What you should know about the New World screwworm fly
For the first time in more than 50 years, the New World screwworm fly has emerged as a potential threat to the $112 billion U.S. cattle industry. Two cases involving the flesh-eating parasite have been confirmed in South Texas.
The fly had been a persistent warm-weather foe for American cattle ranchers from the 1930-1960s until the U.S. eradicated the pest by breeding sterile male flies and dropping swarms of them from airplanes to mate with wild females. According to the U.S. Department of Agriculture, the most recent cases reported southwest of San Antonio, Texas, were the first in that state since 1966.
Texas officials have established a quarantine zone aimed at prohibiting the movement of any warm-blooded animals, including pets, outside that zone without inspection. Several other states have independently implemented strict emergency import precautions and restrictions on livestock and warm-blooded animals. The parasite could spread rapidly as millions of cattle and livestock are transported across the U.S. daily.
Unlike most fly larvae, New World screwworm maggots feed on living tissue rather than dead material. Larvae burrow into flesh, leading to rapid tissue destruction, severe pain, trauma and non-healing wounds. Untreated cases can lead to death within two weeks.
“Any warm-blooded animal can be infected with the fly larvae (Cochliomyia hominivorax) that causes the disease, even people and birds,” said Dr. Manuel F. Chamorro, professor of internal medicine in the College of Veterinary Medicine’s Department of Clinical Sciences and a diplomate of the American College of Veterinary Internal Medicine (Large Animal). “However, infection is more common in cattle, swine, horses, small ruminants and dogs. Infection of people and birds is less common.”
When a female New World screwworm is attracted to an animal’s wound or a natural opening, no matter how small, it will lay hundreds of eggs on the site. Eggs hatch within 24 hours, and the larvae burrow into tissue and feed for up to a week. Larvae drop to the soil, pupate, and emerge as adults. The entire life cycle can be completed within three weeks.
“Any skin or mucosal wound (cuts, lacerations, abrasions, etc.) or body opening has the risk of becoming infected,” Chamorro said. “For example, in newborn calves, screwworm flies may lay their eggs in the umbilical opening and cause infection and disease there. Recent parturition, animals that suffered an injury (open wound), and animals that recently have had surgery or procedures such as dehorning, branding, and castration are at increased risk of the disease.”
Livestock and pet owners should look out for maggots or white egg masses in wounds or body openings, such as nostrils, ears, genitalia or the navel of newborn animals. Other potential warning signs include wound swelling and discharge, foul odor or the smell of decay in affected wounds, pain at the touch, avoidance of food and water, and separation from the herd or isolation from other animals or people.
Chamorro said animals may show signs of systemic illness within a few days of an infestation emerging. An infestation can become life-threatening to adult animals within a few days, but the threat accelerates for newborns.
“In neonates the course of the disease could be much faster and become fatal quickly,” he said. “Therefore, paying close attention to the signs and risk factors and seeking veterinary expertise as soon as these signs/factors appear is the best way to prevent fatalities.”
Chamorro said individuals should immediately contact their veterinarian if they suspect the presence of screwworms. Other recommended points of contact include Alabama’s Office of the State Veterinarian and the USDA.
Community Vaughan Large Animal Teaching Hospital