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HUMAN BOT FLY INFESTATION, MYIASIS, OF THE SCROTUM
BY:
Michael Rashid, MD
Resident of Urology
The University of Texas Medical Branch at Galveston
Gabriel Rodriguez, MD
Assistant Professor of Urology
The University of Texas Medical Branch at Galveston
Cutaneous infestation by Bot Fly larvae, Dermatobia hominis, is endemic to tropical regions of Central and South America which typically victimizes small rodents and livestock. Because human myiasis is exceedingly rare even in endemic areas, correct diagnosis outside the endemic area is difficult because of unfamiliarity with the disease. Lesions produced by this insect are easily mistaken for cutaneous furunculosis. Though the disease is self-limited it commonly causes significant pain to it’s victims. In humans, early correct identification would avoid unnecessary treatment with antibiotics and allow earlier removal of larvae. To our knowledge, we present only the second case of scrotal myiasis caused by Dermatobia hominis in literature to date. The larvae were excised under local anesthesia with excellent results.
Case History:
48 Y/O WM presented to the urology clinic with complaints of “Pain in my scrotum”. He described the pain as intermittent, intense, and abrupt scrotal pain which began after recent travel to Costa Rica. He and his family had traveled to Costa Rica in late November to observe the Volcano Arenal eruption from the Los Lagos observatory. At night, while he was changing his clothes he felt a “mosquito” bite on his scrotum. He complained of immediate scrotal pain, which resolved quickly and without any signs of skin trauma.
Upon returning home he began noticing two”lumps” in his scrotal skin which would cause intermittent episodes of intense sharp shooting pain throughout his scrotum and perineum. Nearly four weeks from the initial bite, he noticed that these tow tiny lesions on his scrotum had “bloody discharge” but no signs of infection. Two other physicians told him that his symptoms were related to a skin infection. He was placed on oral antibiotics for a presumed scrotal cellulitis and told to follow up in the urology clinic. His scrotal exam was significant for two distinct furuncular lesions with a central pore. One located mid-raphae and the second on the right hemiscrotum. No fluctuance, however bloody discharge could easily be expressed with gentle pressure. The lesions measured approximately 1.5cm x 1.0cm
The patient had both lesions surgically excised revealing two live Human Botfly larvae. Pathology demonstrated infestation with fly larva (Myiasis) with inflammation, identified as Dermatobia Hominis.
Discussion:
Diagnosis of edemic tropical disease can prove difficult to physicians in other parts of the world. In this case of myiasis (parasitism by insect larvae), the patient developed painful lesions of the scrotum which were thought to be furuncles by other physicians.
In order Diptera family Cuterebridae, there are a number of fly species which utilize living mammals to incubate larvae. Parasitism of livestock and wildlife is common though human infestation with these species is usually accidental. However, one species does specifically target humans. Commonly known as the botfly or torsalos,
Dermatobia hominis is endemic to forest and jungle regions of Central and South America. Adult flies will capture other insects, such as mosquito’s, and will lay eggs on them. These insects then act as vectors when they land on warm-blooded mammal. The larvae sense the increase in temperature and hatch. Once deposited, the larvae burrow into the subcutaneous tissue and grow for up to 6 weeks. Some species migrate through the host body to continue its growth at a separate location, but D. hominis does not migrate. At maturity, the larvae emerge, fall to the ground, and pupate into adult flies.1
Lesions produced by bot fly larvae often are mistaken for infectious lesions. 2,3,4 The initial penetration is usually painless and unnoticed by the patient. Infestation with multiple larvae is common. As the larvae grow, a subcutaneous mass becomes evident and the lesions are puritic. A pore, called a punctum, is present in the center of mass. The punctum is used for ventilation and excretion of waste. Seroganguinous fluid can be expressed and sudden paroxysmal episodes of sharp sever pain are usual. 3 On physical exam, the lesions seem to be non-tender. Without treatment, larvae emerge and lesions heal with good cosmetic result, however patients must endure painful episodes for up to 8 weeks.
Treatment involves removal of the entire larvae and may be achieved with several different methods described in the literature. Traditional tribal treatments include applying thick tree sap, raw meat, or animal fat to occlude the punctum and force the larvae out in search of air. This method of using occlusive dressings in combination with manual extraction has been described in the literature with success.4 However, attempts to extract larvae with traction run the risk of leaving fragments behind. Warm compresses or injecting lidocaine under the larvae also have been reported to force the parasites to the surface.3
Lesions in this case involved the scrotum. The patient attempted to lure the insects out by suffocation without success. Because of its redundancy, scrotal skin lesions are particularly amenable to surgical excision with good cosmetic result. Surgical excision of scrotal infestation may therefore be the treatment of choice.
References:
1 Harwood RF and James MT. Entomology in Human and Animal Health. Macmillan Publishing. 1979.
2 Bowry R and Cottingham RL. Use of ultrasound to aid management of late presentation of Dermatobia hominis larva infestation. Journal of Accident and Emergency Medicine. 1997; 14:177-78.
Johnston M and Dickinson G. An unexpected surprise in a common boil. Journal of Emergency Medicine. 1996; 14:179-781
Gewirtzman A and Rabinovitz H. Botfly infestation (myiasis masquerading and furunculosis. Cutis. 1999; 63:71-72