University of Kentucky Martin-Gatton College of Agriculture, Food and Environment specialists and state partners are asking livestock owners and veterinarians to stay alert as New World screwworm (NWS) activity continues in parts of Central America and Mexico. No animal cases have been confirmed in the United States during the current outbreak, and Kentucky herds remain unaffected. However, experts warn that cattle producers should still remain vigilant. 

The NWS fly is an obligate parasite whose larvae invade living tissue, “screwing” into wounds with mouth hooks that tear through healthy flesh. Untreated animals can suffer severe lesions, secondary infections and death.  

Cattle draw most of the attention; however, sheep, goats, horses, companion animals, wildlife and humans can also be affected. Even small wounds, such as tick or insect bites, ear tags, castration or dehorning sites, can attract egg-laying females. Livestock owners also need to be aware that natural openings such as the nose, eyes, ears, anus and genitalia can also be a draw. As larvae feed, wounds enlarge quickly and often carry a strong odor of decaying tissue. 

NWS sits at the intersection of animal health, human health and the environment. One Health is the framework that links those pieces. In practice, this means veterinarians, physicians, epidemiologists, wildlife biologists, producers, laboratories and university researchers share information, coordinate guidance and act quickly when something suspicious, such as NWS infections, appears. 

“Cattle are large animals that may go some time between health checks,” said Hannah Tiffin, assistant professor in the UK Department of Entomology. “Flies have ample time to lay eggs and allow the maggots to cause gaping wounds in several animals before the infestation is noticed. This is why knowing what to look for is so important, so that infestations can be caught and treated quickly before more animals are affected.” 

Why One Health is front and center 

In Kentucky, that work is already in motion. The Kentucky Department for Public Health (KDPH), Kentucky Department of Agriculture (KDA), Kentucky Department of Fish and Wildlife Resources, Kentucky veterinarians from the Centers for Disease Control and Prevention and USDA Animal and Plant Health Inspection Service meet weekly with UK faculty to review new information on animal-to-human threats. These standing meetings now support screwworm monitoring, clinician alerts, laboratory routing and rapid reporting. The same playbook helps with ticks, mosquitoes and other vector-borne issues that affect people, livestock, pets and wildlife. 

“Although the current risk to the general public is very low, there is the concern that people who travel to countries where this fly is either endemic or becoming more common are at increased risk,” said Kathleen Winter, associate professor in the UK College of Public Health’s Department of Epidemiology and Environmental Health and current state epidemiologist and director for the Division of Epidemiology and Health Planning with the KDPH.  

The UK Department of Entomology has an updated fact sheet to help producers and animal owners recognize NWS and distinguish it from other wound-associated flies. It can be found at https://entomology.o2.boa.ca.uky.edu/ef519.  

“It’s important to note that not all fly larva (maggots) are screwworms,” said Jessie Lay, extension animal health veterinarian in agriculture and natural resources. “While common blow fly larvae feed on dead tissue near the skin’s surface, screwworm larvae tunnel into healthy tissue, which is why they’re such a concern. We have not seen an animal case in the United States yet during the current outbreak, but are closely monitoring the situation at the southern border. For consumers, it is also important to note that beef is safe to eat. Larvae do not persist on carcasses and processing facilities inspect animals at harvest.”  

Where NWS stands now 

The United States has considered NWS eradicated since 1966 after sterile-insect releases, coined Sterile Insect Technique (SIT), collapsed wild populations. A permanent barrier zone in Panama, established in 2006, helps keep the pest to the south. When detections move north of that line, regional partners release additional sterile male flies to suppress local populations and re-establish control. 

During the past year, NWS has been confirmed north of the barrier in Central America and into southern Mexico. That movement raises the chance of re-introduction via animal movement through legal and illegal trade, stray or feral animals crossing borders, wildlife dispersal and travel.  

“NWS is an old foe,” Tiffin said. “It used to affect animals, especially cattle in Texas and throughout the South but thanks to the advent of SIT, it was mostly eradicated from the US in the 1960s.” 

However, despite the human infection reported in Maryland, Tiffin says that human infections are nothing to keep one up at night. 

“Human infestation is rare,” Tiffin said. “You would typically know if you had maggots in a wound, in your nose, or other body part, usually prompting immediate medical attention in people who are mentally and physically capable of seeking help.” 

Market context 

The stakes are not purely clinical. A 1976 Texas outbreak infested nearly 1.5 million cattle and more than 300,000 sheep and goats before it was contained, causing large producer and statewide costs. Now recently, live-animal import policies were adjusted as conditions changed in Mexico — first paused in late November 2024, restarted at a reduced pace this February and halted again on May 11 — tightening feeder supplies. 

“Kentucky producers are watching feeder supplies and prices closely,” said Kenny Burdine, extension professor in the UK Department of Agricultural Economics. “When news of a travel-associated human case in Maryland hit, futures dipped for a brief time the day of the announcement, but the response was limited because no animal cases have been confirmed in the United States. There really wasn’t a supply or demand impact on the cattle market. Fundamentals still drive the market, though placements have been harder to predict since Mexican imports were first paused.” 

What producers, veterinarians and clinicians can do 

Producers should check animals often for deep, rapidly enlarging wounds with maggots, foul odor, persistent irritation or head-shaking. Pay close attention to small wounds, the navel of newborn calves and natural openings of the head and perineum. Call a veterinarian right away if something looks unusual; early detection supports animal welfare, protects neighboring herds and wildlife, and speeds any needed response. 

When transporting livestock, follow USDA requirements for livestock, equids and pets and watch for agency updates while outbreaks persist in Mexico. 

If one sees a suspected animal case in Kentucky, they should contact the office of the state veterinarian at (502) 573-0282 or USDA-APHIS Veterinary Services (Kentucky) at (502) 848-2040. 

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Writers: Jordan Strickler, jstrickler@uky.edu; Jennifer Elwell, Jennifer.Elwell@uky.edu        

University of Kentucky alum and former trustee Carol Martin “Bill” Gatton bestowed a transformational $100 million gift to the college through The Bill Gatton Foundation. It is the largest gift to the university in its history.   

Four Pillars of The Bill Gatton Foundation’s gift are (1) Scholarships and other initiatives for Student Success, (2) Companion Animal Program, (3) 21st Century Capital Projects and New Initiatives Fund and (4) Faculty Research and Innovation/Research Challenge Trust Fund Program.  

The Martin-Gatton College of Agriculture, Food and Environment is an Equal Opportunity Organization with respect to education and employment and authorization to provide research, education information and other services to individuals and institutions that provide equal opportunities for qualified persons in all aspects of institutional operations and do not discriminate on the basis of race, color, national origin, ethnic origin, religion, creed, age, physical or mental disability, veteran status, uniformed service, political belief, sex, sexual orientation, gender identity, gender expression, pregnancy, marital status, genetic information or social or economic status.