Wildlife Wednesday Free Event: State Enlists Hunters to Watch for Screwworm

By Ben Neary

NMWF Conservation Director 

New World Screwworm is on the move and New Mexico officials are asking hunters to be on the lookout for any sign it has infested the state’s wildlife populations.

Dr. Sean McCartney, a veterinarian with the New Mexico Livestock Board, will give a free presentation on the screwworm, its lifecycle and the threat it poses to wildlife on Nov. 12 in Albuquerque. His talk is the latest presentation under the New Mexico Wildlife Federation’s “Wildlife Wednesday” series, held the second Wednesday of each month.

“We think that wildlife will be one of the earliest signs of infestation in an area,” McCartney said. “And wildlife can be a great way for it to propagate in an area and remain in an area for a long time. And so, we rely on hunters and the messaging to hunters to be on the lookout for things like that because often that’s the best way to at least have surveillance in an area to get an idea of what the wildlife look like.”

The screwworm is the larva of a fly that’s native to the American Southwest and to Mexico and other countries to the south. Unlike most fly larvae that eat the flesh of dead animals, the screwworm larva eats its way into living tissue anytime the fly can find a cut or other break in the skin where it can lay its eggs. The larvae can infest livestock, pets, wildlife including birds, and in rare cases, people.

The U.S. Department of Agriculture has stated that animals suffering from screwworm may exhibit the following signs: irritated behavior; head shaking; the smell of decay and the presence of fly larvae (maggots) in wounds.

The New Mexico Department of Game and Fish has issued instructions for hunters who believe they’ve encountered a suspected case of screwworm in a game animal:

1. Do not eat, handle or transport the animal.

2. Take a GPS location.

3. Take pictures of the wound, larvae and/or affected animal if possible.

4. If you can, collect a sample safely. Using gloves or tweezers, place several larvae into a sealed container (small jar or plastic vial) with 70% alcohol.

5. Label with date, GPS location, species and your contact info.

6. Report and submit immediately to the New Mexico Department of Game and Fish, the New Mexico Livestock Board or the New Mexico Department of Agriculture.

Screwworm was eradicated in the United States in the mid-1960, with the exception of a few small outbreaks since then. Until recent years, officials have kept it from spreading from Central America into Mexico. The United States has worked closely with Panama to control its spread by releasing millions of sterile male flies to keep the females from breeding successfully.

In recent years, however, screwworm has been moving north. Federal officials this summer blocked the importation of cattle from Mexico. State officials say that a case was confirmed in September in Sabinas Hidalgo, in the state of Nuevo León, less than 70 miles from the U.S.-Mexico border.

Mexico has typically exported hundreds of thousands of head of cattle into the United States through ports of entry in New Mexico and other states every year. The current importation ban is having significant economic effects on both sides of the border, costing jobs at feed lots and trucking operations and also prompting increases in the cost of beef in the United States. 

While cattle importations are banned, state and federal officials in New Mexico and other border states are setting up thousands of traps to monitor for fly populations and working to breed sterile flies to block any fly migration into the country. 

“It’s a native fly to the area so it’s able to disperse naturally without human intervention, especially in wildlife but also in any animal that’s feral or cattle out on the range for that matter – they’re quite vulnerable to this fly,” McCartney said.. “So they can perpetuate on their own.”

Fly populations tend to move farther north in the summer and then die back in the winter due to the cold, McCartney said. “So it won’t survive in the adult form outside for very long.”

In Mexico, the fly currently is moving north undetected in movements of cattle that may be legal or illegal, McCartney said. He said it also may be moving north in humans.

“There’s a lot of people moving into Mexico in recent history,” McCartney said. “And that’s kind of when it was starting. Humans aren’t really the preferred host because they tend to notice it quickly and it’s not terribly easy to perpetuate an infestation on a human. It’s not impossible either.

“One of the most vulnerable things is wildlife, and it’s hard to tell because Mexico isn’t reporting what kind of wildlife surveillance is occurring,” McCartney said. “That’s one of the biggest ways to naturally propagate this fly outside of your control.”

McCartney’s free presentation starts at 5:30 p.m., Wed., Nov. 12 at the Marble Brewery NE Heights Taproom, 9904 Montgomery Blvd., NE, Albuquerque.