New World Screwworms

In May, the U.S. Department of Agriculture suspended livestock (cattle, horses, and bison) imports from Mexico following detection of the New World screwworm (NWS) in Veracruz. While the detection was roughly 600 miles from the southern tip of Texas, many producers are wondering if they need to be concerned about the health of their livestock. 

The New World screwworm was eradicated in the U.S. in the 1960s. However, it has occasionally reemerged in Central America and Mexico. The NWS is controlled only through the sterile insect technique (SIT), a pest control method involving the release of sterilized male insects into the wild to mate with wild females, preventing reproduction and thus reducing the pest population. This approach, along with regular active surveillance and livestock inspections, has proven highly successful.

The New World screwworm (Cochliomyia hominivorax) is a parasitic fly found in the Western  Hemisphere. It deposits its eggs in the living tissue of fresh wounds on warm-blooded animals. The resulting larvae (maggots) consume the host’s flesh, creating serious wounds. The name screwworm comes from the feeding behavior of the maggots, which burrow (or screw) into wounds. Using sharp mouth hooks, they tear into the host’s tissue, causing extensive damage. As additional eggs hatch and more larvae feed on the living flesh, the wound grows larger and deeper. If left undetected and untreated, this can result in severe or often fatal harm to the animal.

Adult New World Screwworm fly

Adult New World screwworms are metallic blue blow flies with large orange eyes and three distinct stripes running along the top (thorax) just behind the head. They closely resemble the related secondary screwworm (Cochliomyia macellaria), which is also a metallic blue blow fly with three stripes; however, in the secondary screwworm, all three stripes start at the same point behind the head. Unlike the New World screwworm, adult secondary screwworms do not lay eggs on living animals, and their larvae do not infest them. Adult secondary screwworms do not pose a threat to animal health.

Adult flies of interest can be photographed and emailed to the Texas A&M AgriLife Extension Service at screwworm@ag.tamu.edu. 

Routine monitoring and evaluation of all livestock is essential for maintaining herd and flock health. Livestock producers should stay alert for signs of possible infestation, such as:

• Foul-smelling wounds with visible maggots
• Animals biting or licking at wounds
• Lesions in navels, ears, dehorning or branding sites
• Unusual restlessness or lethargy

Producers should be proactive and take preventive measures by treating wounds promptly and maintaining sanitation. 

Any mammals or birds (wild or domesticated) with signs of irritated behavior or head shaking, those that express a smell of decay but are alive, or those that show evidence of fly strike and/or the presence of fly larvae (maggots) in wounds should be isolated and reported immediately to your local veterinarian or the Texas Animal Health Commission, (800) 550-8242.

For additional information, please visit https://agrilifeextension.tamu.edu/new-world-screwworm/.  

If you have questions or concerns, please contact me, (903) 473-4580 or email Sarah.Latham@ag.tamu.edu. To view upcoming events or additional information please visit https://rains.agrilife.org/ or follow Rains County AgriLife on Facebook.