How to spot New World screwworm in your livestock

Livestock owners work hard to keep their animals healthy, but even small wounds or injuries can create opportunities for parasites to cause serious health problems.

One of these parasites is the New World screwworm, which has recently appeared in areas close to the United States, raising concerns for nearby livestock.

Jennifer Schleining, a clinical professor and head of the Department of Large Animal Clinical Sciences at the Texas A&M College of Veterinary Medicine and Biomedical Sciences and member of the AgriLife Research New World Screwworm Task Force, explains how livestock owners can identify and manage the parasite.

Digging into the details

New World screwworm is a type of fly whose larvae feed on living tissue, unlike most maggots, which feed on dead tissue. The flies lay eggs in open wounds and once the larvae hatch, they invade the tissue, causing pain, infection, and, if left untreated, death.

“The larvae basically eat the animal from the outside in,” Schleining said. “It can happen very fast if you don’t catch it early.”

While the screwworm was once a major concern for U.S. livestock, decades of control efforts have kept it largely out of the country. However, recent cases reported just south of the border serve as a warning for livestock owners to take preventive steps and act quickly if signs appear.

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“For most of our generation, it’s something we’ve never had to deal with before,” Schleining said. “It’s a very bad parasite to have in livestock, and it can affect humans as well.”

Any warm-blooded animal can become infected, but livestock such as cattle, horses, goats and sheep face the highest risk.

“Horses and livestock are more affected because they’re outside all the time,” Schleining said. “Feral hogs can also carry the parasite, and since we can’t treat wild populations, monitoring our own animals is even more important.”

Newborn calves are especially vulnerable, as flies can easily lay eggs in their umbilical area.

“In areas with hard freezes, like the Midwest, calving often happens before flies become active,” Schleining said. “But in Texas, where flies persist year-round, we can’t rely on that seasonal break. That makes our calving season a higher-risk period for screwworm infestations.”

Flying under the radar

Keeping a close eye on your animals is the best way to catch problems before they get serious.

“With New World screwworm, you don’t see the whole maggot — just little white dots inside a wound,” Schleining said. “That’s the key difference from normal maggots that feed on dead tissue.”

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As larvae burrow deeper, animals may become restless, lose weight or develop a foul odor from the wound.

“From the time the eggs hatch, it can be only one to two weeks before the animal is severely damaged or even dies,” Schleining said. “That’s why quick identification and action are critical.”

The best protection comes from daily observation, proper wound care and fly control measures.

“Pour-ons, pyrethrins and fly tags can help reduce fly populations,” Schleining said. “But nothing replaces daily checks and good wound care. An ounce of prevention is definitely worth a pound of cure here.”

Livestock owners can reduce risk by keeping injured animals in areas where they can be monitored closely and covering wounds to prevent flies from laying eggs.

“If you see a wound, after treating it, it is best to cover it to keep flies away,” Schleining said. “If possible, bring a cow closer to the barn so you can check it often.”

Maintaining clean pastures and removing carcasses or other attractants also helps prevent infestations.

Get ahead of the swarm

Screwworm is a reportable disease, meaning any suspected case should be reported immediately to a veterinarian.

“Call your vet right away if you suspect screwworm,” Schleining said. “We are required to report it to the state veterinarian at the Texas Animal Health Commission. Quick reporting allows proper containment and treatment.”

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Treatment typically involves cleaning wounds, removing and killing larvae, and applying insecticidal medications under veterinary supervision.

“Humans can be affected, too, so be careful when handling animals with wounds, especially if you have an open wound yourself,” Schleining said. “If something seems wrong, don’t hesitate to contact your doctor.”

For the latest guidance and resources, livestock owners can visit the website .

Although New World screwworm is not currently established in the U.S., staying alert, monitoring animals closely and following preventive measures will help owners respond quickly to any concerns and keep livestock healthy.