New World Screwworm
New World screwworms (NWS) are fly larvae (maggots) that can infest livestock and other warm-blooded animals, including people. The NWS (Cochliomyia hominivorax) is a parasitic fly native to the Western Hemisphere. Female flies lay eggs in wounds or body orifices. They most often enter an animal through an open wound. Unlike other flies, NWS maggots feed on the animals’ living flesh and, if not treated, infestations can be fatal. Animals at greatest risk include those that have recently given birth, have open wounds, or have undergone surgery or management procedures (dehorning, castrating, branding, etc.) While NWS 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. Cases began spreading in 2023, heading north from Panama, and have reached southern Mexico, increasing the risk of reintroduction into the United States. They are controlled only through the release of sterile males, known as the sterile insect technique. Any suspected cases should be reported to the Mississippi Board of Animal Health.
See locations of recent confirmed cases of the New World Screwworm Outbreak in Central America
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Signs
Screwworms are typically found on warm-blooded animals that have a draining or enlarging wound. The name screwworm refers to the feeding behavior exhibited by the maggots and they burrow (screw) into the living flesh of affected wounds.
Screwworm eggs are a creamy and white color and deposited in a shingle-like manner on or near the edges of wounds. The larvae are cylindrical with one pointed end and one blunt end and have rings of dark brown spines around the body. Adult flies look similar to common house flies or may be slightly larger. They can be distinguished by these characteristics:
- Orange eyes
- Metallic blue or green body
- Three dark stripes along their backs
New World Screwworm Larva
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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 a foul 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 7 to 14 days may die. Signs of screwworm infestation in animals include:
- Wounds in living animals infested with maggots
- Blood-tinged discharge from the infested wounds
- Foul odor
- Discomfort
- Depressed/no eating
- Decreased milk production
- Seclusion from rest of herd or flock
Human Health Guidance
While the occurrence of screwworms in humans is less common than in livestock or other mammals, it is possible for humans to become infested. People are urged to keep wounds clean and closely monitor open cuts and wounds for the presence of maggots. People infested with screwworm usually have significant discomfort or itching at the wound site. Anyone who suspects the presence of screwworms should contact their primary physician immediately.
Animal Health Guidance
Carefully inspect animal wounds and hides for signs of screwworm. Owners should immediately consult their veterinarian and MBAH (601) 359-1170 if a case of screwworm is suspected.
Treatment
Screwworm infestation is treated by cleaning the site and topical application of a pesticide or other medication effective at killing the screwworm larvae. 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 report to the Mississippi Board of Animal Health (State Veterinarian’s Office).
Report Suspected Cases
Anyone who suspects the presence of screwworms in animals or has questions or concerns can contact:
- By phone during office hours: (601) 359-1170
- By phone after hours: 1-888-646-8731
- By email: msstatevet@mdac.ms.gov
History
In the 1950s, the U.S. Department of Agriculture’s (USDA) 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, and until the current outbreak, a permanent sterile fly barrier at the Darien Gap between Panama and Colombia prevented the flies from moving back into Central America.
The last outbreak in the U.S. occurred in September 2016, when the presence of New World screwworm was confirmed in Key deer from a wildlife refuge on Big Pine Key in Monroe County, Florida.
Resources
- Information on the Screwworm Eradication Program (PDF – USDA APHIS)
Screwworm Information Sheets
- Screwworm Brochure (English Version) (PDF – USDA APHIS)
- Screwworm Brochure (Spanish Version) (PDF – USDA APHIS)
- Screwworm Poster (PDF – USDA APHIS)
- Screwworm Factsheet (PDF – USDA APHIS)
- Screwworm Pest Card (PDF – USDA APHIS)