Florida Department of Agriculture and Consumer Services

New World Screwworm

New World screwworms are fly larvae (maggots) that can infest livestock and other warm-blooded animals, including people. They most often enter an animal through an open wound or, in the case of newborns, the navel. They feed on the animal’s living flesh and, if not treated, infestations can be fatal. While New World screwworm (Cochliomyia hominivorax) has not been widely present in the United States since the 1960s, it is still found in most of South America and in five Caribbean countries.

The adult screwworm fly is the size of a common housefly, or slightly larger, but different in color and appearance. The screwworm fly has orange eyes and a metallic dark blue to blue-green or gray body. It also has three dark stripes running down its back, with the middle stripe shorter than the outer two. A female screwworm fly typically mates once in her lifetime and lays her eggs on or near an open wound or the mucous membranes of an animal’s nose, mouth or ears. In her lifespan, the screwworm fly can produce thousands of offspring. The eggs hatch into larvae within a day and then feed on the animal’s tissue for five to seven days before maturing. The mature larvae then tunnel into the ground and emerge as adults, ready to mate and continue the cycle.

While they can fly much farther under ideal conditions, adult flies generally do not travel more than a couple of miles if there are suitable host animals in the area. New World screwworm is more likely to spread long distances when infested animals move to new areas and carry the pest there.

See dates and locations of recent New World screwworm detections.


In the 1950s, USDA’s Agricultural Research Service developed a new method to eradicate the pest using a form of biological control called the sterile insect technique. Infertile male flies are released in infested areas. When they mate with local females, no offspring result. With fewer fertile mates available in each succeeding generation, the fly breeds itself out of existence.

USDA began using this technique in Florida in 1957 and eradicated the flies from the entire southeastern United States by 1959. The technique was next applied to the more extensively infested Southwest in 1962. By 1966, self-sustaining screwworm populations were eliminated from the United States. However, there were still infestations in Mexico, and animals crossing the border were causing some reinfestations in the United States. USDA worked with Mexican officials to reduce the flies in that country. After 1982, there were no more reinfestations in the United States, and Mexico was officially declared free of screwworms in 1991. USDA then partnered with the countries of Central America to eradicate New World screwworms.

Today, USDA and its partners maintain a permanent sterile fly barrier at the Darien Gap between Panama and Colombia to prevent the establishment of any screwworm flies that enter from South America.

Screwworm Response Efforts

In late September 2016, the presence of New World screwworm was confirmed in Key deer from a wildlife refuge on Big Pine Key in Monroe County, Florida. Florida Commissioner of Agriculture Adam H. Putnam declared an agricultural emergency [ Adobe PDF Document 720.62 KB ], and the Florida Department of Agriculture and Consumer Services (FDACS) established an Animal Health Checkpoint at mile marker 106 to ensure that animals traveling north out of the Keys did not have screwworm.

FDACS worked closely with the U.S. Department of Agriculture and other partners on control and response efforts to eradicate the screwworm. With no screwworm finds since January 10, 2017, the Animal Health Checkpoint at mile marker 106 was closed on Saturday, March 18, 2017. Sterile fly releases in the Homestead area were discontinued as of March 21, 2017. Sterile fly releases continued in the Infested Zone through April 25, 2017. 


Screwworms are typically found on warm-blooded animals that have a draining or enlarging wound.

Almost any type of wound can become infested with screwworm larvae, including those caused by feeding ticks, castration, dehorning, branding, shearing, barbed wire fences, and even shedding of antler velvet in deer. The navels of newborn mammals can also become infested.

It can be very difficult to see the early stages of screwworm larvae feeding in a wound. The most obvious sign is a change in the wound’s appearance — as larvae feed, the wound gradually enlarges and deepens. An infested wound also gives off an odor and some bloody discharge.

Even if the actual wound on the skin is small, it could have extensive pockets of screwworm larvae beneath it. Infested livestock usually show signs of discomfort, and they may go off their feed and produce less milk. Typically, these animals will separate themselves from the rest of the flock or herd and seek shady or secluded areas to lie down. Infested animals that are not treated in seven to 14 days may die.

While human cases of New World screwworm are rare, they have occurred. People infested with screwworm usually have discomfort or itching at the wound site.

Guidance for Human Health

While the occurrence of screwworms in humans is less common than in livestock or other mammals, people are urged to keep wounds clean and closely monitor open cuts and wounds for the presence of maggots. Anyone who suspects the presence of screwworms should contact a physician immediately.

Guidance for Animal Health

Carefully inspect animal wounds and hides for signs of screwworm. Any suspected cases of screwworm should immediately be reported to 1-800-HELP-FLA (1-800-435-7352).

Screwworm eggs are creamy and white and deposited in a shingle-like manner on or near the edges of superficial wounds. The larvae are cylindrical with one pointed end and one blunt end and have rings of dark brown spines around the body. Female screwworm flies are larger than a housefly with a dark blue to blue-green body and reddish-orange head.


Screwworm infestation is treated by cleaning the site and topical application of a pesticide. Treated wounds should be inspected daily until completely healed. The examining veterinarian will remove larvae from the wound with tweezers, place them in alcohol, and submit them to USDA’s National Veterinary Laboratory for positive identification.

Report Suspected Cases

Anyone who suspects the presence of screwworms or has questions or concerns can contact FDACS at 1-800-HELP-FLA (1-800-435-7352).

To report a sick or injured Key deer including a suspected screwworm infestation, call 1-888-404-FWCC (3922) ext. 7 or (305) 470-6863 ext. 7.

Additional State Screwworm Entry Requirements

Below are states that have imposed additional entry requirements related to the New World screwworms in the Florida Keys.


Eradication Efforts

Press Releases

Download Outreach Materials

Screwworm Information Sheets

Additional Resources

Sources: CDC, USDA, Iowa State University