BUENOS AIRES/CHICAGO — According to four sources familiar with the situation, a case of the flesh-eating screwworm parasite has been discovered in a person in Maryland who had recently traveled to the United States from Guatemala.

The individual, who received treatment in Maryland, marks the first confirmed case this year in the US of the New World screwworm. This parasite, which feeds on cattle and other warm-blooded animals while they are alive, is part of an escalating outbreak moving northward from Central America and southern Mexico.

Beth Thompson, the state veterinarian of South Dakota, revealed to Reuters on Sunday that she was informed of the case within the last week by a source with direct knowledge of the situation in Maryland.

A second source, who preferred to remain anonymous, mentioned seeing emails sent by an executive of the industry group Beef Alliance on August 20 to approximately two dozen individuals in the livestock and beef sectors. The emails informed them that the US Centers for Disease Control and Prevention had confirmed a human case of screwworm in Maryland, involving a person who had traveled from Guatemala to the US.

During a call last week with the CDC, state veterinarians were made aware of the human case in Maryland, as confirmed by one source. Additionally, a Maryland state government official verified the case.

CDC deferred questions to Maryland in a call with state animal health officials, as mentioned by Thompson.

"We learned about this through other channels and then had to approach the CDC to provide us with details," she stated. "Initially, they were not forthcoming at all and redirected us back to the state for confirmation regarding the situation of this traveler."

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

This confirmed case follows shortly after US Department of Agriculture Secretary Brooke Rollins and other government officials visited Texas to announce the construction of a sterile fly facility as part of the ongoing efforts to combat this pest.

Understanding Screwworms

Screwworms are parasitic flies whose females lay eggs in wounds on warm-blooded animals. After hatching, the screwworm larvae burrow through living flesh using their sharp mouths, ultimately leading to the death of the host if left untreated.

The feeding behavior of the maggots resembles a screw being driven into wood, hence the name "screwworms."

While screwworms can be devastating to cattle and wildlife, infestations in humans are rare. However, infestations in either animals or humans can be fatal.

Treatment is laborious and involves the removal of hundreds of larvae along with thorough wound disinfection. Early treatment of infestations is typically survivable.

According to the August 20 emails from the Beef Alliance executive, the positive human case of New World screwworm in Maryland was confirmed on that day.

A subsequent email from the same group stated that due to patient privacy laws, no further details were available. The affected person received treatment, and preventive measures were implemented in the state.

Market analysts asked a livestock economist at Texas A&M University to prepare a report for Rollins on the industry impacts of the border closure to Mexican cattle. This closure, in place since November to prevent the arrival of screwworm in the US, has largely affected the industry.

One of the emails mentioned that the CDC was obligated to report the positive New World screwworm case to both Maryland health officials and the Maryland state veterinarian, and that the CDC also informed other agriculture stakeholders.

The beef industry executive expressed hope that the limited awareness among industry representatives and state veterinarians would reduce the likelihood of the case being leaked, thus minimizing market impact.

Requests for comments from a representative of the Beef Alliance went unanswered.

Impact on Beef and Cattle Futures

A confirmed screwworm case in the US is likely to unsettle the beef and cattle futures market, which has experienced record-high prices due to tight supplies. The US cattle herd is currently at its smallest size in seven decades.

The case and the lack of transparency surrounding it could pose a political challenge for Rollins, who had made repeated promises to keep screwworm out of the country. Despite setting traps and deploying mounted officers along the border, the USDA has faced criticism for not acting swiftly to increase fly production.

In June, Rollins initially 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. The facility is expected to be operational in two to three years.

A USDA spokesperson did not respond immediately to requests for comments.

Mexico has also taken measures to restrict the spread of this pest, which can cause livestock deaths within weeks if left untreated. In July, the Mexican government disclosed the construction of a $51 million sterile fly production facility in the southern part of the country.

Currently, the only operational plant is located in Panama City and can produce a maximum of 100 million sterile screwworm flies per week. The USDA estimates that releasing 500 million flies weekly would help push the fly back to the Darien Gap, the rainforest stretch between Panama and Colombia.

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

In July, Mexico reported a new case about 370 miles (596 km) south of the US border in Ixhuatlan de Madero, Veracruz. This prompted the USDA to halt livestock trade through southern ports of entry, following previous import halts in November and May.

On average, the US imports over a million cattle annually from Mexico for fattening in feedlots and beef processing.

Screwworms were eradicated from the US in the 1960s by releasing large numbers of sterilized male screwworm flies. These sterile flies mated with wild female screwworms, leading to the production of infertile eggs.

The USDA estimates that a screwworm outbreak could cost the Texas economy, the largest cattle-producing state in the US, about $1.8 billion due to livestock deaths, labor expenses, and medication costs.