On the patient’s return home, all of his insect bites healed except two. Weeks later, an incision provides some answers.

Three weeks after returning from a vacation in Guatemala, 58-year-old Mr. R presented to my rural northern California health clinic with two lesions—one on his right upper arm and the other on his right lower leg. Mr. R believed he had been bitten by an insect while swimming in a lake. He complained of pain and burning in and around both lesions. Mr. R indicated that he suffered multiple insect bites during his stay in Guatemala but all of them had healed except these two.

Desperate for a solution, Mr. R tried antibiotic ointment and hydrocortisone cream, but the lesions persisted with pain, burning, and slow bleeding. He reported no itching, scratching, or picking at the lesions, nor did he complain of fever, diaphoresis, chills, or GI symptoms. The area was not edematous.

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Mr. R had no recent tick bites, fatigue, or joint pain, and he had not used any new skin products, vitamins, or medications. His past medical history was negative for eczema, psoriasis, allergic dermatitis, or other dermatologic disease. He denied any IV drug use. He also had no history of immunocompromise or diabetes mellitus. In generally good health, Mr. R rarely visited our clinic. He was retired and traveled out of the country a few times each year.


On his first visit to the clinic, Mr. R was in good spirits, appeared in no discomfort, and seemed appropriately concerned about the nonhealing nature of his skin lesions. He was muscular, lean, and had good hygiene. The lesion on Mr. R’s lower leg was raised, erythematous, indurated, and approximately 1.5 cm in diameter. Bright red blood drained from the center of the lesion, which appeared punctate in nature. There was no purulent discharge and no excoriation or other signs of scratching.

The lesion on the lateral surface of the skin overlying his deltoid muscle was similar in nature and measured 1.0 cm in diameter. On inspection with a magnifying head lamp, no foreign body was visible within or around the lesions. Mr. R had no axillary or inguinal adenopathy, nor did he have any rashes or skin lesions elsewhere on his body. All vital signs were normal.


In rural northern California, many residents live off the grid and spend a great deal of time outdoors cutting firewood and working on their property. It is not uncommon to see patients present with a tick or insect bite that becomes secondarily infected. Wounds of patients in this area are frequently culture-positive for methicillin-resistant Staphylococcus aureus (MRSA). In addition, a quick glance at my dermatology atlas caused me to consider cutaneous leishmaniasis (given the patient’s recent travel to Guatemala), but his symptoms and presenting signs seemed incongruent with this diagnosis. I chose to treat Mr. R for simple cellulitis with a 10-day course of trimethoprim-sulfamethoxazole (Bactrim) and hydrocortisone 2.5% (for the pruritus). I advised Mr. R that we would probably need to culture the lesion if it persisted and perhaps perform a punch biopsy to identify the source of the infection.

Sixteen days later, Mr. R returned to the clinic still complaining of pain and burning. He indicated that he had discontinued the Bactrim three days after his last visit because it upset his stomach and he “did not feel like it was helping.” Mr. R seemed much more anxious and concerned about the lesions and asked if I would “cut them open to see what was inside of them.” Instead, I told him I would seek the advice of a dermatologist in Eureka, which is 60 miles north of our clinic. After discussion, the dermatologist recommended three 3-mm punch biopsies and checking for bacteria, atypical fungi, mycobacteria, and leishmaniasis. He also advised making an incision at the time of the punch biopsy to more closely check for possible foreign bodies.

The following day, a punch biopsy was performed on each lesion. I also made an 8- to 10-mm linear incision in the skin overlying each lesion. I attempted to express any matter that might be within and looked for possible foreign bodies. There was no discharge other than blood and no foreign bodies.

Six days after the biopsy, Mr. R told me he was very concerned that the lesions were not healing. He said he had contacted the CDC, and he suggested I do the same. I told him that I was sorry that his condition was causing him such anxiety and discomfort and said I would be happy to get in touch with the CDC.

That same day, a representative from the CDC’s parasite division, who had spoken with my patient earlier, advised that I re-biopsy the lesions for leishmaniasis.

My very anxious patient returned for the second biopsy two days later. He was still complaining of intense burning and pain at the site of the lesion. Using materials provided by the CDC, I performed a second punch biopsy on both lesions. After being hounded by the relentless insistence of my patient, I agreed to make another incision into the lesion on his right lower leg to check again for any foreign bodies. Using a #11 scalpel blade, I made a 1-cm incision into the lesion on his leg. On close inspection, I noticed a 0.25- to 0.5-mm piece of black tissue protruding from the incision.

With forceps, I removed what appeared to be a white, 4- to 5-mm larva (which was moving). I placed the organism on sterile gauze and proceeded to make another incision in the patient’s right upper arm. Unlike the lesion on the leg, there was no suspicious tissue or foreign body. What I found instead was a channel or sinus tract parallel to the surface of the skin. I blindly probed this channel with my forceps and removed a second and larger organism (which was also moving). I found no other foreign bodies. After applying antibiotic ointment to both lesions, I approximated the edges of the incisions with Steri-Strips.

Under a magnifying head lamp, each organism was a creamy-white color, shaped like a flask, and featured very small, black spinelike appendages that encircled the entire length of its body. There also were two black fangs at what appeared to be the insect’s head. I contacted the CDC to determine how best to preserve and transport the specimens for identification. I was instructed to submit the two organisms in a formalin mix.


Two weeks later, the CDC contacted me to report that it had identified the specimens as larvae of Dermatobia hominis, more commonly known as the botfly. The lab also reported that the test for leishmaniasis was negative. The D. hominis botfly larva is described as having rows of short spines, which explains the burning and stinging reported by my patient. Using these spines, the larva anchors itself within the host tissue.1 The rows of spines and the two fanglike hooks were diagnostic for D. hominis. The biopsy showed inflammation of the tissue surrounding the larvae but no secondary bacterial infection. The day after the larvae were removed, my patient reported that the burning and stinging had dissipated significantly.


The parasitic botfly larva develops within the skin of such hosts as cows, dogs, pigs, sheep, and occasionally humans. The female botfly attaches her eggs to the underside of mosquitoes and ticks. After hatching 5-15 days later, the embryo embeds itself in the tissue of an appropriate host via the puncture of a bite or through a hair follicle or other breach of the skin. After entry, the larva creates a chamber called a “warble” within the tissue of the host, leaving a punctate opening through which the larva can breathe and excrete waste.2 This punctate lesion becomes a chronic, nonhealing lesion. Five to 10 weeks later, at the final stage of development within the host, the larva creates an even bigger opening in the skin through which it emerges and falls to the ground.1

Other conditions included in the differential diagnosis of botfly larval infestation include cutaneous leishmaniasis, folliculitis, boil, atopic dermatitis, sebaceous cyst, and cellulitis (my original diagnosis for Mr. R). Key subjective findings would be travel to Central or South America and a burning sensation that appears more severe than the mild-to-moderate nature of the skin lesion would suggest.

Treatment consists of removing the larva. Within the clinical setting, surgical excision is the most common approach. Home remedies that have historically been used include occluding the lesion with fingernail polish, petroleum jelly, and even a piece of raw meat. The theory behind these treatments is that oxygen deprivation will force the larva to move to the surface of the skin.Having only been in practice six months at the time of

Mr. R’s first visit, my professors’ oft-cited admonition was still fresh in my mind—“don’t chase zebras.” Thanks primarily to this case, I have since learned that it is appropriate to consider zebras when a patient has traveled to a tropical area. I have also learned the importance of listening to the patient. Mr. R kept insisting there was “something inside” him. The idea seemed far-fetched at the time, but the patient knew that something was not right.

There are photographs of botfly larvae pinned to the bulletin board above my desk to remind me of two things—almost anything is possible, and patients often know more about what’s going on than we do.

Ms. Christian is a physician assistant at Redwoods Rural Health Center in Redway, Calif.


1. Czachor JS, Elder BH, Sutherin SR. Travelers beware the bot fly! J Travel Med. 1995;2:264-266.

2. Northern Land Institute at the University of Northern British Columbia. Flies. Available at www.unbc.ca/nlui/wildlife. Accessed October 1, 2007.