New World Screwworm Update 2025

New World Screwworm 2025 Update: 

On May 11, 2025, the USDA suspended livestock (including horses) imports along the southern border with Mexico due to the increased threat of New World Screwworms in Mexico and Central America.  
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• Read Dr. Wendy’s blog from 2016 during the Key Deer outbreak in Florida below. 
• We have also added the USDA’s bulletin with pictures to help you identify suspected cases. 
• Dogs, cats, backyard chickens and other yard pets can be infected. If you suspect an infection, contact your veterinarian immediately
• To report a suspected wildlife infection, contact the Florida Fish and Wildlife Conservation Commission 888-404-FWCC (3922) [email protected]
• UF News - USDA halts animal imports, UF experts explain New World Screwworm risks

Originally posted on 11/2/2016

A Halloween Story ——— Cochliomyia hominivorax - “ eater of man “

Screwworms are the larval stage of the blue bottle fly aka blow fly with a scientific name of Cochliomyia hominivorax - “ eater of man “. 
The screwworm has been eradicated since the 70s, but its recent discovery in a population of Key Deer has the USDA and veterinarians on high alert to contain them. They are a potential danger to cattle and sheep, but also to people, horses, pets, and wildlife. 

An adult female fly will “blow” her eggs (100-400) onto a fresh wound. The eggs will hatch in 12-24 hours. Screwworms differ from other larvae in that they will burrow into and eat the healthy tissue. Other maggots eat only necrotic tissue, so do not cause damage to the host. Screwworm infestations can cause deep, large wound tracts in flesh and bone. Any wound is an entry point, even a small cut, the navel stump of a newborn or the mucous membranes of the nose, mouth, and eyes. After 5 -7 days, they drop into the soil and pupate for only 7 days in a warm climate like South Florida. The adults can breed within 3 days after emerging from the pupa. They will mate 1 time in their life, lay eggs, and die within 2-3 weeks. 

Screwworm infestation was a huge problem for ranchers in the southern US, and in 1958, the USDA released millions of sterile males. The theory was that the sterile males would outcompete the wild males and decrease the population. Males eat only vegetation, not flesh. The program was successful, and the screwworm was eradicated in the US and much of Mexico by 1972. Small populations remain in South and Central America and the Caribbean, and this outbreak is most likely from an animal or person traveling from an infected area to the Keys.

Treatment includes removing the larvae manually, applying insecticide, and cleaning and bandaging the wound, so stock up on SWAT ! 

If you are in South Florida this winter, check your pets and horses. Call your veterinarian immediately if you suspect screwworms in a wound on your animal or if you have a very smelly, slow-healing wound.  

The larvae have dark pigment on their trachea, so they are easy to identify. Other maggot species do not have this. You may see a bottle fly similar to the adult shown here. There is a very similar species, C. macellaria - common name green bottle fly. C. macellaria are common in the US and only eat necrotic tissue.  They are used by forensic scientists to establish time of death.

USDA New World Screwworm Identification & Lifecycle