What is a Screwworm?
The scientific name of the New World screwworm, also commonly known as the American primary screwworm, is Cochliomyia hominivorax. As its common name suggests, the screwworm is endemic only to the Americas. Before its eradication from the United States the screwworm devastated livestock populations, cost ranchers and consumers millions of dollars, and inflicted horrible pain and suffering on infected animals and humans. A U.S. Department of Agriculture (USDA) study performed in 1935 reported that screwworms cost the state of Texas $10 million that year. A 1985 publication states, "Using the 1957 loss estimate, which would be many times higher at today's prices, the Southeast Screwworm Eradication Program has saved the livestock industry of Florida and adjacent states 20 million dollars a year for the past 25 years, a total savings that now exceeds half a billion dollars and will continue" (Meadows, 10).
Screwworms were a threat to the health of humans, livestock, and animals in the wild. While any open wound invited a screwworm attack, humans were most often infected in the nose or sinus cavity. In fact, the word hominivorax translates to "humans being devoured." It was in livestock, however, that the screwworm inflicted the most damage. Animals had to be constantly monitored for wounds, as events such as the branding of cattle and the birth of a newborn animal were invitations for screwworm attacks. One of the most commonly affected places was the navel of a newborn animal. Ranchers also took precautions against animals cutting themselves and having open wounds by removing barbed wire and protruding nails from their land.
The screwworm lives for approximately 21 days in warm weather, but can maintain a longer life cycle in cooler climates. Although the male screwworm fly mates continually throughout its lifetime, the female fly mates only once, laying her eggs along the edges of wounds on warm-blooded animals. If the wound is not treated, the egg masses hatch into larvae, which burrow into the host's flesh--hence the name "screwworm"--and consume its living tissue and fluids. As the larvae feed, they enlarge the wound and attract additional female flies, which deposit their own egg masses into the wound. For several days the screwworm larvae feed on the host's flesh, gaining nourishment for the next stage in their development. If the wound remains untreated, secondary infections are nearly inevitable and it is unlikely that the host will survive.
After the screwworm larvae gain sufficient sustenance, they leave the host by dropping to the ground and burrowing shallowly into the soil. There they form a dry outer shell and enter the pupal stage. It is during this stage that the pupae experience a metamorphosis into adult flies. When the flies emerge from the ground they soon mate and the cycle begins again. Screwworm flies are blue-green, with three dark stripes on their backs, and orange eyes. They are about twice the size of a housefly.
Control and Treatment
Devil's Island, off the coast of French Guiana, was the site of the first reported screwworm case in 1858. It was not until 1933, however, that the screwworm fly was recognized as its own species. Emory Clayton Cushing, an entomologist with USDA's Bureau of Entomology and Plant Quarantine, and Walter S. Patton of the University of Liverpool, United Kingdom, are credited with establishing that the screwworm fly is not the same species as the more common blowfly (Cochliomyia macellaria), which feeds on dead animals. Following publication of this discovery, research began to focus on control of the pest.
Before 1933 the screwworm was concentrated in the southwestern part of the United States. A shipment of infested livestock brought the screwworm from the southwestern to the southeastern United States in 1933. County agents and researchers conducted numerous reports and surveys to assess the damage and to try to determine the living habits of the screwworm. In 1934, the screwworm could be found in Mississippi, Alabama, North Carolina, South Carolina, Georgia, and Florida as well as in the southwestern United Sates.
Initially, the only two effective wound treatments were benzol and pine tar oil. Benzol killed the worms while the pine tar oil repelled flies and aided in healing the wound. These treatments required daily application and confinement of the wounded animal. After further study, scientists developed a better remedy which they named Smear 62. It was a thin paste formed from a combination of diphenylamine, benzol, turkey red oil, and lamblack. Although this worked better than the previous combination, it still did not kill the fly. A full scale eradication of the screwworm was desperately needed.
Sterile Insect Technique
Two factors combined in the 1930s to form the basis for the USDA's Screwworm Eradication Program. These were Edward Fred Knipling's theory that the screwworm population could be eradicated by inundating the normal population with sterile males (known as the Sterile Insect Technique), and Raymond C. Bushland's development of a method for rearing large numbers of the insects for research on a diet of ground meat, beef blood, water, and a small amount of preservative, such as formalin, rather than having them feed on live animals. Researchers still needed to develop an effective method to sterilize the screwworm.
Between 1947 and 1950, scientists made unsuccessful attempts at chemical sterilization of the insect at the Kerrville, Texas, research laboratory. During this time the screwworm spread as far north as South Dakota via infested livestock shipments.
In 1950, Alfred W. Lindquist drew Knipling's attention to an article published in January of the same year by the Nobel Prize winner Hermann Joseph Muller. Muller's article investigated the sterilization of fruit flies by radiation and Knipling immediately saw this as a potential solution to the problem of mass sterilization of screwworm flies. Bushland arranged to use hospital x-ray equipment to test whether radiation could effectively and efficiently sterilize large numbers of screwworm flies. The experiment was a success, and cobalt-60 gamma ray equipment from Oak Ridge National Laboratory made the mass sterilization possible.
Sanibel Island and Curacao
At this time, tests began on Sanibel Island, just off the coast of Florida, utilizing the Sterile Insect Technique to eradicate screwworms from the island. Sterile flies were released in large quantities from airplanes, flies were trapped to determine the ratio of sterile to fertile, and wounded goats were monitored for screwworm infestations. On May 8, 1953, the Sanibel Island tests were terminated and the Sterile Insect Technique was pronounced a success. Due to a slight reinfestation of the island, however, it was decided that a location farther from the mainland was needed in order to determine whether a full-scale eradication program would be possible. In January of 1953, a letter from Benjamin A. Bitter, a veterinarian on the island of Curacao, Netherlands Antilles, to the director of the Bureau of Entomology and Plant Quarantine, brought this island to the attention of Knipling. Curacao is approximately 40 miles off the coast of Venezuela and the next closest island is 30 miles away. It was here that scientists truly achieved a successful eradication. They released flies from planes at a rate of 400 sterile males per square mile, per week. In ten weeks, screwworm flies were eradicated from Curacao.
Fly Production Facilities
A larger fly production plant was built in Bithlo, Florida, in 1956. In 1957, the Florida legislature appropriated funds for a full-scale Southeast Eradication Program. The need for this program in Florida was great. The northern areas of the southeast and the southwest of Florida remained insect-free throughout the coldest winter months. This area became infested each spring due to the movement of livestock and insects from the warmer southern areas, where the screwworm could overwinter. If the overwintering zone in central and south Florida could be eradicated then the entire southeastern United States would benefit. Scientists conducted a pilot test over a 2000 square mile area near Orlando, Florida, where they released 500 sterile male flies per square mile per week for a total of 1 million sterile flies each week. The end result was an eradication of 70 percent.
In 1958, Sebring, Florida, became the home of the famed "fly factory." This new plant could produce 50 million sterile flies per week. At that time an effective eradication was desperately needed as noted by Senator Holland in his speech entitled, "Science Pays Off," presented at the dedication of the Sebring plant: "Although losses due to screwworm in the Southeast fluctuate from year to year, the annual losses in livestock production are estimated at $20 million, and according to all reports, last year was the worst we have experienced since this pest entered the Southeast twenty-five years ago."
The Southeast Eradication Program was deemed a success in 1959. The perseverance of scientists and citizens had paid off. Any further reinfestations that occurred were efficiently controlled and the focus of screwworm eradication turned toward the southwestern United States.
