REDFIELD, Iowa (DTN) -- New World screwworm (NWS) may be a new issue in the U.S. in 2026, but it really isn't all that new. Currently, there are 29 NWS cases in Texas and New Mexico, mostly in cattle, sheep and goats, but two cases have also been found in dogs.
Although this pest has been absent from our country for several decades, it is important to draw on experience and apply our current knowledge to prepare for the future.
An article from the August 1965 edition of Progressive Farmer celebrated the eradication of screwworm from the U.S. Even back then, the agricultural heads from the U.S. and Mexico were working together to solve the issue through a coordinated effort and the use of the sterile fly release. They wanted to push the screwworm fly completely out of the hemisphere. They were successful -- until it got past the Darian Gap in 2022 and gradually made its way up through Mexico and eventually the U.S.
Livestock producers in the Northern states may never have to deal with NWS; but given how many more livestock and people move around today compared with the 1960s when the pest was last established in the U.S., it's wise to understand the risks and be prepared. While I'm not a veterinarian, I do have an animal science degree and have written many articles about NWS, which is where I've gathered all this information.
Here are 10 things to know about NWS:
1. What NWS does to animals. New World screwworm larvae eat living tissue, not dead tissue. This is different than many of the other fly species. NWS flies lay eggs in wounds on live animals; the larvae hatch and feed on living tissue. Small wounds can become large within days, giving off a putrid odor and causing severe pain.
2. How the life cycle starts. The female NWS fly can enter the body to lay eggs at any opening in the skin. Wounds, navels on newborns, castration sites, dehorning sites, ear tag locations, injection sites, and even something as small as a tick bite. Delaying some of these practices or treating animals to prevent a problem is a good idea.
3. What animals are affected. All warm-blooded animals are susceptible to NWS infestation. It's most commonly been reported in cattle, sheep and goats, but pigs, horses, dogs, cats and wildlife are susceptible too. Humans can be affected, but it's less common.
4. What needs done regularly. Inspecting and observing animals often is important, especially those in areas near NWS infestations. Look at wounds and for discharge and foul smell. These animals may isolate themselves from the herd, be restless, lose their appetite, have head shaking and tail switching.
5. How to prevent the spread. Prevention is the key to NWS spread. In this case, good animal husbandry is needed, especially when it comes to wound care. If you see a wound, treat it immediately. Some say to cover the wound so NWS eggs can't be deposited. The use of approved insecticides and repellents can be done with the help of your veterinarian. Preventing injuries is important as well.
6. What to do if a case is suspected. Report a suspected case immediately. Don't move the animal but instead call your veterinarian and tell him or her what your concerns are. The veterinarian will help determine the next steps after inspecting the animal. He or she will contact state animal health officials and USDA. This is also when it is good to have a premise identification number, which can be used to contact other producers if an animal health emergency is found.
7. What happens to your farm. If NWS is found on a farm, a 12-mile diameter infested zone is drawn. According to Texas Animal Health Commission officials, other animals in that zone are not on a stop-movement restriction but must be inspected, treated and have proper paperwork before moving to another location. The exception to this is animals going to slaughter. They must still be inspected and have the paperwork but wouldn't be treated.
8. How to stop NWS. Besides the use of insecticides to treat affected animals and prevent infection, the primary tool used for eradication of NWS is the sterile insect technique. This was developed in the 1950s and is what has been used to eliminate the NWS fly numerous times. Since the NWS fly only mates once, a wild female fly mates with a sterile male fly, and the population gradually dies out. The number of sterile flies being produced is increasing with the opening of a facility in Mexico and the construction of a facility in South Texas.
9. How to protect your animals. Biosecurity and surveillance are important for all producers to use. The NWS fly can only travel 12 miles per day in flight when looking for a host. The spread is more likely traveling on infested animals. This could be on a domestic or wild animal. Good biosecurity and inspection can help prevent the spread of NWS as well as other diseases.
10. We will survive -- don't panic. While this is a serious situation and everyone who owns animals needs to be looking at their animals daily and preventing a problem from arising, it also is a time to help each other. This is not contagious. It is treatable and the animals can survive. It is also not a food safety issue.
Eat meat and enjoy summer grilling season. We've beat NWS before and we will again.
DTN Headline News
DTN Blogs & Forums
