New World Screwworm outbreak in Mexico halts US livestock imports amid threat

A threat to American livestock – the New World Screwworm (NWS) fly, which has been considered eradicated from the country since 1966 — has reemerged as a potential danger following an outbreak in Mexico.

The news triggered a shutdown of cattle, horse and bison imports along the southern border, as U.S. Department of Agriculture (USDA) Secretary Brooke Rollins announced in an X post on Sunday.

“Due to the threat of New World Screwworm I am announcing the suspension of live cattle, horse, & bison imports through U.S. southern border ports of entry effective immediately,” she wrote in the post. 

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“The last time this devastating pest invaded America, it took 30 years for our cattle industry to recover. This cannot happen again.”

What is the New World Screwworm?

The NWS is a fly that is endemic in Cuba, Haiti, the Dominican Republic and some South American countries, according to the USDA’s Animal and Plant Health Inspection Service (APHIS).

While the flies themselves are found in forests and other wooded areas, they will seek hosts like cattle or horses in pastures and fields, per the above source.

A female fly lays eggs in a wound or orifice of a live, warm-blooded animal. The eggs then hatch into larvae (maggots) that burrow into the flesh, causing potentially deadly damage.

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Screwworms are named for their maggots’ behavior, as they burrow into the flesh similar to how a screw is driven into wood.

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“Maggots cause extensive damage by tearing at the hosts’ tissue with sharp mouth hooks,” according to the APHIS website. This can then enlarge the wound and attract more flies to lay eggs.

In rare cases, the larvae can feed on people, the Centers for Disease Control and Prevention states.

These infestations can be very painful and can cause serious, potentially fatal damage to their hosts by causing myiasis, a parasitic infection of fly larvae in human tissue. 

Risk factors and prevention

Screwworms are often found in South America and the Caribbean.

“People who travel to these areas, spend time among livestock animals, sleep outdoors and have an open wound are at greater risk of becoming infested with NWS,” says the CDC.

“Wounds as small as a tick bite may attract a female to feed.”

People who are immunocompromised, very young or very old, or malnourished are also at a higher risk of infection, the above source stated.

Those who have had recent surgery are also at a higher risk, “as the flies will lay eggs on open sores,” according to the CDC.

Potential impact

If another outbreak were to occur in the U.S., “pets, livestock, wildlife and even humans may suffer and die from screwworm myiasis,” the USDA warned.

The USDA estimates that livestock producers in the southwestern U.S. lost between $50 million and $100 million annually due to NWS in the 1950s and 1960s until it was successfully eradicated.

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“Presumably, these higher losses in the Southwest were due to higher livestock populations, larger geographic area and/or greater potential for NWS to overwinter,” stated the report.

While the USDA eradicated NWS in 1966, there was an outbreak contained within the Florida Keys in 2016. It affected only in the endangered deer population and was eradicated by March 2017, per APHIS.

Greg Wehner contributed reporting.