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.