Spread of flesh-eating parasite in Mexico puts US on alert

A flesh-eating parasite spreading across Mexico and Central America has placed the U.S. on high alert, but government officials say several key steps have been taken in case an outbreak occurs here.

According to a June report from the USDA Animal and Plant Health Inspection Service (APHIS), the New World screwworm (NWS) is a fly larva that infests the living tissue of warm-blooded animals, including humans, that causes a painful condition known as myiasis.

According to the Center for Disease Control (CDC), the name screwworm refers to the feeding behavior of the larvae, known as maggots, as they burrow, or “screw,” into healthy tissue.

When that happens, the CDC says extensive damage could occur as the maggots tear into their host’s tissue with sharp mouth hooks. That wound can then become larger and deeper as more larvae hatch and feed on the tissue.

The USDA said those infestations of NWS can be fatal if left untreated.

Where is the New World screwworm now?

According to the APHIS, the NWS is currently known to exist in parts of every country in South America except Chile, as well as in parts of the Caribbean.

The APHIS said Panama reported a dramatic increase in the number of NWS cases in 2023, and since then has spread northward through Central America.

And in November 2024, the NWS reached the southern state of Mexico.

As of the June report, the APHIS said the NWS has not been spotted in the U.S.

What’s being done to prevent the New World screwworm from entering the U.S.?

The APHIS said there are two ways officials can try and prevent the spread of the NWS inside the United States.

One of the ways is by preventing contact between NWS flies and non-infested animals, including the prevention and treatment of wounds in non-infected hosts, as well as by quarantining and treating infested livestock.

Another way to try and prevent the spread of NWS is by stopping the reproduction cycle altogether.

This can be done through the detection and treatment of infested animals, and with the use of the Sterile Insect Technique (SIT).

That technique exposes NWS pupae to gamma radiation to create sterile NWS flies. These are then released into the wild from either ground level or the air, with sterile males overwhelmingly mating with wild female flies that then lay unfertilized eggs.

The APHIS said that because female NWS often flies only mate with one male during its lifecycle, the population of NWS flies is progressively reduced and then eradicated.

How does it spread?

The CDC says New World screwworm infestations begin when a female fly lays its eggs on open wounds or on other parts of the body and mostly affects livestock.

However, it can also infest birds and humans.

The female flies are attracted to the smell of a wound or other opening, and wounds as small as a tick bite could attract the female.

One female, according to the CDC, can lay 200-300 eggs at a time and could lay up to 3,000 eggs during its 10- to 30-day lifespan.

Those eggs then hatch into larvae and burrow into the host’s flesh.

What are the symptoms?

As we stated above, infestations are very painful.

The CDC says some of the symptoms of infestation include (are you ready?) unexplained skin lesions that don’t heal, bleeding from open sores and feeling larvae movement within a skin wound or sore, or in the nose, mouth or eyes.

An infected host may also see maggots around or in the open wounds.