A flesh-eating pest affecting livestock, pets and wildlife, known as New World screwworm (NWS), is resurging across Central America and Mexico. The pest was eradicated from the United States in 1966¹, but some additional cases did occur in U.S. cattle during the next 16 years. Since 2023 this pest has been making its way back north. It is now present in central Mexico.

What is NWS?

New World screwworm isn’t a worm at all, explains Kirk Ramsey, professional services veterinarian with Neogen. “It’s truly just the larvae form of a species of blow fly. Like any flies, they will lay eggs in an area where their larvae can feed.”

Typically, this group of flies is associated with decaying flesh of dead animals.

“The New World screwworm is a very strange species within this group, as they feed on living tissue exclusively to survive,” says Jonathan Cammack, assistant professor and state extension specialist for livestock entomology and parasitology at Oklahoma State University.

Further explaining why the NWS is a concern, Ramsey adds, “While feeding off of live tissue, they continue to burrow deeper into the flesh, essentially consuming tissue as they move in.” An infestation of NWS can cause extensive damage as the larvae tear into living tissue with sharp mouth hooks and dig deeper into the wound², which separates them from other insects.

Old vs. new

Although there is an Old World screwworm, Cammack confirms the two are not the same.

“The reason it’s called New World is to differentiate this particular species from one that is present on the other side of the world that has a similar habit,” he explains.

Cammack adds that the two species are very distantly related, and both have evolved this ability. “This is not a new pest, it’s just their distribution on the planet.”

Impact of NWS

Infestations of NWS were more prominent in Jamaica, Cuba and across South America until increasing detections were noted north of its geographical barrier, the Darién Gap in Panama, in 2023³. According to the Animal and Plant Health Inspection Service (APHIS), NWS detections in 2023 in Panama, specifically, increased from an average of 25 cases per year to more than 6,500 cases in one year.

Producers in the United States are bringing their attention back to this parasite as it travels farther north. Not only can an infestation cause economic loss, but the effect NWS can impose on livestock and wildlife is severe and often deadly if not discovered and treated.

Signs of NWS

Since NWS feeds on live flesh, it only takes a tiny break in the skin to allow an infestation. Looking for evidence that NWS may be present can be a challenge.

“It could be something very, very small and you wouldn’t even know it was happening, because these larvae underneath the skin are eating away at the muscle and tissue below,” Ramsey adds.

Producers should monitor their animals regularly and be alert for the following signs of NWS:

• irritated behavior;
• head shaking;
• the smell of decay; and
• presence of fly larvae (maggots) in wounds.

Producers who suspect animals to be infected with NWS should contact their veterinarian.

Controlling NWS

The only effective solution found to control and eradicate NWS is through the release of sterile male flies called the sterile insect technique (SIT). Giving further history on the technique, Cammack adds, “Researchers determined this particular species only mates one time throughout their life, and they could sterilize male flies then release them into the environment.”

After the sterile male flies would mate with the wild females, the females would lay unfertilized eggs that would not hatch. Cammack explains that the principle of the program was to overload and outnumber the population of wild male flies, so the sterile male flies could be present in the environment to mate with the females instead. This method, along with implementing a fly control solution, has proven to be successful in mitigating their effect.

For producers, the first line of defense against NWS is making sure animals are protected through a fly control program. Utilizing products that are specifically labeled to treat screwworms can also help control a potential outbreak.

Ramsey gives a tip to apply products labeled to treat screwworms to an open wound or an area where an open wound would be, to help eliminate flies or fly larvae. Something even as small as a scratch from barbed wire or site where a tick had been feeding can become an entry point for these larvae.