting up mosquito netting, wearing long-sleeved clothing, and sleeping indoors.
Typically D. hominis myiasis results in a furuncular myiasis that causes no serious morbidity other than localized symptoms and anxiety. However, more serious complications, such as blindness, can occur, and infestation in infants can be fatal if migration into the brain occurs through entry over an open fontanelle.2 Secondary infection is possible, although bacteriostatic factors produced by the larva make this an infrequent complication.2
Clinicians in nontropic areas must broaden their differential for skin lesions to include many exotic diseases. Encountering a disease acquired halfway across the world emphasizes the importance of taking a good travel history regardless of where the clinician is practicing.
Dr. Arnold and Dr. Michener are majors in the U.S. Air Force previously stationed in Afghanistan. Dr. Arnold is currently stationed at Robins Air Force Base, Ga., and Dr. Michener is currently stationed at Scott Air Force Base, Ill.
1. Maier H, Hönigsmann H. Furuncular myiasis caused by Dermatobia hominis, the human botfly. J Am Acad Dermatol. 2004;50(2 Suppl):S26-S30.
2. Sampson C, MaGuire J, Eriksson E. Botfly myiasis: case report and brief review. Ann Plast Surg. 2001;46:150-152.
3. Chin RL. Cellulitis due to botfly larvae. N Engl J Med. 1997;337:429-430.
4. Sajjad N, Biederman G. Case report: insect bite reveals botfly myiasis in an older woman. Am Fam Physician. 2005;71:1262-1263.