The New World Screwworm (NWS) fly, Cochliomyia homnivorax was eradicated from the United States in 1966. Prior to eradication, it cost the U.S. livestock industry more than $100 million annually. In 2023, increased cases of Screwworm infestation were noted in Central America and began advancing north.
Screwworm larva - Cause of costly infestations in horses and livestock.
Horses become infected with the larvae through a break in the skin - trauma, horse fly or tick bite or at mucous membranes - eyes, vulva, prepuce, etc., making diligent fly protection a high priority.
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In February 2025, the first cases were identified in southern Mexico and more cases have been identified further north, creating a concern to the U.S., especiallywith border states such as Texas and New Mexico.
Unfortunately, with the increase in international travel, this pest can hitch a ride and travel into the U.S. and become established before it can be stopped. On May 13, 2025, Secretary of Agriculture Brooke Rollins announced that the border was closed for cattle, bison and horses coming out of Mexico due to the NWS threat. Sheep and goat imports into the U.S. were already restricted due to Scrapie disease.
Unfortunately, this fly has no boundaries, and it is hard to control wildlife movement across countries. Therefore, we must be diligent in our efforts to educate producers and pet owners as to what an NWS fly looks like and the signs of an NWS infestation by its larvae, i.e. myiasis.
Cochliomyia homnivorax
An adult fly is up to 10mm long, body is metallic blue, bluish purple to blue-green, eyes are reddish in color and large. The fly does not bite; however, the harm comes when the female lays eggs near an open wound and the larvae infest the wound. An adult fly only survives for about two weeks but she is sexually mature within two to three days after developing into an adult.
Once fertilized by a male, she is capable of laying an egg mass several times during her lifetime near open wounds. These egg masses contain 100 to 300 eggs, which develop into larvae within 10 to 12 hours, which then feed on live tissue.
The female NWS fly only mates once in her lifetime. One of the methods of control is to release sterilized male NWS which breed the females, which are thenunable to lay viable eggs. This is the method which has been used in the U.S. and other countries to eradicate
NWS flies.
The larvae go through three stages and grow larger with each stage. The larvae can survive and eat live tissue for up to seven days. The head of the third stage (instar) larvae has hooks on it, which enables the larvae to tear at the host tissues during feeding. They also have barbs on their sides which allow them to secure their position within a wound. “Myiasis” describes a wound that is infested with fly larvae.
When these larvae develop into the third stage larvae, they exit the wound, burrow into the soil and develop into pupa. Pupa can develop into adults or they can hibernate within the soil until conditions are more conducive for development into an adult fly.
Horses can become infected with the larvae through any break in the skin (trauma, horse fly or tick bite, etc.) or at their mucous membranes (eyes, vulva, prepuce, etc.). Therefore, it is essential that the horse owner is diligent in fly protection and observation. If they observe any suspicious lesions, they should contact their veterinarian, State Animal Health Official or
USDA Area Veterinarian in Charge immediately
Identification/Notification
Once an animal becomes infected by NWS and left untreated, the animal may die within one week due to secondary infections. Animals identified with myiasis should receive veterinary medical assistance along with collection of larvae within the wound submitted for identification.
If you observe an animal with myiasis, collect the larvae from the lesion and place in a sterile container with 70% ethanol or alcohol and send to the nearest National Animal Health Labatory Network (NAHLN lab for diagnosis. Do not place the larvae in formalin. The link for the parasite submission form if larvae are found can be found here.
Treatment
Clean and debride the wound and apply permethrin spray. Ivomec injectable can be given to livestock as a treatment. Also, topical permethrins could be used in horses and other livestock to assist with fly control.
JULY 2025 | 3
Screwworm (Cochliomyia homnivorax). Eradication Techniques
The only proven technique which worked back in the ‘60s was releasing sterilized male screwworm flies which mated to the females and rendered their eggs infertile.
For more information about NWS:
• https://www.aphis.usda.gov/sites/default/files/
aphis-sop-detection-nws-in-animals.pdf
• https://www.aphis.usda.gov/sites/default/files/
factsheet-nws-private-veterinarians.pdf
• https://www.kyagr.com/statevet/disease-
prevention.htm
Praess release by Maxwell H. Gluck Equine Research Center, Lexington, Kentucky USA,- Article by Beth Johnson, DVM Director of Regulatory Field Services, Kentucky Department of Agriculture, Office of State Veterinarian