New World screwworm (NWS) infestation is caused by NWS fly larvae (Cochliomyia hominivorax). These maggots eat flesh of warm-blooded animals (including horses and humans) by using hooks on the larva to burrow into and tear tissue. Screwworms can be transmitted to people.
The parasite was identified in Mexico in late 2024. Currently, NWS is not present in the United States and is considered to be a foreign animal disease. Suspected cases must be reported to state and federal animal health officials. This parasite is common in tropical and subtropical regions in Central and South America. Once the disease is confirmed in the United States, there may be movement restrictions and requirements for horses. United States Equestrian Federation will collaborate with state and federal animal health officials to ensure safe movement and competitions.
Transmission & Incubation Period
The screwworm fly lays eggs in open wounds. Eggs are creamy white and are found at the edges of the wound. Larvae invade the wound and subsequently develop into adult flies which transmit the parasite to new wounds.
Screwworm larvae emerge from the eggs in 12 to 24 hours. After about seven days of feeding, larvae drop to the ground, burrow into the ground and pupate. The adult screwworm fly emerges from the soil after at least seven days. Shedding of larvae continues until treatment is administered.
Clinical Signs and Diagnosis
The parasite is seen in open wounds which cause pain and irritation to the animal. The presence of maggots in wounds create a foul odor. The infestation can be severe and potentially fatal if untreated.
As this is a foreign animal disease, any suspect cases of unusual maggots should be reported to state or federal animal health official. It is important to note that there are other flies that can be associated with wounds.For official identification, the larvae should be sent to the National Veterinary Services Laboratory. Once a diagnosis is confirmed, the animal health official will complete the investigation and establish the appropriate diagnostic testing, treatment, and biosecurity protocols.
Treatment and Prognosis
Animals infested with NWS should be treated according to the recommendations of their veterinarian and state and federal animal health officials. Typically, woulds must be cleaned and maggots must be surgically removed, and the area should then be treated with approved topical and systemic insecticides. Larvae removed from the wound must be destroyed as directed by federal or state animal health officials and not allowed to enter into the environment. Systemic administration of antibiotics is recommended if the wound becomes infected. Treatment of the environment may also be necessary.
Good with prompt treatment; poor if untreated.
Control and Eradication
The primary measure for eradication of screwworm is the use of sterile insects, which involves rearing and releasing large numbers of male flies sterilized by gamma radiation. The sterile males then mate with wild females, who then lay unfertilized eggs. Since females only mate once, the screwworm population declines with each generation until it dies out.
Currently, the only sterile fly production facility is in Panama, butthe U.S. Department of Agriculture is investing in converting a fruit fly facility in Mexico to produce additional sterile flies, and is investigating the creation of a domestic production and dispersal facility in Texas.
