New World screwworm (NWS) infestations occur when the female fly lays eggs on an animal’s open wounds and other vulnerable areas. The larvae hatch and invade the flesh of living mammals, and occasionally birds. The U.S. Department of Agriculture (USDA) successfully eradicated NWS from the United States in 1966 through the use of the sterile insect technique, which involves releasing sterile flies into the environment to gradually reduce the population. Currently, NWS remains endemic in Cuba, Haiti, the Dominican Republic, and various South American countries, posing a continual risk of reintroduction to the United States. Over the summer 2024, NWS advanced steadily through Central America. On November 22, 2024, NWS was detected in southern Mexico, indicating that parasite is rapidly moving north. 

NWS can infect any mammal, including all species of livestock, pets, wildlife, and humans. The species derives its name from the larvae’s feeding habits, as they burrow into wounds, resembling a screw being driven into wood. These maggots inflict significant damage by tearing into the host’s tissue with their sharp mouth hooks. 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 7-10 days. Producers or owners who suspect this disease should contact their veterinarian immediately to evaluate the animal or herd, collect samples, and report to veterinary authorities. 

Adult screwworm flies are about the size of a common housefly (or slightly larger). They have orange eyes, a metallic blue or green body, and three dark stripes along their backs. 

Keep in mind, NWS infestations are often hard to detect in the first 1-3 days. The larvae burrow below the skin; they usually are not seen crawling on the surface. As many as 200 parasitic worms can be found below the surface of what appears to be a small opening in the skin. 

Protect Your Herd: 

Preventing an infestation of NWS is key. Treatment can be difficult, and eradication is expensive. NWS are attracted to the open wounds and mucus membranes where they lay their eggs, and they can be carried by both domestic and wild animals. We recommend taking steps to quickly identify and treat open wounds and keeping wildlife away from your cattle to the extent possible. 

• Frequently inspect cattle and pay close attention to tick bites, cuts, and dehorning/branding sites. Immediately treat any open wounds, scratches, or scabs. Screwworms in wounds are killed by direct application of a wound dressing. 
• Closely monitor new calves and mothers. NWS often lay eggs on the navel areas of calves, and the vulva and perineum of cows. On bulls and steers, monitor the sheath/prepuce as well.  
• Pay close attention to nasal passages and eyes for signs of larvae (maggot) infestation. 
• Minimize access of wild animals (birds, feral hogs, etc.) to cattle to the greatest extent possible. 
• Practice good biosecurity measures like frequently cleaning clothing, boots, vehicles, and equipment on your farm or ranch. 

Animals suffering from NWS infestation may exhibit the following signs: 

• Bloody or light-colored drainage from a cut or wound. 
• A cut or wound that rapidly enlarges for seemingly no reason. 
• White or cream-colored runny substance (the eggs) in and around a wound. 
• Presence of fly larvae (maggots) in wounds. 
• Irritated behavior and signs of pain. 
• Head shaking. 
• Reduced appetite. 
• Fever and other signs of secondary infection. 
• Foul odor or smell of decay. 

If you see any of these signs in your cattle, contact your veterinarian immediately. 

If you think you have found a screwworm, contact your veterinarian and then report it immediately to your state animal heath official and APHIS office. This will allow APHIS and partner agencies to respond quickly and remove the screwworms before a population becomes established.