We have a “new” bug to worry about. An actual insect (not really a bug) and one that isn’t quite new. For more than half a century, the New World screwworm was considered one of veterinary medicine’s greatest success stories. After causing enormous losses to livestock throughout the southern United States, the parasite was eradicated from this country in 1966 through an innovative international control program. Unfortunately, after spreading north through Central America and Mexico over the past several years, the screwworm has again reached the United States.
Despite its name, the New World screwworm is not actually a worm. It is the maggot of a metallic blue-green colored fly, Cochliomyia hominivorax, a blowfly. Unlike most flies, whose maggots (larval stages) feed on dead tissue, screwworm larvae invade living flesh. Adult female flies are attracted to even small wounds, surgical incisions, tick bites, navels of newborn animals or mucous membranes such as the prepuce or even mouths. They lay hundreds of eggs, which hatch within about a day. The larvae burrow head-first into living tissue using screw-like mouth hooks (hence their name), creating painful, rapidly enlarging wounds that become deeper as the larvae feed. Untreated infestations can be fatal. It’s the stuff of horror movies.
Through a comprehensive effort, the parasite was eliminated from the United States using a remarkable example of biological control: the sterile insect technique. Millions of male flies were raised in specialized facilities, sterilized using radiation, and released over infested areas. Because female screwworm flies mate only once during their lifetime, mating with a sterile male produces no offspring. Releasing enough sterile males causes the wild population to collapse.
This program progressively pushed the fly southward through the United States, Mexico and Central America until a permanent biological barrier was maintained in Panama. For decades, this barrier protected North America.
But it’s back. Why? Since 2023, the parasite has spread rapidly north through Central America into Mexico. Epidemiologists and veterinarians believe the principal driver has been the movement of infested livestock across borders, allowing the parasite to travel far faster than adult flies could disperse naturally. Former USDA officials have stated that the loss of experienced personnel complicated early response efforts, although experts agree that the resurgence resulted from multiple factors, including infected livestock movement through Central America and insufficient sterile-fly production capacity. At the same time, only one major sterile-fly production facility remained in operation in Panama, limiting the number of sterile flies available to contain the outbreak.
The United States has responded by expanding sterile-fly release operations, establishing additional dispersal facilities and planning new production capacity, but rebuilding this capability takes time.
We are talking cattle, but what about here in Washington, DC? The fly can’t survive the winter but can infect any mammal, including dogs and cats. With national and international travel of pets, the infection and return of the parasite to DC is a possibility. Dogs with recent surgical wounds, skin injuries, tick bites, ear infections or draining skin lesions are at greatest risk. Reports from affected countries have described dogs developing rapidly enlarging wounds containing numerous active larvae. Because the larvae continue feeding on living tissue, the damage progresses much more quickly than with ordinary maggots. Prompt treatment is essential and usually includes removal of the larvae, wound care and treatment of secondary bacterial infections.
Pet owners and livestock owners should watch carefully for wounds that fail to heal, enlarge rapidly, produce bloody or foul-smelling discharge or seem unusually painful. This is especially important in indoor/outdoor cats. Animals may constantly lick or bite at the area, appear restless or become depressed. In some cases, tiny white larvae may be visible deep within the wound. Any suspicious wound in an animal that has recently traveled from an affected region ‒ or that occurs while the parasite is spreading ‒ should be examined by a veterinarian immediately. Early recognition greatly improves the outcome and helps protect other animals.
The New World screwworm is considered an animal disease of major importance. Anyone in DC who suspects an infestation should immediately contact the United States Department of Agriculture’s Animal and Plant Health Inspection Service (USDA APHIS) Area veterinarian in charge or notify the District of Columbia Department of Health, Animal Services Division.
Do not attempt to discard larvae before consulting a veterinarian, as laboratory identification is essential for confirming the diagnosis and initiating an appropriate response. Keep them in a sealed bag or jar to prevent loss.
The return of the New World screwworm is a reminder that disease control requires constant vigilance. While today’s risk to pets in the District remains extremely low, rapid reporting, prompt veterinary care and continued international cooperation remain our best defenses against one of veterinary medicine’s most destructive parasites.
Dan Teich, DVM, is the founder of District Veterinary Hospitals.
