The genius breakthrough against the flesh-eating screwworm - Inside the U.S. Agriculture Research Service’s strategy
New world screwworms, which were officially declared eradicated from the United States in 1966, have returned raising concerns among cattle ranchers.
The new world screwworm used to cost American ranchers as much as $100 million a year. The tremendous damage they caused led to a massive campaign to rid the nation of the parasite.
Beginning in the 1950s, using a new technique developed by researchers at the Agriculture Research Service, the United States successfully eradicated the screwworm, not only from the nation but nearly from all of North America. However, the pesky parasite has managed to push north once again and make its way to the United States.
How the U.S. once won the war on the flesh-eating screwworm
The new world screwworm was once endemic to the southwest of the United States and killed thousands of livestock each year. The parasite lays its eggs in the wounds of warm-blooded animals and then the larvae, unlike those of other insects, devours healthy live flesh resulting in infections and even death of the host.
The first step to eradicating the pest came when Raymond C. Bushland, an entomologist at the Agriculture Research Service, developed a method for mass rearing the screwworm fly in 1937, which allowed for laboratory research to be conducted on the parasite. Then fellow ARS scientist Edward F. Knipling came up with the theory that by introducing sterile male screwworms into wild populations their numbers could be controlled.
The Sterile Insect Technique (SIT) was put into practice for the first time on Sanibel Island off the coast of Florida in 1951. While the technique worked, its proximity to the mainland meant there was a steady reinfestation of the parasite.
Then in 1953, the experiment was conducted again, this time on the island of Curaçao north of Venezuela, believed to be sufficiently distant from South America to prevent screwworm fly migration. 400 sterile male screwworm flies per square mile were airdropped over the island and within 10 weeks it was a success.
The first place in the U.S. where the screwworm fly was eradicated was in the southeast where it had been accidentally introduced by infected livestock. Thanks to a severe winter and 50 million sterile males airdropped per week, by 1959, roughly a year from beginning the effort, the parasite had been eradicated.
Attention then turned to the Southwest. The United States officially declared the screwworm eradicated from the nation in 1966. Although there were some major outbreaks into the 1970s as in rare cases the flies can travel up to 180 miles, as well as occasional infected livestock getting through inspection controls.
Mexico reported its last outbreak in 1993 and by 2006 the screwworm fly had been eradicated from all of North America down to the Darien Gap in southern Panama. However, lapses in monitoring, disruptions to sterile male releases, and smuggling of infected animals allowed the parasite to breach the protective barrier and they have once again appeared in the United States.
There are concerns that if the screwworm infestation spreads, it could cost the Texas economy alone as much as $1.8 billion per year.
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