U.S. officials have warned that the flesh-eating parasite that burrows into its host\’s skin through open wounds is making a comeback in Central America.
The New World screwworm (Cochliomyia hominivorax) primarily infects cattle and other livestock, but it has also been known to infect humans. The worm can cause New World screwworm myiasis, a potentially fatal condition in humans with no known treatment.
For decades, countries across the Americas have invested billions of dollars to control the flesh-eating parasite, which is native to South America and the Caribbean. But cases of screwworm have been on the rise and spreading northward since 2023.
Between the 1930s and 1950s, screwworms were a major problem for cattle farmers in southern U.S. states, causing producers to lose up to $100 million each year, according to the U.S. Department of Agriculture (USDA). By the mid-1960s, the parasite was nearly eliminated thanks to sterilization efforts to prevent screwworm flies from breeding, and a New World Screwworm Barrier Zone was established along the U.S.-Mexico border.
By 1986, the parasite was largely eliminated in Mexico, but it continued to circulate in South American and Caribbean countries where screwworms are endemic. Now, after a positive detection in Mexico on Nov. 22, 2024, the screwworm could begin to make a comeback, the USDA said in a statement on Dec. 6.
\”Since 2006, the United States and Panama have maintained a barrier zone in eastern Panama intended to prevent NWS [New World Screwworm] from moving northward from South America to screwworm-free areas in Central and North America,\” the department said in the statement. \”However, since 2023, cases have been increasing in number and spreading north from Panama to Costa Rica, Nicaragua, Honduras, Guatemala, and now Mexico.\”
The parasite spreads when adult female screwworm flies lay their eggs in wounds or open pores of live, warm-blooded animals. According to the U.S. Centers for Disease Control and Prevention (CDC), these flies can lay up to 300 eggs at a time, potentially laying thousands during their 10- to 30-day lifespan.
Even a wound the size of a tick bite is large enough to attract female flies to lay their eggs. These eggs then hatch into larvae, which burrow into the wound and eat the surrounding flesh with their pointed, hooked mouthparts, essentially eating their host from the inside out. The wound becomes deeper and larger as more worm-like larvae hatch from their eggs.
These infections are very painful. They also make their host vulnerable to secondary infections.
Myiasis generally refers to infection of live vertebrate animals with fly larvae, including the screwworm. Mortality rates vary greatly between different species, but a large study conducted by the World Health Organization found that the mortality rate in human cases was about 3 percent.
According to the C.D.C., people living in rural areas who often work with animals are most susceptible to infection, though anyone with wounds or open sores from recent surgery can be affected.
According to the C.D.C., there is no approved treatment for screwworm infections other than physically removing the larvae from the infected tissue. Instead, the agency notes that the best way to prevent screwworm myiasis is to avoid exposure. This includes cleaning and covering open wounds, especially when in contact with livestock and wild animals, and using insect repellents registered by the Environmental Protection Agency (EPA).
To prevent the spread of this parasite in the U.S., the USDA\’s Animal and Plant Health Inspection Service (APHIS) is banning the importation of livestock, including horses, into the U.S. from Mexico, \”until further information is received from Mexican veterinary officials regarding the size and scope of the infestation.\”