What to know about New World screwworm as cases detected near the US
Cases have been confirmed about 60 miles from the U.S. border.
Federal health officials reported on Monday that cases of a flesh-eating parasitic infection continue to be detected near the United States.
The U.S. Department of Agriculture (USDA) said a New World screwworm case was confirmed in the Mexican state of Nuevo León, approximately 62 miles from the Texas border. This marks the northernmost active case in Mexico.
USDA said the current risk to livestock, other animals and people in the U.S. remains very low and there is currently no evidence of NWS among animals in the U.S.
Here's what you need to know about the infection, how it's treated and how it can be prevented:
What is New World screwworm?
New World screwworm (NWS) is a species of parasitic fly that feeds on live tissue and can cause myiasis, which is an infestation of larvae, or maggots, according to the Centers for Disease Control and Prevention (CDC).
They can infest many types of animals, including livestock, pets, wildlife and, in rare instances, humans.
"These maggots actually feed on the tissue in the wound ... and they can destroy that tissue very substantially," Dr. William Schaffner, a professor of preventive medicine at Vanderbilt University Medical Center in Nashville, told ABC News. "And it's as though they bury themselves. They grind themselves into the tissue as though they were screwing themselves down into the tissue."
A female NWS will find a living host and land in an open wound -- even as small as a tick bite -- around the eyes, ears, nose and mouth. The fly will lay between 200 and 300 eggs, according to the CDC.
After the eggs hatch, the maggots burrow further into the tissue and cause painful infestations.
"It can cause deadly wounds in animals," Dr. Todd Ellerin, chief of infectious diseases at South Shore Health in Massachusetts, told ABC News. "So, it can basically kill cattle, kill livestock. ... Rarely, it can lay an egg in human wounds that can then become secondarily infected with bacterial infections and cause kind of severe infections. It's usually not deadly in humans."
Eggs hatch into maggots that burrow into the wound to feed on the living flesh. After feeding for about seven days, larvae drop to the ground, burrow into the soil, and emerge as adult screwworm flies.
NWS is not typically found in the U.S. and is present in countries in Central and South America as well as the Caribbean, according to the CDC.
"The screwworm likes hot, kind of moist environments," Ellerin said. "With climate change, that's why Central America and Mexico are at higher risk than the U.S., but that's why the southern states that border there are at higher risk than the northern states."
NWS infection can cause several symptoms including skin lesions that don't heal or worsen over time, painful wounds or sores, bleeding from open sores, feeling or seeing maggots in wounds or a bad odor from the site of the infestation, the CDC says.
How is it treated?
There is currently no drug-only cure for NWS infestation. If someone believes they may be infested -- or sees and feels maggots in a wound or anywhere else in the body -- they should contact a health care provider immediately, according to the CDC.
Dr. Marcus Pereira, medical director of the transplant infectious diseases program at NewYork-Presbyterian/Columbia University Irving Medical Center, told ABC News that a wound that smells worse, is bloody, is experiencing discharge or causing more pain could be a sign of an infestation.
A physician will have to remove the maggots, which may require surgery, the CDC says. The agency said patients should not try to remove or dispose of the maggots themselves.
"There are no antibiotics or medications that one can get to attack the larvae, so often the treatment really is surgical," Pereira said. "You have to remove the larvae and the wound along with it. So often, you really need sort of an urgent surgical intervention for these things."
The CDC warns not to throw live maggots in the trash or outside because it could lead to NWS spreading in the area.
How to prevent infection
The experts told ABC News that anyone traveling to parts of the world where NWS infestation is common should keep any open wounds covered and use an insect repellent approved by the Environmental Protection Agency.
People who are spending time outdoors should wear loose-fitting, long-sleeved shirts and pants, hats and socks, according to the CDC.
Using a sterile insect technique, NWS was eradicated from the U.S. in 1966, according to the USDA.
Schaffner said sterile male files that can't impregnate females were created in a laboratory.
"And you made millions of these sterile male flies, and then you released them in nature," Schaffner said. "They mate with the females. The females mate only once. The females do not become pregnant and so they don't produce any eggs. And doing that, you will then eliminate that species from the land area in which you have released these millions of sterile male screw flies."
Schaffner added this is also more ecologically sound than using pesticides, which can harm other species or plants.
Last week, Agriculture Secretary Brooke Rollins visited Texas for the groundbreaking of $600 million facility to produce sterile flies and eradicate NWS.
Once completed, the facility will be able produce 300 million sterile flies each week, but it won't be fully built until 2028.
How concerned should we be?
Experts told ABC News there's no cause for major concern in the U.S. currently.
Since the U.S. eradicated screwworm in 1966, the country has been relatively free of the parasite aside from a 2017 outbreak in the Florida Keys.
Last year, a case was detected in a Maryland resident who had recently returned from travel to El Salvador.
"This is principally an issue regarding livestock," Schaffner said. "The occasional human can become infested. But it's potentially an economic problem."
Crystal Joseph, MD, MS is an anesthesiology resident at Beth Israel Deaconess Medical Center/Harvard Medical School and a member of the ABC News Medical Unit.
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