Myiasis

(This 817th Buffalo Sunday News column was first published on November 26, 2006.)

I had the good fortune two years ago to attend Wayne Gall's University at Buffalo medical entomology course. Dr. Gall introduces graduate students in the Department of Social and Preventive Medicine to insects that have an effect on human health.

The best parts of this course were the case studies. We students considered them his H-Files, the H for health department. In some cases Dr. Gall was able to share personal experiences as the New York State Department of Health Regional Entomologist associated with the state Arthropod-Borne Disease Program.

Consider a few of Dr. Gall's stories about bot flies, just one of the many kinds of insects he discussed. As he did in class, I have carefully modified them to insure the privacy of the individuals involved.

Several years ago an Erie County resident traveled to Central America. A few days after he returned, a dime-sized itchy welt appeared on his shin. When he rubbed it, the skin "slid off" and a brown sticky discharge emerged. The patient described toothache-like painful gnawing at his leg muscles, but a succession of doctors diagnosed his problem as an infection, possibly an imbedded thorn and "not a bug." Finally, after the entire leg swelled, a surgeon excised a human bot fly larva that had penetrated the skin into the leg muscles. According to the report, when he removed the larva, the surgeon exclaimed, "Holy cow, you're right!" and "the nurse's eyes got as big as saucers." The 3.5-inch wound required nine sutures. Thankfully, this human bot fly, Dermatobia hominis, does not occur in this country.

A human bot fly taken from a wound

Photo by Jim Kalisch, University of Nebraska, Lincoln

Bot flies here are thick-bodied bee-like insects of the genus Cuterebra. Local species attack rodents and rabbits but, although rarely, humans as well. Their larvae pass through three stages called instars as they grow.

In September 2002, after working around and under an old cabin, a middle-aged Cattaraugus County man experienced swelling near his eye. A doctor removed a presumed insect stinger from the man's eyebrow next to two small papules. A week later the man returned to the doctor with a second-instar Cuterebra larva that he had expelled by squeezing one of the papules. Presumably, the bot fly eggs had hatched on the ground under his cabin and the warmth of his body attracted them to him rather than a rodent or rabbit.

Almost exactly two years later an elderly Cattaraugus County woman awoke with chest pain she described as "terrible burning". The sensation subsided when she ate breakfast, but returned a few hours later. This pattern of recurrent pain declining at meals continued for a week. She was diagnosed as suffering gastric esophageal reflux disease, but finally at the end of the week she coughed up an object into her mouth. When she removed it and held it in her hand, the object "reared up and wiggled." It was another second-instar Cuterebra larva. After this episode, her bronchial problems gradually resolved.

In this case the woman owned two pet cats that frequently slept on her bed. One of them went outside regularly. Although no bot flies were identified on either pet, cats are occasionally infested by Cuterebra and might have been the source of her problem.

All of these episodes are examples of what is called myiasis, the invasion of a living vertebrate animal by fly larvae. Of course, that vertebrate designation includes us as possible myiasis victims.

A final H-File involves still another bot fly genus. In April 2004, one of our soldiers in Afghanistan reported to an optometrist with what he described as an "itchy" eye. The doctor removed 18 first-instar sheep nose bots from the soldier's eye. This kind of bot fly normally hovers around the muzzle of domestic animals and squirts first-instar larvae into their nose and eyes. Evidently this soldier served -- temporarily thank goodness -- as an alternate host, because these larvae normally crawl into the nasal passages and sinus cavities of sheep and goats where they complete their development and are finally sneezed from the infested animal's nostrils.

My classmates and I found these unusual H-File episodes occasionally frightening, but always fascinating. Dr. Gall will be teaching another section of this course spring semester.-- Gerry Rising