Will New World Screwworms Return to the U.S.

The U.S. Department of Agriculture (USDA) recently confirmed the New World screwworm (NWS) infestation was eradicated from the United States decades ago using an innovative sterile insect technique.

However, as of October 21, 2024, this flesh-eating worm remains endemic in Central America and is constantly at risk of being reintroduced into the United States. 

While NWS primarily infects animals, the USDA says a fly (Cochliomyia hominivorax) about the size of a typical house fly transfers a parasite to warm-blooded animals, including humans. Its larva consumes living flesh after hatching from an egg deposited in a host.

“A female fly can lay hundreds of eggs in a single wound, which can hatch out into hundreds of larvae, causing extensive tissue destruction, debilitation, and a risk of death,” said Rebecca Chancey, M.D., during a U.S. Centers for Disease Control and Prevention (CDC) briefing last week.

The CDC wrote in late September 2024 that people are at higher risk for NWS when traveling to Central America, being around livestock in rural areas where the flies are, and having open wounds. This year, NWS animal cases have been reported in Panama, with 18,553 positive cases, Costa Rica, 6,938, and Nicaragua, 3,307.

The CDC also confirmed last week that people in Panama (79), Costa Rica (33), Nicaragua, and Honduras were infected with NWS in 2024, elevating concern about a northward spread of this parasite.

For example, the U.S. Embassy in Costa Rica confirmed a local human death attributed to the NWS infection on June 21, 2024, the first in Costa Rica in over three decades.

The USDA began efforts to remove the NWS in 1957 and largely succeeded in pushing them out of the U.S.

That effort expanded southward to Mexico and reached Panama in the early 2000s. The last confirmed NWS outbreak in the U.S. was in the National Key Deer Refuge in Big Pine Key, Florida, in October 2016.

Eradicating NWS is only possible through the sterile insect technique. Developed by the USDA, this method releases sterile flies into an area where a known population has become established. The sterile male screwworm flies mate with fertile female screwworm flies, causing the population of screwworm flies to decrease.

According to the U.S. Commission for the Eradication and Prevention of Screwworm, the current emergency response is based on these pillars:

• Increase the production of sterile pupae from 20 million to 96 million per week.
• Increased area and ground dispersal averaged more than 60 million sterile flies.

The CDC writes that NWS infestations are painful. Infestation begins when a female fly lays eggs on open wounds. People may see maggots (larvae) around the nose, eyes, or mouth.

People who suspect they are infested with NWS should seek immediate medical treatment, following the CDC guidelines updated in June 2024. Treatment requires the extraction of all NWS larvae from the patient’s body.

Additionally, the CDC confirmed no vaccines are available to protect people against NWS infections.