Have you heard about the potential threat of screwworm entering the United States?   

In December 2024, Mexico confirmed its first case of screwworm. Unfortunately, screwworm is spreading northward in Mexico and is now less than 700 miles away which is not far when it comes to fly movement. On May 13, 2025, the US Department of Agriculture (USDA) closed all land border crossing stations along the Mexico/US border, stopping all movement of animals from Mexico into the United States. Recently, Dr. Rosemary Sifford the Chief Veterinary Officer for USDA, is quoted saying “It's hard to say exactly when, but (imports will resume) for sure before the end of the year, unless something really dramatically changes.”   

What is the risk of screwworm to horses? 

Screwworm myiasis (infestation) is caused by fly larvae (maggots) that feed on living tissue of any warm-blooded animal. Humans can be infected with screwworm larvae. The larvae are deposited on wounds or superficial body cavities such as the nose and sheath of horses where they feed and grow. Untreated wounds often develop multiple infestations which are more likely to kill the animal.  

Has screwworm ever entered the United States? 

Screwworm was declared eradicated in 1966 from the U.S. using the sterile insect technique, which involves releasing sterile male flies to disrupt reproduction, gradually reducing the pest population. There was an outbreak of screwworm in 2016 in Florida, which was controlled by sterile flies and was successfully limited to the Florida Keys. This 2016 screwworm eradication effort in Florida was estimated to cost over $10 million for implementing control measures including the release of 154 million sterile flies and other control measures, such as animal checkpoints. 

Why is the US concerned about a screwworm incursion? 

If screwworm were to spread into the United States, it would result in significant economic losses and threats to animal health and welfare. The critical control measure is the release of sterile flies. Currently, the production of sterile flies is limited to one facility in Panama which is working at maximum production. Depending on the prevalence of screwworm in Central America and Mexico, the facility may be unable to produce enough sterile flies for expanded distribution to the United States. Thus, US control efforts would need to focus on surveillance for early detection and treatment of infected domestic and wild animals as well as movement restrictions to ensure only healthy treated animals are moved.  

How could a screwworm detection in the United States impact Equestrian Sports? 

State and federal animal health officials would be responsible for implementing control and eradication efforts, namely quarantines and movement restrictions. Horses infected with screwworm or horses in a geographic area with screwworm present could be placed under quarantine and movement restrictions, at the discretion of state animal health officials. States may restrict equine movement and activities such as equestrian events in a geographic area. As the USDA has not provided specific quarantine or movement restriction parameters, states are developing their own plans and protocols for responding to screwworm incursion.  

What is the Federation doing to prepare for a potential screwworm incursion?  

The Federation veterinarians are working collaboratively with the states to develop a response plan which would allow healthy safe competition to continue in the event of a screwworm detection in the United States. Any protocol for events during a screwworm incursion would be at the discretion of the state veterinarian in the venue state. For this type of disease situation, entry requirements to the event could include a requirement for veterinary examination for issuance of a certificate of veterinary inspection which includes an antiparasitic treatment (ivermectin/moxidectin) statement and examination on arrival. The federation will continue to work with states on a screwworm response plan for equestrian events. 

For more information on screwworm visit our initial USEF Alert here.

Or the disease fact sheet at the Equine Disease Communication Center here.