This School of Veterinary Medicine professor answers important questions related to the New World Screwworm to help prepare livestock owners.
The re-emergence of New World Screwworm is raising fresh concerns for livestock producers and animal health officials across the region. Known for infesting open wounds and feeding on live tissue, this parasite can cause serious harm to animals and create major challenges for the people who care for them.
Although the screwworm was wiped out from the U.S. in the 1960s, recent cases just south of the border have animal health authorities paying close attention. For cattle producers, that means staying alert. Screwworm infestations can spread quickly, lead to significant losses and require serious effort to manage.
Dr. Jennifer Koziol, associate professor of food animal medicine and surgery at Texas Tech University’s School of Veterinary Medicine, answers some common questions to help ranchers be on the lookout for screwworm flies and larvae, and help them protect their livestock.
Where did it come from?
The New World Screwworm (Cochliomyia hominivorax) is native to tropical and subtropical regions of the Americas. It was historically found throughout South and Central America, and parts of the southern U.S. Thanks to eradication efforts, it was eliminated from the U.S. in 1966, but it remains endemic in places like Cuba, Haiti, the Dominican Republic and parts of South America. The screwworm was eliminated as far south as Panama by 2000.
The New World Screwworm began re-emerging above the biological barrier in Panama in 2022 and has continued moving north through Central America and Mexico. In 2024 a case was reported in Mexico and since then multiple cases have been diagnosed in Mexico prompting the increased awareness and efforts of U.S. Department of Agriculture (USDA).
What does it look like?
Adult screwworm flies resemble common houseflies but are slightly larger. They have metallic blue or green bodies, orange eyes and three dark stripes on their backs. The larvae (maggots) are what cause the damage. They burrow into wounds and feed on living tissue, which gives them the name "screwworm" due to their screw-like burrowing behavior.
How does it affect livestock?
Screwworm larvae infest on open wounds in livestock, feeding on live tissue. This leads to pain and distress, severe tissue damage, secondary infections and death if untreated. Infestations can spread quickly and affect multiple animals, especially in warm climates where flies are more active.
What are early signs?
Early signs of screwworm infestation include unusual irritation or behavior (e.g., head shaking), foul odor from wounds due to tissue decay, visible maggots in wounds and wounds that worsen over time, even if initially minor. If a livestock owner has concerns, they should contact their local veterinarian.
Is there anything we can do to prevent it?
Yes, there are prevention strategies. The first being the sterile insect technique (SIT). This technique requires the release of sterilized male flies to reduce breeding. This method has been highly effective in eradication efforts.
It is also encouraged for producers to inspect their animals regularly for wounds and signs of infestation. When wounds are found, be sure to treat them promptly and use insect repellents or wound dressings. It is important to be mindful of routine animal husbandry techniques that create wounds such as ear-tagging, branding, castrating, dehorning and tail docking. If there are suspected cases, be sure to report them to animal health authorities.
What does it mean for the cattle industry?
The impact on the cattle industry can be severe. It can cause economic losses due to treatment costs, animal deaths and reduced productivity. It could also increase the demand for labor and care, cause trade restrictions if infestations occur and cause beef prices to increase due to reduced supply.
During past outbreaks, damages exceeded $132 million in a single year, affecting over a million animals. The USDA Economic Research Services estimated that the current cost of an outbreak would be $1.9 billion annually for Texas alone.
Understanding what the screwworm is, how it spreads and what signs to watch for is key for protecting individual animals and for preventing broader outbreaks that could impact the entire industry.