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Suffering to End Suffering: The Heroic Effort to Eradicate the North American Screwworm

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By: Dr. John Richard Schrock  Date: 01/16/2012 Category: | Farm and Ranch Almanac |

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It was a gruesome infection. From pioneer days until the mid-20th Century, the North American screwworm was a scourge of cattle. Any little cut from thorns, any open wound left from birthing, any eye infection that wept, would soon harbor the larvae of the dreaded primary screwworm, Cochliomyia hominivorax.

The adult female fly is attracted only to living flesh, and the eggs she laid hatched into maggots that burrowed into the wound. Their feeding and secretions expanded the wound, providing more space for more eggs. What was a small innocuous cut soon grew into an extensive infection that caused much suffering and could kill the animal.

And the fly was not picky. She laid eggs on wounds beyond cattle: in pigs and sheep and wild deer. The screwworm had been a natural parasite on wild animals; up to 75 percent of newborn deer died from the infections. The herds of cattle raised by ranchers merely expanded their population. And the fact that the screwworm fly infested wild deer meant that ranchers could not control the fly by herding their domestic animals through “dips.” Infected wild animals would always provide a reservoir of screwworm flies to re-infect the cattle herds.

Then researchers discovered a special weakness of this fly: unlike most insects, the screwworm only mated once. And nuclear technology provided a way to sterilize flies with radiation, enough to make them infertile but not enough to harm their reproductive behavior. Entomologists reared immense numbers of screwworm flies in a huge “fly factory” near Brownsville Texas and later at the southern end of Mexico. The inactive fly pupae, waiting to change from maggots into adults, were at a perfect time to be dosed with radiation and dropped from airplanes. This new technique, new in the 1950s and 1960s, was called “sterile release.”

We had spread huge amounts of pesticides to kill insects. But because natural selection soon selected resistant strains, not one insect species has ever gone extinct from pesticides. But for the few species that only mate once, the distribution of huge numbers of sterile insects for several generations drives the species to local extinction.

The screwworm caused huge losses of livestock across the southern United States and south into Central and South America. When the initial experimental release of sterile flies eradicated the screwworm from the island of Curacao, the U.S.D.A. launched a campaign to raise the fly, sterilize them, and drop them from airplanes in a slow sweep across the United States from Florida westward. By constantly bombarding both farm and wilderness areas with sterile flies, the few surviving fertile flies could no longer find fertile mates.

The key to the process was knowing when there are no more fertile flies laying eggs. Only then could they move the battlefront forward. Since this fly only comes to open living wounds, not to dead meat, it was necessary to use “sentinel” sheep with open wounds—purposely inflicted cuts—in order to detect if there were any wild flies left. No other system would work.

The screwworm was driven from the United States and south through Mexico. The domestic and wild animals of North America and Mexico have now been free from this pest for two farming generations.

The amount of money saved annually in the cattle industry by the eradication of the screwworm approaches $400 million annually. The amount of suffering that has been prevented, both among domestic animals and among wild deer and relatives, is also immense. But only the veteran ranchers and wildlife officers remember those gruesome infections.

Just as we know that we have to suffer the momentary pain of a vaccination in order to avoid the much greater suffering of serious infectious diseases, a small number of sentinel sheep had to endure surface wounds in order to wipe out the screwworm fly. For those who would never condone this very limited suffering by the sentinel sheep, they must confront the fact that inaction would have allowed the ongoing and far more massive suffering of both domestic and wild animals in the future. Researchers are still trying to formulate a “bait” that will attract the fly and substitute for sentinel sheep, but that would be little justification for 50 years of inaction.

This also reveals a paradox about science. When we make progress in science, we often eliminate the experience base that gave us the drive to make that progress. When we suffered from contaminated water, we supported chlorine and ozone water treatment. Now, among the new generation that has always had reliably clean water, some want to end water treatment. Our children had dental cavities so we fluoridated the water and dramatically reduced tooth decay. Now many in our new generation without cavities see no reason to fluoridate the water. Teachers can teach such abstract facts—just as you can read the historical account above—but abstract discussions and historical pictures do not rise to the impact of living with widespread water-borne illness, rampant cavities and false teeth, or terribly infected cattle and deer. The rural folks of the 1950s and 1960s had the direct experiences necessary to take action to improve the condition of their animals and wildlife. With a new generation where fewer have grown up in contact with the countryside and with wildlife, it is a reasonable question to ask if today, we would still act to eradicate the suffering caused by the screwworm? 

As science teachers, it is important for our students to understand that the biggest benefactors of agricultural and biomedical research with animals—are the animals themselves.

Reference: “Autocidal Control of Screwworms in North America” by R.H. Richardson, J.R. Ellison, and W.W. Averhoff, Science, Vol. 215, 22 January 1982, 361–370.
Photos from U.S.D.A. education slide series circa 1970s.

Dr. John Richard Schrock, Emporia State University, NAIA Board Member

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About The Author

All Authors Of This Article: | Dr. John Richard Schrock |