A carnivorous parasite with a tooth-covered head is poised to take over an entire country: who is at risk?

In February of last year, Costa Rica declared a renewed state of emergency due to the emergence of a carnivorous parasite. Now, the larvae of the New World screw-worm fly (Cochliomyia hominivorax) have once again revealed their tooth-covered heads, this time in southern Mexico—experts are concerned that they may cross the border into the United States, as reported by IFLScience.

It is known that the screw-worm fly larva was found in only one cow in southern Mexico, near the Guatemalan border. Its discovery disrupts a 34-year eradication period in the country, and experts believe the appearance of the parasite is linked to illegal cattle trafficking in Central and South America.

Sixty years have passed since the parasite was eradicated in the U.S., and now officials are warning that the screw-worm fly could again cross the U.S. border. According to Jeremy Radachowski, the regional director for Mesoamerica at the Wildlife Conservation Society, illegal cattle trade in Mesoamerica, the southern border of North America stretching to the Pacific coast of Central America, follows cross-border routes that start in Nicaragua and pass through Honduras and Guatemala. From there, its route extends through Mexico and could reach the U.S.

Experts also note that unregulated cross-border movement has created a rapid corridor for the parasite, allowing it to travel 1,126 kilometers: from the Nicaragua-Honduras border to Catemaco, Mexico—this took only two and a half months.

As a result, representatives from the Texas Parks and Wildlife Department issued a statement warning residents of South Texas about the risk of infection. Locals were also asked to report any potential cases of infection.

Symptoms of infection by screw-worm fly larvae

Researchers have also identified the main clinical signs of infection. These include:

• irritation or depressed behavior;
• loss of appetite;
• head shaking;
• the smell of decaying flesh;
• presence of larvae in wounds;
• isolation from other animals or people.

What is the screw-worm fly and why is it dangerous?

According to researchers, there are two types of screw-worm flies—both are obligate parasites in their larval stages. The New World screw-worm fly, or true screw-worm fly, inhabits much of North and South America.

The particular danger of these screw-worm flies lies in the fact that they lay eggs on the wounds of living mammals, rather than on carrion. Members of this species seek out any wound or orifice and lay between 100 to 350 eggs on their edges.

In less than 24 hours, the eggs hatch and the larvae burrow into the wound in groups. Subsequently, the infection spreads rapidly among groups of animals, as the odor from the wound infected with larvae attracts other females, which also lay eggs. As a result, more and more screw-worm flies are drawn to the area.

About a week after hatching, the larvae, which by this time have grown from 2 millimeters to 1.5 centimeters, stop feeding and drop to the ground, where they burrow and pupate. After 6-7 days, depending on the climate, they emerge to the surface to continue the cycle.

The infection by screw-worm flies is further complicated by the increased likelihood of various types of fly larvae entering the wound, which raises the chances of infection. Researchers note that without proper treatment, infection by screw-worm flies can kill an animal within one to two weeks. According to scientists, newborns are the most vulnerable to these parasites.

Eradication of the screw-worm fly in the U.S.

About 60 years ago, the U.S. Department of Agriculture implemented a strategy to combat the parasite that proved to be extremely effective. At that time, experts employed a strategy known as the sterile insect technique. This involves sterilizing male flies held in captivity using radiation.

The sterilized males were then released into wild populations: approximately 3 million sterile flies were added to populations twice a week in one of the affected areas. Since screw-worm flies mate only once in their lifetime with a sterile male, this prevents the female from laying viable eggs.