For the first time in nearly 60 years, a parasite known as the flesh-eating New World screwworm has been detected in animals in the United States.
The most recent tracking from the U.S. Department of Agriculture (USDA) shows that seven animals have been infected: five cattle and one goat in Texas, and a dog in New Mexico.
The USDA is leading a coordinated emergency response to contain the outbreak.
“This is not a food safety issue — the U.S. food supply remains safe,” the agency said in a recent announcement. “The current risk to animals and people in the United States is very low.”
What Is Screwworm Infection?
Screwworms are actually a type of maggot, or larva. They grow into screwworm flies, which are similar in size to common houseflies, sometimes slightly larger. Female screwworm flies lay eggs in open wounds of warm-blooded animals.
When these eggs hatch, the maggots burrow into wounds, “feeding as they go like a screw being driven into wood,” according to the USDA.
Unlike other maggots that eat dead flesh, screwworms can cause severe damage to living tissue as they tear in with sharp mouth hooks.
Wounds can grow and deepen as more maggots hatch and feed on living tissue. As the infestation expands, the injury grows more painful and foul-smelling.
“The infections can be horrific and they can absolutely kill the host animal, particularly if they’re very young, or very old,” says Sally Anne DeNotta, DVM, PhD, an associate professor in the department of large animal clinical sciences at the University of Florida’s College of Veterinary Medicine in Gainesville.
Screwworms Primarily Affect Livestock
Screwworms can infest farm animals including cattle, goats, and sheep. Livestock make ideal breeding grounds because they provide an abundance of hosts in close quarters, and they often have open wounds caused by standard animal management practices, including branding, dehorning, and birthing.
Wild animals including deer, bighorn sheep, and black bears can also get infected with screwworm, as well as pets — particularly those that go outside, have open wounds, and live near livestock.
Can Screwworms Infect Humans Humans?
Screwworms rarely affect humans, but some people are more susceptible than others.
Humans most at risk of New World screwworm infection include those who:
- Work closely with livestock or wildlife animals in an area with screwworm activity
- Travel to regions and countries where screwworm is consistently present, such as South America, Cuba, Haiti, and the Dominican Republic, or to areas currently experiencing an outbreak
- Have open sores or wounds that may attract screwworm flies
- Are unhoused or sleeping outdoors in areas experiencing screwworm activity
Unlike many infectious diseases, screwworm doesn’t spread from animal to animal or from animals to humans, says Catherine Troisi, PhD, a professor of management policy and community health at the UTHealth Houston School of Public Health.
“Infected animals aren’t contagious,” says Dr. Troisi.
Beef Remains Safe to Eat Food and an Emergency Response Is Underway
Beef hamburgers, steaks, and other beef products remain safe to eat, according to Dr. DeNotta.
“The public should know that this is not a risk to the U.S. food supply and federal authorities are responding aggressively to eradicate the problem,” she says.
Public health authorities had already been preparing for this event, Troisi says. Infestations are prevalent in South and Central America and the Caribbean, and the U.S. government has known for at least a year or two that screwworm has been moving northward.
USDA efforts to control the current outbreak include releasing sterile male flies in areas where infestations are occurring or may occur.
“Releasing sterile male flies will prevent the female from laying eggs, ending the cycle,” says Sharon Nachman, MD, the chief of pediatric infectious diseases at Stony Brook Children’s Hospital in New York. “Enough sterile male flies have to be released at the correct time and place to help stop this parasite from moving further into U.S. farmland.”
Such sterile flies, though, are in short supply at the moment.
“The only manufacturing of these sterile flies is in Panama, but they have limited capacity to produce them. We will need many millions more to stop this infestation,” says Troisi.
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