Screwworms Reach the Mainland

Screwworms, destructive parasites in livestock and other animals, were eradicated from the United States during the second half of the 20th Century, and programs to prevent their return have worked well. But, since screwworm adults are flies, their potential for migrating remains a concern, and that concern was realized last fall when whitetail deer in the Florida Keys became infested with the parasites. With eradication efforts underway on the islands off the South Florida coast, the USDA's Animal and Plant Health Inspection Service (APHIS) this week confirmed the presence of the New World screwworm (Cochliomyia hominivorax)

in a stray dog near Homestead, in Miami-Dade County, on the mainland of Florida.

Screwworm flies lay their eggs on warm-blooded animals, often at the site of an open wound. The larvae then burrow into the animal's flesh, potentially causing extensive damage and death. The adult flies typically do not travel long distances, but the pests can spread through the movement of infested animals or by the adults hitching rides in vehicles or ships.

In October 2016, screwworm infestation were confirmed in Key deer from National Key Deer Refuge on Big Pine Key, Florida. Key deer are an unusually small subspecies of whitetail deer unique to the Florida Keys

In the 1950s, USDA scientists Raymond Bushland and

Edward Knipling

developed the "sterile insect technique" to control screwworm flies. The technique involves raising large numbers of male flies, made sterile using X-rays, and releasing the sterile flies into the wild in infested areas. The sterile males mate unsuccessfully with wild females, and breeding success eventually becomes so low that the species is eradicated from the target area. The technique helped eradicate the New World Screwworm from the United States, Mexico and much of Central America and the Caribbean. Since then, the sterile insect technique has been used to help control a variety of pests affecting crops, livestock and humans.

According to the latest report from the USDA, 13 Keys have had known infestations since October, mostly in the key deer, with five confirmed infestations in domestic animals. State and federal animal-health and wildlife officials have engaged in aggressive efforts to eradicate screwworms from the Keys, including the release of sterile flies. Monitoring efforts have shown reductions in adult fly populations and fewer cases of infected deer.

Appearance of the parasite on the mainland of Florida creates some concern that the flies could spread and eventually affect cattle and other livestock. As seen with the Key deer, wildlife can serve as hosts and facilitate reproduction and dissemination of the insects.

Control efforts in Florida likely will keep the outbreak confined and eventually eradicated. If allowed to spread, the screwworm could affect much of the southern United States, as it did prior to eradication. The Texas Animal Health Commission (TAHC) has reminded veterinarians and producers and stakeholders to report any animal suspected to be infested with screwworm larvae, by calling the TAHC at 1-800-550-8242 or contacting the nearest TAHC regional office.

Veterinarians can

submit collected samples to the State-Federal Laboratory in Austin at 8200 Cameron Road Suite A186, Austin, TX, 78754, 512-832-6580, or the National Veterinary Service Laboratories in Ames, Iowa.