By Lucy Ray, County Extension Coordinator and ANR Agent, Morgan County
By now, you’ve probably heard of the New World Screwworm or seen the headlines on various mainstream media outlets proclaiming that a “Flesh Eating Parasite Invades US!!”. Once you weed through the headlines, what does the New World Screwworm mean for those of us in livestock and specifically horses? At the time of this article, there are 25 confirmed cases in the United States in Texas and New Mexico. The majority of those cases are in cattle, although sheep, goats, and dogs have also been affected.
New World Screwworm (NWS) has been present in the southwestern part of the US, since 1842, and became a significant problem in the Southeastern states in 1933. Throughout the 1930s and 40s, this pest continued to cause substantial damage to the cattle and sheep industries. The female of this fly species mates once and then lays eggs in any wound in a mammal. This differs from other blowfly species which lay their eggs in carrion. Twelve to 24 hours after laying the eggs, the larvae emerge and feed on living tissue of the host. As they feed, they tear the tissue and enlarge the wound. This destruction of tissue emits a distinctive odor, which then attracts more females to lay eggs and exacerbates the problem. Severe infestations of screwworm can result in the death of the host. NWS was declared eradicated from the United States in 1966. As of June 2026, NWS is again a problem in the United States. The United States Department of Agriculture has a plan in place for the most recent infestation of NWS. Although there is no vaccine against screwworm, several products formulated to treat larvae have received approval. If your animal has an open wound, contact your veterinarian for up-to-date recommendations on the best products to use in your situation. Since screwworm can be a problem in any mammal, infestations can spread between animal species such as pets, livestock, and wildlife.
In attempt to curb the spread of NWS, the USDA halted all imports of cattle from Mexico in June 2025. Although the border was reopened for a short time later in the year, it was closed again as screwworm cases continued to be reported farther north in Mexico. The US border remains closed to Mexican cattle and livestock today. On June 4, 2026, the first case of NWS in domestic animals was reported in the United States in 60 years. This case was in south Texas in a young calf. In response, on June 10th, 2026, Mexico closed the border to all United States horses, cattle, sheep, pigs, goats, and all other forms of livestock. This appears to be a political move, as Mexico has nearly 28,000 cases of NWS.
Currently, there are no reported cases of screwworm in horses in the United States. So what does screwworm mean for the horse industry? At this point, we are still waiting to find that out. One concern is horse slaughter. Currently there are no federally inspected horse slaughter facilities in the United States, and have not been since 2007. For many years, horses destined for the slaughter pen have been shipped to either Mexico or Canada. Currently, Canada has temporarily closed its borders to livestock (including horses) from the United States as well. Although proponents of banning horse slaughter celebrate the closed borders, slaughter remains a significant economic force in our industry by providing a price floor for equines. Without an outlet for these animals, the supply in this country will increase. It remains to be seen if an increased supply of horses will result in a decrease in the value of performance and pleasure horses or an increase in the number of neglected or abandoned animals. Another concern is the effect that the movement restriction of horses will have on shows and competitions. Last year’s outbreak of Equine Herpes Myeloencephalopathy is a key example of economic impacts that medical issues in horses can have on the competition world. Currently there are protocols for movement of any livestock coming from inside a NWS Infested Zone. The Georgia Department of Agriculture requires any animal from inside an infested zone to obtain a Georgia entry permit and follow the NWS Movement Guidance which is found here: New World Screwworm National Continuity of Business. Animals coming into Georgia from outside an infested zone but in a listed state or county (includes all of Texas), must have a Certificate of Veterinary Inspection within 7 days of travel. All animals must be inspected and found free of NWS. If attendance at horse shows and other competitions is affected by reduced movement of animals, this could pose another economic impact to the horse industry.
What can you do as horse owner? The old saying that the “best thing you can put on your animals is your eyes”, certainly holds true here. Keep an eye on your horses—particularly during the hot summer months when flies are more active. Practice an integrated approach to fly control. Consult last year’s newsletter article on fly control for further reading: https://equine.caes.uga.edu/2025/08/buzz-off-smart-strategies-for-summer-insect-control-in-horses/.
However, remember that conventional flies lay eggs in manure while the screwworm fly is not affected by manure management since it uses a living animal as its host. If you are in the breeding business, realize that one of the most ideal spots for the female screwworm fly to lay eggs is in the umbilical site of a newborn foal. Keep in mind that any wound, even a tick bite, can act as a host site for the female screwworm and manage your horses accordingly. The label of any product is the law and there are very few products labeled for controlling screwworm infestations. Any suspected screwworm infestations should immediately be reported to the Georgia Department of Agriculture.
USDA is keeping an up-to-date website for all things screwworm here at screwworm.gov, including currently labelled insecticides They have an excellent fact sheet on screwworm The Facts About New World Screwworm. This also includes a map with locations of current and past cases as well as the quarantine zones. The Georgia Department of Agriculture also has a lot of good and current information located here https://agr.georgia.gov/new-world-screwworm. If you see an animal that you suspect has New World Screwworm contact the office of the Georgia State Veterinarian at 404-656-3667 or email animalhealth.agr.georgia.gov. And as always, your local county Extension Agent is always happy to help you and can be contacted at 1-800-ASK-UGA1. Together we can mitigate the spread of NWS and once again eradicate it from the United States.
Sources:
History of screwworms in the U.S. | Veterinary Medicine Extension | Washington State University