Invasion of the Screwworm
The New World screwworm is marching northward from Central America at an alarming rate. The fly’s larvae eat the flesh of livestock and have moved some 1,400 miles from southern Panama to southern Mexico in about two years. The northernmost sighting is currently about 700 miles south of the U.S. border.
Why it matters: Screwworms are disastrous for ranchers, whose cattle can become infected when the flies lay eggs in cuts or wounds, after which their resulting larvae burrow, or screw, into that flesh. The resulting infection can cause weight loss, impaired milk production and even death from secondary infections. The fly larvae can infect humans too, especially those who work closely with livestock, for which there is no treatment other than surgery.
What can be done: Throughout Central America, agricultural departments keep fly populations down by releasing millions of sterile male flies per week. Female screwworms mate only once in their lifetime, so the infertile males reduce the size of the next generation of flies. In May, ethicists and entomologists wrote in a paper in Science that the screwworm is a good candidate for complete elimination that won’t adversely affect local ecology. Scientists could use genetic engineering to insert a deadly mutation into the fly’s sperm and egg cells, which would be passed on to the next generation.
What the experts say: Consistent use of the sterile male flies technique eradicated the screwworm from the U.S. in 1966 and from regions north of the Darién Gap (the zone that connects Central and South America) in 2006. But the flies are spreading again. “I don’t know how it got away so quickly,” says Maxwell Scott, an entomologist at North Carolina State University. “There had to be some movement of infested livestock, particularly through the middle [of Central America].... It just moved too fast.”