Screwworms: an endangered (thank God!) species

by Mary Kasulaitis

A recent article on screwworms in Arizona Wildlife Views, put out by the Arizona Game and Fish Department, inspired me to do a little reminiscing about that dreaded insect. Once upon a time I overheard some folks in Arivaca complaining about flies. These were different, not ordinary house flies: an airplane had dropped a little box filled with flies just a little too close to their house and by accident, the flies went inside. As soon as I realized what they were talking about I was angry: what are a few flies compared to the devastation done by screwworms, not just to cows, but wildlife and people as well. Perhaps they didn’t realize just what screwworms are.

Those flies were a kind of parasite of the species Cochliomyia hominivorax, which in its larvae stage eats live flesh: yours, mine, cows, deer, whatever. The flies lay their eggs in any kind of open wound. The eggs hatch and the larvae begin to feed on the flesh. Unlike maggots, which they resemble but which eat dead flesh, screwworms eat living flesh. An untreated wound, which has screwworms in it, can grow in size until the animal dies of the infestation or a secondary infection.

In the old days, ranchers had to check their cattle frequently for screwworms. Baby calves often had infestations in their umbilical cords. Branding and castrating created wounds that had to be watched carefully. Black smear, made of pine tar, was used to coat the wounds to keep the flies away until they healed. A great deal of time was spent checking the cattle to see if they were healthy, and hunting in all the corners and under bushes to find those who went into hiding if they had screwworms and didn’t feel well. That’s what cowboys spent most of their time doing. You may remember reading Eva Wilbur-Cruce’s memoir, in which she tells of having to clean screwworms out of a wound. My father never made me do anything like that when I was a child, but I vividly remember watching him do it. He poured pink liquid screwworm killer onto the larvae in a wound that was about eight inches across, the heifer having eluded the cowboys for some time, using his finger to scoop them out, and when all were removed and the wound clean, covering the edges with black smear to protect it from another infestation. Not a fun process and one that was repeated frequently every year.

Once my dog had gotten a barbed wire cut on her foot. It was small, so I thought it would heal by itself. She kept licking, so I finally took a good look at it. There were six screwworms in the cut, eating away. Licking hadn’t dislodged them, nor had it kept them from hatching.

Obviously, wild animals were just as susceptible, and less likely to get treatment. David E. Brown, in his Wildlife Views article, told of finding a buck, the top of whose skull had been almost eaten away by the worms. He had had them for weeks, probably ever since his antlers were in velvet. He was in intense discomfort, if not agony, and had to be euthanized.

You don’t even have to have a wound to attract screwworm flies. The Nogales’ Oasis newspaper recounted the following story in 1906: a Mr. A. Wilson came into the hospital for treatment of an unknown ailment, which he had contracted in Mexico. Turns out, he had screwworms in his nose. “Eighty-five worms were removed, after which the sick man was allowed to rest…The worms had burrowed into the cavities of the nose, into the cheeks, into the roof of the mouth and well into the throat…At the second sitting about fifty were removed. On the evening of the same day what remained of the worms were taken out and the patient was able to rest and recuperate.” That is a gross story, I must admit, but there are some things that ought to be remembered. Screwworms were an ever-present threat until just recently. “Never Again,” as they say.

It was because of economic losses to the agricultural industry and wildlife that the U.S. Dept. of Agriculture worked out a way to eradicate the pest. Apparently, there was more trouble with screwworms in the 1950s in the Southeast and Florida than there was here. Brown recounts: “Using the fact that the female screwworm mates only once in her life, entomologists …devised an ingenious method to ensure that her eggs never hatched. Using Atomic Age technology developed in the years immediately after World War II, scientists in a laboratory in Mission, Texas, began irradiating from 50 million to 70 million screwworm pupae per week with gamma rays from radioactive cobalt. Now sterile, the newly emerging flies were boxed up and dropped from airplanes at a density of between 200 and 1,000 flies per square mile. Because the sterile flies greatly outnumbered the native flies, the sterilized males did most of the breeding. All, or nearly all, of the females mated with sterile males and deposited only infertile eggs. Within a few years, the screwworm was declared eradicated in Florida.” In 1962, the program expanded into Texas, New Mexico and then Arizona. The Federal government began to realize that this project would have to include Mexico, so it shouldered the burden of paying for an expanded effort. For years, whenever a case of screwworms was suspected, the rancher would collect a few of the larvae and drop them into a special container that was mailed to a laboratory. (It isn’t easy to tell screwworm larvae from maggots with the naked eye.) If identification was made, an airplane would soon be dropping sterile flies into the area. This went on for years, because according to Brown, Sonora was not declared screwworm-free until 1982. The program extended down into Central America, and I expect it will continue to go on. There is absolutely no redeeming value to this species of fly.

Mr. Brown notes that he wishes he’d kept one of the little gray boxes with the red target printed on it; the ones they dropped from planes. For sentimental reasons, he said. I have one, which my father kept, for the same reasons. I also have a bottle and packaging that you would use to send the sample into the laboratory. That was one government program that made him a happy man. Screw worm eradication would never have happened except the U.S. Dept of Agriculture saw the need and implemented a successful program.

References: “Turning on the worm that turned,” by David E. Brown, Arizona Wildlife Views, May-June 2001.

“Had screwworms,” The Oasis, April 14, 1906.