Editor’s note: This story was updated June 9 to reflect more recent information from the U.S. Department of Agriculture.

As concerns grow about the potential spread of screwworm from Mexico to the United States, where it has been eradicated since 1996, the U.S. Department of Agriculture has closed the country’s southern border to importing horses and other livestock.

“The United States has ordered the suspension of livestock imports through ports of entry along our southern border after the continued spread of the New World screwworm in Mexico. [Mexican Secretary of Agriculture and Rural Development Julio Berdegué Sacristán] and I have worked closely on the NWS response; however, it is my duty to take all steps within my control to protect the livestock industry in the United States from this devastating pest,” U.S. Secretary of Agriculture Brooke L. Rollins said in a May 11 press release announcing the decision. “The protection of our animals and safety of our nation’s food supply is a national security issue of the utmost importance. Once we see increased surveillance and eradication efforts, and the positive results of those actions, we remain committed to opening the border for livestock trade. This is not about politics or punishment of Mexico, rather it is about food and animal safety.”

The border closure—which USDA representatives have not put a timeline on but expect will be lifted before the end of the year—marks an escalation from measures put in place by USDA late last year, which required the livestock crossing the border be pre-treated with ivermectin and remain in a quarantine facility for seven days before being released. USDA officials have said the closure will be re-evaluated every 30 days.

This week, the U.S. Equestrian Federation issued a press release to update members on the spread of screwworm and how it could affect horse sports in the U.S.

“If screwworm were to spread into the United States, it would result in significant economic losses and threats to animal health and welfare,” USEF wrote in its press release. “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, U.S. 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.”

The USDA announced May 27 that it is investing $21 million to help renovate an existing sterile fruit fly production facility in Mexico.

“When operational, this facility will produce 60-100 million additional sterile NWS flies weekly to push the population further south in Mexico,” the department announced in a press release. “Given the geographic spread of NWS, this additional production capacity will be critical to our response.”

While state and federal officials are still developing response protocols, USEF said that if screwworm is detected in the U.S., horse owners should expect quarantines and movement restrictions to be put in place in areas where it is present.  

“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,” the federation noted.

“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,” USEF stated. “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 about screwworms and how to protect your horses, click here.