The screwworm has reappeared in the United States, and scientists are tracking its spread.

The article "Meat-eating parasite has reappeared in the US, and scientists are tracking its spread" first appeared on the AZ Animals website.

A flesh-eating pest has made an unwanted comeback in the United States, prompting a response from various agencies. The New World screwworm was discovered in a calf in Texas in June 2026. We explain why this is bad news and how advanced genetic technologies and epidemiological surveillance can provide a solution.

Why worry about the New World meat worm?

New World blowfly ( Cochliomyia hominivorax) Belongs to the Calliphoridae family and is a parasitic blowfly. Instead of feeding on dead or decaying tissue, its larvae burrow into the living flesh of warm-blooded animals, including livestock and humans. Females lay eggs in open wounds or natural body orifices. By feeding, the larvae create or enlarge the wounds. This causes pain and often leads to secondary infections.

As a result, livestock can become severely ill or even die, causing significant economic losses. Historically, screwworm outbreaks have cost the U.S. agriculture industry hundreds of millions of dollars annually, and experts warn that if the current outbreak is not contained, significant economic losses could occur. A public health concern is that the larvae can also infect wounds and mucous membranes in humans, causing illness and, in extreme cases, death.

Another alarming feature of the New World screw fly is its ability to spread geographically. Adults can travel kilometers in search of hosts, but they are also carried by livestock, pets, birds, and other wild animals.

Eradication and return

Screwworm larvae are not just a modern problem. In the first half of the 20th century, they caused significant economic losses before being eradicated by 1966. This was achieved through the innovative use of the sterile insect technique (SIT). Agencies released large numbers of captive-bred sterilized males. As a result, females, which typically mate only once, were unable to produce offspring. Thanks to regional cooperation, eradication was widespread throughout Central America. However, the situation was different in South America, and the flies persisted in Panama. Unfortunately, these fly populations have always posed a potential threat of reintroduction. This likelihood has been increased by modern factors, including increasing global connectivity, cross-border livestock trade, and climate-related changes in vector distribution.

Several sporadic cases of reintroduction of screwworm larvae have been reported in the United States. These include an outbreak in the Florida Keys in 2016-2017. During an ongoing regional outbreak in Central America, in August 2025, a Maryland resident returning from El Salvador was diagnosed with the first travel-related human case of screwworm infection in decades. In response to the increased threat, surveillance measures were strengthened and a new sterile screwworm production facility was established in Texas.

Experts knew that screwworm larvae would again spread northward starting in 2023. They had reappeared in Central America and Mexico. However, they did not expect them to reach the United States so quickly. The source of the infestation in Texas is still unclear.

Learning from the mistakes of the past

To combat this emerging threat, scientists are using a combination of proven methods and new technologies. Efforts led by the U.S. Department of Agriculture (USDA) include the use of sterilizing insect techniques (SIT) alongside pesticides and animal movement quarantines. Additionally, work is currently underway to obtain approval from the U.S. Environmental Protection Agency for the use of a genetically modified, all-male strain, which is undergoing field trials. Previously, males sterilized by irradiation were used for SIT.

Furthermore, researchers from the University of California, San Diego, in collaboration with the San Diego County Health and Human Services Department (HHSA), have developed a dashboard to monitor the risk of New World screwworm infestations. This dashboard provides situational awareness of the screwworm, generating regional risk profiles. It incorporates data on confirmed sightings and media coverage, combining them with data on the suitability of the environment for the screwworm and its larvae. The advantage of the system is that it eliminates the need to wait for traditional reporting mechanisms, which involve laboratory testing, the results of which are often delayed by days or even weeks. By incorporating media reports, results are timely and can be acted upon more quickly.

How to Stay Ahead of Blowflies with One Health

It's important to be prepared to combat screwworm, and this tool allows experts to identify potential threats before they occur. They can now predict where cases are likely to occur and prepare before the fly actually appears.

Another important aspect of this approach is interagency teamwork. This is a pest that affects both human and animal health. Therefore, a One Health program, which recognizes the interconnectedness of human, animal, plant, and environmental health, is crucial. Its members include epidemiologists, infectious disease specialists, veterinarians, nurses, healthcare workers, and laboratory specialists.

The screwworm program collaborates with veterinarians serving livestock, companion animals, and wildlife. It is the latest project of Resilient Shield, an initiative of the University of California, San Diego, and the Centers for Disease Control and Prevention (CDC)-funded center led by Aronoff-Spencer and her co-directors. It provides public health professionals with tools to combat this threat, bringing together the expertise of public health professionals, academia, and industry.

The article "Meat-eating parasite has reappeared in the US, and scientists are tracking its spread" first appeared on the AZ Animals website.