Dangerous screwworm species found in U.S., poses little threat

NORWELL, MASS. — An imported dog carried a larvae species never before detected in the Western Hemisphere into the country. The threat to humans and animals is minimal, veterinarians say.

Maggots collected from a 1-year-old Labrador were identified by the United States Department of Agriculture (USDA) National Veterinary Services Laboratories (NVSL) as Old World screwworm, Chrysomya bezziana.

However, "this particular case is not a major threat because of the stage of development of the maggots at the time of their discovery, and especially because of the time of the year and locality of the finding," says William Smith, DVM and USDA Animal and Plant Health Inspection Service (APHIS) lead veterinarian in the New England area.

Multiple other Old World species exist throughout the United States, but only in the form of facultative myiasis flies. All native obligate myiasis parasites — or New World screwworms — were wiped out nationwide during an aggressive eradication effort through the 1950s and 1960s, Smith says.

Imported from Singapore on Oct. 26, the dog was taken to a veterinarian the next day, where unusual larvae were pulled from a tail wound and sent to NVSL. The samples were identified as a species of Old World screwworm on Nov. 1, Smith says.

Because both Old World and New World screwworms are tropically adapted, they can't survive in temperate areas, especially during winter.

"Most of the maggots in this case were only half-grown when collected, and even in ideal ambient conditions they would not be capable of surviving away from the host and transforming to flies without feeding for probably another two days or so," Smith says. "On top of that, environmental conditions in Massachusetts at this time of year are not suitable for completion of development."

However, reporting these cases is vital to ensure the disease remains contained in the United States, to protect both animals and people.

Although two human cases of screwworm were identified recently, both were found in people returning from trips to Latin America where the parasites are much more prevalent.

While no other cases associated with the dog from Singapore were found or are anticipated, increased awareness of the disease may heighten veterinary surveillance. This could lead to the NVSL receiving an influx of maggots for evaluation and identification, but all are expected to be domestic facultative myiasis flies.

Just to be safe, "Veterinarians, upon suspecting screwworm infestation or any other foreign disease or pest, should immediately contact their state and federal veterinarians. And remember to collect ectoparasite samples, including maggots, in alcohol and not in formalin," Smith says. "Prompt response and action aids in rapid diagnosis and control."