Mexico screwworm case triggers US emergency response

The recent discovery of a New World screwworm (NWS) infection in a cow in southern Mexico has prompted action by the federal government to prevent the deadly parasitic pest from entering the country.

The U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) announced December 13 that it will release $165 million in emergency funding to protect U.S. livestock, pets, and wildlife.

Meanwhile, animal health officials are calling on veterinary clinicians to be vigilant for signs of NWS infection, particularly those along the U.S.-Mexico border.

Eradication efforts

The New World screwworm can infect any living mammal, including humans, and, in rare cases, birds. Screwworm infestations occur when the female fly lays eggs on a host’s open wounds and other vulnerable areas. The larvae burrow into the flesh, resulting in severe damage, infection, and often death.

NWS 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 the Florida Keys, but it was limited to the Key islands.

Screwworm is endemic in Cuba, the Dominican Republic, Haiti, and in other Central and South American countries, posing a continual risk of reintroduction to the U.S. Over the past two years, NWS has spread rapidly through Panama and into Costa Rica, Nicaragua, Honduras, and Guatemala.

On November 22, Mexico’s chief veterinary officer alerted APHIS about a confirmed NWS case in a cow near Mexico’s border with Guatemala.

APHIS has worked since 2006 with Panama to maintain a biological barrier in the Darien Province through the Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG). However, increasing cattle movements and agricultural expansion have allowed NWS to breach the barrier in recent years. Sterile flies are now being released via aerial and ground operations at key locations in Central America.

“The current outbreaks in Central America demonstrate the need for USDA to increase its investment in NWS eradication and prevention,” said Jenny Lester Moffitt, USDA undersecretary for marketing and regulatory programs, in the announcement. “If NWS were to spread to the United States, it would result in significant economic losses and threats to animal health and welfare. This funding will allow for a coordinated emergency response to control the outbreak and prevent NWS from spreading to the United States.”

The emergency funds will support intensified surveillance, animal health checkpoints, and collaboration with Mexico and Central American countries to establish a new barrier on the Isthmus of Tehuantepec and reestablish the Panama barrier. These measures involve eradicating NWS in affected areas through the sterile insect technique.

Surveillance measures

APHIS is also tightening restrictions on imports of animal products originating from or transiting through Mexico. The agency is urging livestock producers along the U.S. southern border to monitor their animals for signs of screwworm. Early signs include enlarging or draining wounds, discomfort, and the presence of larvae or eggs near superficial injuries, according to APHIS.

“The detection of New World screwworm in southern Mexico highlights the importance of collaboration between countries to keep this parasite from entering the United States,” Dr. Fred Gingrich, executive director of the American Association of Bovine Practitioners. “This disease can be devastating to cattle, as well as other species, and continued monitoring, treatment and prevention is important.

“AABP is supportive of efforts by USDA to temporarily halt the live importation of cattle and bison from Mexico and efforts to increase funding for sterile male flies and treatment programs to protect our cattle herd.”

Dr. Kathy Simmons, chief veterinarian for the National Cattlemen’s Beef Association, says that while screwworm is often thought to be predominantly a problem for livestock and wildlife, the larva of this harmful pest can also infest pets and people.

“The veterinarian serves as the first line of defense against New World screwworm, a reportable foreign animal disease,” she said.

Veterinary vigilance

The USDA is encouraging all producers and pet owners to report potential screwworm cases immediately to veterinarians or local animal health authorities. Additionally, individuals who suspect a screwworm infestation in themselves should seek medical attention without delay.

Screwworm maggots inflict significant damage by tearing into the host’s tissue with their sharp mouth hooks. As compared to normal maggots that only feed on dead and dying tissue, screwworms feed on healthy living tissue. As more larvae hatch and continue to feed, the wounds can expand and deepen, leading to severe, often fatal harm, to the affected animal. Left untreated, myiasis caused by NWS can be fatal within seven to 10 days.

Screwworm infestation can also occur on mucous membranes, such as nostrils, eye orbits, ears, mouth, and genitalia, according to the USDA. Nearly any wound is susceptible to screwworm infestation, including wounds resulting from management procedures such as barbed wire fencing, dehorning, castration, and ear tagging. Navels of newborn mammals are also common sites for screwworm infestation

Characteristics of NWS-infested wounds include drainage, suppuration, discharge of blood and serum, and a distinctive odor. Secondary bacterial infections are also common. Close examination may show shingle-like deposition of fly eggs in marginal or peripheral masses.

Larvae are visible by the third day of infestation and are positioned head-down with their posterior ends at the surface of the wound. In cases where the wound is deep, pocket-like, and the opening small, minor movement within the wound may be the only indicator of infestation.

A version of this story appears in the February 2025 print issue of JAVMA