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Screwworm infection triggers renewed US livestock surveillance

A single travel-associated human case of New World screwworm was confirmed in Maryland in early August, triggering renewed surveillance of livestock in the area.

The case marks the first confirmed detection of the parasitic fly in the US since an animal outbreak in Florida in 2017, according to the US Department of Agriculture (USDA).

The affected traveller had recently returned from El Salvador, where screwworm outbreaks are ongoing. The patient has since recovered, and the agriculture authority says there is no indication of further transmission to humans or animals.

As a precautionary measure, the USDA and the Centers for Disease Control and Prevention (CDC) have implemented a 20-mile surveillance zone around the affected area, which includes parts of Maryland, Washington DC, and Virginia. As of 26 August 2025, all traps set within the area have tested negative for screwworm flies.

“This is not cause for alarm,” the USDA stated, noting that “human risk is low and we have seen several isolated cases in recent years that have not resulted in livestock transmission”.

Screwworm impacts agriculture across Americas

The New World screwworm fly (Cochliomyia hominivorax) poses a significant threat to livestock, particularly in areas with frequent infestations in South and Central America. Female flies lay eggs in open wounds, and larvae feed on living tissue, causing severe myiasis (parasitic infestation) symptoms.

According to NGO Screwworm Free Future, if untreated, mortality rates in animals can reach up to 80%. Infestation leads to reduced meat and milk output, weight loss, and increased veterinary costs.

In South America, the NGO estimates that the screwworm is responsible for $3.5 billion in annual losses due to reduced productivity, prevention efforts, and animal deaths. In Uruguay alone, the economic impact is estimated at $150 million per year.

According to historical data from the USDA Animal and Plant Health Inspection Service (APHIS), a screwworm outbreak similar in scale to one recorded in 1976 in Texas would cost livestock producers an estimated $732 million, with an overall economic impact of $1.8 billion in 2024 terms.

In that 1976 outbreak, 1.49 million cattle and 332,600 sheep and goats were infested. The cost per case in cattle was approximately $452.14 (adjusted to 2024), with losses due to animal death, productivity decline, veterinary treatment, and additional labour.

Eradication success and renewed biosecurity risks

The US eradicated NWS within its borders in 1966, following the successful deployment of the Sterile Insect Technique (SIT). The SIT involves releasing sterilised male flies that mate with wild females, resulting in non-viable eggs.

The New York Times reported that this intervention has saved the US cattle industry $2.3 billion per year by preventing recurring infestations.

A small outbreak in Florida in 2016 affecting endangered Key deer was also eliminated using SIT by early 2017.

However, in 2022, NWS breached the “biological barrier” in Panama, a zone maintained by the US-Panama Commission for the Eradication and Prevention of Screwworm (COPEG), prompting a resurgence across Central America and southern Mexico, with affected areas spreading further North over time.

US invests $750 million in new sterile fly facility

To bolster biosecurity, US Agriculture Secretary Brooke Rollins announced a five-part plan in June, including a $750 million sterile fly production and dispersal facility in Edinburg, Texas. The facility is expected to take 18 to 36 months to become operational.

Mexico is also building a $51 million sterile fly production plant in the country’s south, as part of bilateral containment efforts.

In parallel to SIT, researchers are trialling gene-editing methods. The Institut Pasteur of Montevideo and Uruguay’s National Institute of Agricultural Research (INIA) are developing genetically modified male flies that pass on female-infertility traits. This self-limiting method could reduce the need for mass releases of sterile males.

Meanwhile, Texas agricultural officials are deploying a chemical attractant called Swormlure-5, which mimics the smell of open wounds. Trials show that it can eliminate up to 90% of flies in targeted areas within weeks. These lures are used to suppress populations ahead of SIT deployment.

Livestock sector advised to increase vigilance

While the USDA has not confirmed any livestock infections within US borders as of late August 2025, producers are urged to monitor animals for signs of screwworm infestation, including non-healing wounds, swelling, foul-smelling discharge, and visible larvae.

“Public health and safety and our joint effort to combat the northward spread of NWS from Mexico into the United States is the top priority,” the Department of Health and Human Services said in a joint statement with the CDC.

Meanwhile, the World Organisation for Animal Health (WOAH) is coordinating with regional governments to develop a new action plan for the Americas, including permanent expert groups and strategies to maintain screwworm-free zones.