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Infectious Disease Pathology Conference
February 14, 2001
Case #1 (SP-00-17770)
Laura Chandler, Ph.D (Clinical Microbiology)
Gabriel Rodriguez, M.D. (Urologic Surgery)
Clinical Summary: This 48-year old male reported to UTMB with two painful lesions on the scrotum. He has no significant past medical history, but reported recent travel to Costa Rica. The lesions were examined and surgically removed.
Diagnosis: Myiasis due to infestation of the skin with Dermatobia hominis (human bot-fly).
Description: Myiasis is an invasion of tissues or organs by larval forms of insects in the order Diptera (true, two-winged flies). This order contains several families of medically important flies, including Sarcphagidae and Calliphoridae (screwworms and blow flies), Oestridae, Gasterophiliadae (botflies). Dermatobia hominis is in the family Cuterebridae and is referred to as the human botfly. The adult flies are 12-19 mm in size, with a yellowish head, dull blue-black thorax, and metallic looking abdomen. The legs are orange, and the wings are brown. This species readily infests several other species besides humans, including wild or domestic mammals, and birds. Infections in cattle and sheep can have significant economic effects
The life cycle of the botfly Dermatobia hominis is quite interesting because of the way in which the hosts become infected. Female flies do not deposit their eggs directly on the host. Rather , the adult female fly (after mating), actively seeks out biting arthropods such as ticks, mosquitoes , or other species of biting flies. The botfly captures the other arthropod, hold it’s wings to prevent escape, and then attaches her eggs (roughly 15-30 at a time) on the abdomen of this other insect. The biting insect then carries around the botfly eggs. When this insect finds a suitable host for its own blood feeding, the botfly eggs are stimulated to hatch by the warmth of the host. A first-stage larva emerges from the egg and is deposited on the skin. The larva will burrow into the skin, either directly through intact skin, or through the bite from the insect vector. It can also enter through hair follicles or damaged areas. The burrowing process takes 5-60 minutes but apparently is not usually noticeable to the host.
The larva will develop at the site of entry, it does not migrate through the body although it does move around under the skin. It’s anterior end is found towards the inside, and the posterior end is located at the skin surface. The anterior end is used to grasp host tissues for feeding. There are two oral hooks which are used for tearing tissue during the feeding process. The curved spines along the body assist with anchoring the larva in the skin. The larva is able to breathe through small spiracles located at it’s posterior end, where the skin will remain open. The pattern of spiracles is one of the features used to differentiate the various species of botfly. The developmental stages take 4-14 weeks. The larva goes through two molts, to a 3rd instar stage, with emerges from the skin and drops to the ground. It pupates in the ground, taking 14-30 days to mature to an adult, which then emerges. Adult botflies do not feed, and live only a short time. The females are able to develop eggs because of the stored resources acquired from the host during larval development. Botflies are obligate parasites of vertebrates: their larvae must undergo development inside a warm-blooded host.
Epidemiology and Ecology: The distribution of Dermatobia hominis botfly includes Mexico, Central and South America. In the U.S., cases usually occur in travelers who have visited endemic areas. Flies live near water, thus coastal and forested or jungle areas are endemic for this species.
Clinical Manifestations: May depend on the location of the fly larva on the host’s body. Usually larvae are found on the arms, legs, back and scalp. Other areas that have been reported are brain, eyelids, tongue, nose, genitalia and buttocks. Approximately 24 hours after infestation, a small (2-3 mm) papule will develop that resembles an insect bite. This will enlarge gradually to 10-35 mm and will be approximately 5-10 mm in height and will be surrounded by an area of induration. A breathing hole will be visible in the center of the lesion. As the larva develops, the lesions may become pruritic and produce a discharge that may be serous, serosanguineous or purulent. A secondary bacterial infection is actually uncommon. Often, pain (a stabbing feeling) is felt by the host; this is a result of the larva tearing off tissues while feeding, and from the spines irritating the tissues as the larva moves around. Usually systemic signs and symptoms do not occur, but occasionally malaise, letharge and insomnia have been reported.
Pathology: Moderate inflammation may occur in the area where the larva is found in the subcutaneous tissues. In general, secondary bacterial infections do not occur.
Diagnosis: Definitive diagnosis is made by removal of the larva followed by indenification by a medical entomologist or trained personnel. Tentative diagnosis can be made if there is a history of recent travel to an endemic area, and presence of non-healing lesions on the skin. The lesion may resemble other infections, so presence of the larva should be confirmed. Identification of the larva is relatively easy and is made by examining the spiracles, mouth hooks and pattern of spines on the body. Dermatobia hominis is easily distinguished from other botflies found in the Western Hemisphere, such as Gasterophilus spp. A similar fly, Cordylobia anthropophaga (also called the Tumbu fly), is found in Africa .Larvae are similar to Dermatobia hominis,but can be distinguished by the spines and spiracles.
Treatment: Removal of the larva. Larvae will migrate out of the skin if their spiracular plate is covered. A number of substances have been used to accomplish this, including nail polish, tape, wax, mineral oil, etc. The larva can be removed by grasping with forceps or applying pressure on both sides, but because of the curved spines, surgical excision may be required. Antibiotic treatment is not usually necessary and should only be used if a bacterial infection has developed.
Harrisons Internal Medicine online. Chapter 393, Ectoparasite Infestations and Arthropod Bites and Stings, Myiasis.
Kettle, D.S. Medical and Veterinary Entomology, 2nd Edition. CAB International, Cambridge, U.K. 1995. Chapters 14 and 15, pages 268-314).