Screwworm has expanded from South America into Mexico. The U.S. Equestrian Federation warns that if not controlled, the screwworm flies will continue to move into the U.S. Should that happen, it could have an impact on shows, and movement of horses may be halted.
Screwworm is a type of fly larva (also known as maggots) foreign to the U.S. They can infect any warm-blooded animal, including humans, as they enter wounds as well as mucus membranes to feed on living tissue. Infested animals may have enlarging, draining, foul-smelling wounds and weight loss. They can die from secondary infection or toxicity in seven to 14 days if not treated.
An adult screwworm fly can travel up to 125 miles before laying hundreds of eggs in a wound or on a mucus membrane. The larvae that emerge from the eggs feed on living tissue and then fall to the ground to pupate.Ultimately, adult flies that reproduce will emerge. The adult female screwworm only mates once and lays those eggs to continue the spread of the parasite. Screwworms can also be transported by animals and people with infected wounds traveling from infested areas.
As it is a foreign animal disease, a state and/or federal animal health official must be notified of suspected cases. If screwworm is diagnosed, animal health officials will quarantine the animal until daily wound care and treatments with larvicides and insecticides have eliminated the screwworm larvae. Animal health officials will further investigate cases to determine if additional control measures, such as insecticidal treatment of the environment or sterile fly release, are warranted.
In endemic countries such as those in South America, control is based on a sterile male fly release program. As the female only mates once in a lifetime, the mating with a sterile male fly will reduce the fly population over time. Additionally, movement restrictions and treatment requirements are implemented to further prevent the spread.
Daily inspection of all wounds for any signs of larvae is critical. If larvae (maggots) are detected in a wound, the horse owner should contact their veterinarian immediately so they can perform an assessment of the type of larvae. If screwworm is suspected, samples will be collected for confirmation.
Any livestock or horses entering the United States from a screwworm-affected country are required to be pre-treated with an ivermectin product, remain in an import quarantine facility for a seven-day observation period, and complete a screwworm examination.Sedation is required for the examination of the sheath of stallions and geldings. If examination reveals no screwworms, the horse will be released.
For more information and images of screwworm, click on this link.