On Nov. 22, 2024, the Chief Veterinary Officer of Mexico notified the United States Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) of a positive detection of New World screwworm in Mexico.
What Is New World Screwworm?
New World screwworm (NWS, Cochliomyia hominivorax) is a devastating pest of livestock and other warmblooded animals, including humans, that is not currently present in the United States. They most often enter an animal through an open wound or mucous membranes and feed on the animal’s living flesh. NWS larvae do not feed on dead tissue or carrion. Untreated screwworm infestations can be fatal.
Where Is NWS Endemic?
NWS is endemic in Cuba, Haiti, the Dominican Republic, and countries in South America. Since 2006, the United States and Panama have maintained a barrier zone in eastern Panama intended to prevent NWS from moving north from South America to screwworm-free areas in Central and North America. However, since 2023, cases have been increasing in number and spreading north from Panama to Costa Rica, Nicaragua, Honduras, Guatemala, and now Mexico.
New Livestock Import Restrictions for Mexico
Given the severity of the threat from NWS, APHIS is currently restricting the import of livestock, including equines, into the U.S. from Mexico, pending further information from Mexican veterinary authorities on the size and scope of the infestation.
Once established, import requirements for horses originating from or that have resided in Mexico within the 60 days immediately preceding export to the U.S. will reflect the current established import screwworm protocol for horses outlined in eCFR :: 9 CFR 93.308 — Quarantine requirements and VSG 13424.1 Procedures for the Import of Equines Into the United States and Approved Quarantine Facilities. This includes pre-export treatment and examination, as well as treatment, examinations, and a seven-day quarantine in a permanent import quarantine facility upon arrival. However, APHIS is aware that horses are sometimes illegally imported from Mexico, and stray equines cross the border.
Clinical Signs and Precautions
An infested equine could introduce NWS into the U.S. Therefore, anyone handling equines in the U.S. should take the following precautions:
- Check your pets and livestock for draining or enlarging wounds and signs of discomfort such as irritated behavior and headshaking.
- Check your pets and livestock for screwworm larvae (maggots) or eggs. Screwworm eggs are creamy and white and deposited on or near the edges of superficial wounds.
This pest can infest a wide variety of wounds, from tick bites to cuts and dehorning or branding wounds. Infestations are very common in the navels of newborn animals, nasal or eye openings, and genitalia.
If you suspect an animal is infested with screwworm, immediately contact your local accredited veterinarian, state animal health official, or APHIS veterinarian.
