by Neogen

A flesh-eating pest infecting 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 over the next 16 years. Since 2023, this pest has been making its way back north and is now present in central Mexico.

What is NWS?

“The screwworm isn’t a worm at all,” says 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 they tear into living tissue with sharp mouth hooks and dig deeper into the wound,² which separates them from other insects.

Old vs. new

Although there is an Old World screwworm, the two are not the same, Cammack says. “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.”

The two species are very distantly related, and both have evolved this ability, Cammack adds. “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 more 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 infection cause economic loss, but the effect NWS can also have on livestock and wildlife is severe and often deadly if not discovered and treated.

Signs of NWS

It takes only a tiny break in the skin to allow an infestation. Looking for evidence NWS may be present can be a challenge, Ramsey explains. “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.”