Veterinary Division - Animal Health Programs
Screwworm is the common name of a pest native to the tropical areas of North, South, and Central America that causes extensive damage to domestic livestock and other warm-blooded animals. The larvae of these pests feed on the raw flesh of the host animal. Rare human cases have been reported. Suspect cases should be reported to the State Veterinarian or Federal animal health authorities.
The United States has been free of screwworm since 1966. USDA estimates that the U.S. livestock industry could suffer $750 million in production losses annually if this pest were reintroduced to the United States. Limited screwworm outbreaks in Mexico during 1992-1993 were contained and eradicated at the cost of several million dollars.
When larvae drop from a wound, they burrow into the soil where they enter the pupal stage, which lasts 5 to 7 days. Adult flies emerge from the pupal cases and are ready to mate within 3 to 5 days. Veterinarians should note that initial cases of screwworm are often detected in pet animals. Animals are infested when the screwworm eggs hatch in the wound of an animal and the larvae feed on the animal's flesh.
The Pest and Its Cycle
The screwworm is an insect that, in its adult stage, is about twice the size of the common housefly. It has orange eyes and a blue-gray or gray body with three dark stripes running down its back.
After mating, the female screwworm fly lays her eggs in the open wounds of animals. One female can lay up to 400 eggs at a time and as many as 2,800 eggs during its 31-day lifespan. These eggs can hatch into larvae in as little as 12 hours.
Screwworm larvae grow by feeding on the flesh of living animals and can grow to be over on-half inch within 5 to 7 days after hatching. The full-grown larvae then drop from the wound and tunnel into the soil, where they form protective cases to house themselves while they pupate. Adult screwworm flies emerge from these pupal cases and are ready to mate again within 3 to 5 days.
Practitioners should be on the lookout for:
- Wounds that may become infested with maggots. Wounds commonly infested include those caused by feeding ticks, castration, dehorning, branding, shearing, wire cuts, sore mouth in sheep, and shedding of the velvet in deer. Navels of newborn mammals are a common site for screwworm infestation. It is very difficult to see early stages of screwworm larvae feeding in a wound; only slight movement may be observed. In some cases, the openings in the skin may be small with extensive pockets of screwworm larvae beneath.
- Bloody discharge from the infested wounds
- Decreased feed intake
- Decreased milk production
- Seclusion from rest of herd or flock. Animals may seek shady or secluded areas to lie down. Animals with screwworm infestations may die 7 to 14 days if wounds are not treated to kill the larvae, especially in cases of multiple infestations. As many as 3,000 larvae may be fund in a single wound. Death results from toxicity and/or secondary infection. Suspect cases should be reported to the State Veterinarian or Federal animal health authorities.
A screwworm infestation is treated with topical application of approved chemicals to kill the larvae. Wounds should be treated for 2 to 3 consecutive days to ensure that all larvae have been killed. The larvae should be removed from the wounds using tweezers.
Treating wounds and spraying or dipping animals with an approved organophosphate insecticide will provide protection against screwworm for 7 to 10 days. However, the most effective way to control screwworm infestation is through eradication.