First human screwworm case in US traced to person in Maryland who traveled from Guatemala, sources say

Buenos Aires/Chicago: A case of the flesh-eating screwworm parasite has been identified in a person in Maryland who had traveled to the United States from Guatemala, according to four sources familiar with the situation.

The person, who received treatment in Maryland, is the first confirmed case in the US of New World screwworm, a parasite that eats cattle and other warm-blooded animals alive, since an outbreak began to escalate and move northward from Central America and southern Mexico late last year.

Beth Thompson, South Dakota's state veterinarian, told Reuters on Sunday that she was notified of the case within the last week by a person with direct knowledge of the Maryland case.

A second source, who asked not to be identified, said they had seen emails sent by an executive of the industry group Beef Alliance on August 20 to about two dozen people in the livestock and beef sectors, informing them that the U.S. Centers for Disease Control and Prevention had confirmed a human case of screwworm in Maryland in a person who had traveled to the U.S. from Guatemala.

State veterinarians learned about the human case in Maryland during a call last week with the CDC, according to one source. A Maryland state government official also confirmed the case.

CDC deferred questions to Maryland on a call with state animal health officials, Thompson said.

“We found out via other routes and then had to go to CDC to tell us what was going on,” she said. “They weren’t forthcoming at all. They turned it back over to the state to confirm anything that had happened or what had been found in this traveler.”

A CDC spokesperson and a spokesperson for the Maryland Department of Health did not immediately respond to requests for comment.

The confirmed case comes just over a week after U.S. Department of Agriculture Secretary Brooke Rollins and other government officials traveled to Texas to announce plans to build a sterile fly facility there as part of efforts to combat the pest.

What are screwworms?

Screwworms are parasitic flies whose females lay eggs in wounds on any warm-blooded animal. Once the eggs hatch, hundreds of screwworm larvae use their sharp mouths to burrow through living flesh, eventually killing their host if left untreated.

The maggots' feeding is similar to a screw being driven into wood, giving the pests their name.

Screwworms can be devastating in cattle and wildlife, and rarely infest humans, though an infestation in either an animal or a person can be fatal.

Treatment is onerous, and involves removing hundreds of larvae and thoroughly disinfecting wounds. But infestations are typically survivable if treated early enough.

The August 20 emails from the Beef Alliance executive, whose content was shared with Reuters, said the positive human case of New World screwworm in Maryland had been confirmed that day.

A follow-up email from the same group said that due to patient privacy laws, there were no other details available. The person was treated and prevention measures were implemented in the state, the email said.

A livestock economist at Texas AM University had been asked to prepare a report for Rollins on the impacts to industry of the border closure to Mexican cattle, according to the emails, a measure that has largely been in effect since November to prevent the arrival of screwworm to the United States.

The CDC was required to report the positive New World screwworm case to both Maryland health officials and the Maryland state veterinarian, one of the emails said, adding that the CDC also notified other agriculture stakeholders.

"We remain hopeful that, since awareness is currently limited to industry representatives and state veterinarians, the likelihood of a positive case being leaked is low, minimizing market impact," the beef industry executive wrote.

A representative for the Beef Alliance did not respond to requests for comment.

Impact on beef and cattle future

A confirmed screwworm case in the U.S. would likely rattle the beef and cattle futures market, which has seen record-high prices because of tight supplies, with the U.S. cattle herd at its smallest size in seven decades.

The case and the lack of transparency around it could also
present a political challenge for Rollins, who pledged
repeatedly to keep screwworm out of the country. The USDA has
set traps and sent mounted officers along the border, but it has
faced criticism from some cattle producers and market analysts
for not acting faster to pursue increased fly production.

Rollins first announced plans for a sterile fly facility at
Moore Air Force Base in Edinburg, Texas -- near where a
production facility to combat screwworm operated during the last
major outbreak 50 years ago -- in June, saying that the facility
would take two to three years to come online.

A spokesperson for the USDA did not immediately respond to a
request for comment.

Mexico has also taken efforts to limit the spread of the
pest, which can kill livestock within weeks if not treated. The
Mexican government said in July that it started to build a $51
million sterile fly production facility in the country's south.

The sole operating plant is in Panama City and can produce a
maximum of 100 million sterile screwworm flies each week. The
USDA has estimated that 500 million flies would need to be
released weekly to push the fly back to the Darien Gap, the
stretch of rainforest between Panama and Colombia.

Screwworms have been traveling north through Mexico from
Central America since 2023. They are endemic in Cuba, Haiti, the
Dominican Republic and countries in South America, according to
the USDA.

Mexico reported a new case about 370 miles south of the U.S.
border in Ixhuatlan de Madero, Veracruz, in July. The USDA
immediately ordered the closure of livestock trade through
southern ports of entry, after previously halting imports in
November and May.

The U.S. typically imports over a million cattle from Mexico
a year to fatten in feedlots and process into beef.

Screwworms were eradicated from the United States in the
1960s when researchers began releasing massive numbers of
sterilized male screwworm flies that mate with wild female
screwworms to produce infertile eggs.

The USDA has estimated a screwworm outbreak could cost the
economy in Texas, the biggest U.S. cattle-producing state, about
$1.8 billion in livestock deaths, labor costs and medication
expenses.