Screwworm case in Mexico prompts emergency response in the United States

(Image courtesy of The United States Department of Agriculture’s Animal and Plant Health Inspection Service)

After the recent discovery of New World screwworm (NWS) infection in a cow located in Southern Mexico, the United States federal government has begun to take action to prevent NWS from entering the country. The US Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) released $165 million in emergency funding to protect pets, livestock, and wildlife in the US.²

NWS can infect any living mammal, including humans, and birds in rare cases. The infestation occurs when a female fly lays eggs on a host’s open wounds and other vulnerable areas. The larvae then burrow themselves into the flesh, causing severe damage, infection, and in many cases, death.

NWS eradication efforts

In 1966, the United States declared that NWS had been eradicated through sterile insect techniques, which involves the release of sterile male flies to disrupt reproduction and gradually reduce the pest population.¹ There was one outbreak since this, which took place in the Florida Keys in 2016 but was limited to only the Key islands

Although eradicated in the US, screwworm is endemic in Central and South American countries, including Cuba, the Dominican Republic, Haiti, and has recently spread rapidly throughout Panama and into Costa Rica, Honduras, Guatemala, and Nicaragua.

Then, in November 22, 2024, The chief veterinary officer of Mexico notified APHIS that the country had a positive detection of NSW in southern Mexico, near the Guatemalan border.²

“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 under secretary for Marketing and Regulatory Programs, in an APHIS release.² “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.”

Current surveillance and vigilance

The funding from APHIS will help support intensified surveillance, animal health checkpoints, and working with Mexico and Central American countries to help establish a new barrier on the Isthmus of Tehuantepec as well as reestablish the Panama Barrier. The measures will also involve eradicating areas affected by NWS through the sterile insect technique.

Along with funding, APHIS will tighten restrictions on imports of animal products originating from, or transitioning through, Mexico. APHIS is also urging livestock producers located on the US southern border to monitor their animals for signs of NWS, which include enlarging or draining wounds, discomfort, and the presence of larvae or eggs near superficial injuries.¹

The USDA is encouraging pet owners and producers to report any potential cases to their local animal health authorities immediately and any person who suspects they have a screwworm infection should seek immediate medical attention.

According to the American Veterinary Medical Association, Screwworm maggots inflict significant damage to its host by tearing its tissue with the maggots sharp mouth hooks.¹ When compared to normal maggots that only feed on tissue that is either dead or dying, the screwworm feeds on healthy living tissues. If infected, more larvae hatch and will continue to feed, potentially expanding and deepening the wounds that can cause severe, and sometimes fatal, harm. If these infections are left untreated, NWS-caused myiasis can be fatal within 7-10 days.¹

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

The third day of infection is when the larvae are visible and are positioned head-down with their posterior ends at the surface of the wound.¹ If the wound is deep, pocket-like, and the opening small, minor movement within the wound could be the only indication of infestation.

Anyone interested in learning more about NWS should visit the APHIS website.

References

1. USDA Approves Emergency Funding to Protect U.S. Livestock and Animals from New World Screwworm. News release. The United States Department of Agriculture’s Animal and Plant Health Inspection Service. December 13, 2024. Accessed January 16, 2025. https://www.aphis.usda.gov/news/agency-announcements/usda-approves-emergency-funding-protect-us-livestock-animals-new-world#:~:text=WASHINGTON%2C%20December%2013%2C%202024%20%E2%80%93,and%20to%20increase%20USDA's%20ongoing
2. Nolen RS. Mexico screwworm case triggers US emergency response. American Veterinary Medical Association. Published December 20, 2024. Accessed January 16, 2025. https://www.avma.org/news/mexico-screwworm-case-triggers-us-emergency-response