What is the Screwworm, Why is it a Threat, and What is the Federal Government Doing About it?

In light of Reuters revealing that there was recently a Maryland case of a New World Screwworm in a person who had traveled to Central America, the Federal government is taking steps to address the issue and prevent an outbreak. The U.S. Department of Health and Human Services (HHS) has recently authorized the Food and Drug Administration (FDA) to issue Emergency Use Authorizations (EUAs) for animal drugs to combat the New World Screwworm (NWS), a parasitic threat that has resurfaced and is progressing towards the United States. This declaration, issued on August 19, 2025, aims to protect the nation’s livestock, pets, and food supply from the potential devastation of this parasite. The New World screwworm infests warm-blooded animals, causing severe tissue damage that can be fatal, and while the risk to humans is currently low, its northward progression from Central America since 2022 has raised significant concerns for animal health and national security.

The U.S. Department of Agriculture (USDA) is also taking proactive steps, with a congressional member introducing legislation to direct the Secretary of Agriculture to study and report on strategies for combating the New World screwworm and enhancing the nation’s preparedness for potential outbreaks. This bill, H.R.3806, introduced on June 6, 2025, by Representative Monica De La Cruz, has been referred to the House Committee on Agriculture. The legislation underscores the growing concern among policymakers about the potential economic and agricultural impacts of the pest.

The New World screwworm, scientifically known as Cochliomyia hominivorax, is a parasite that was previously eradicated from North America and Central America decades ago. Historically, it caused significant losses to livestock populations, costing ranchers and consumers millions of dollars. The flies are attracted to open wounds on warm-blooded animals, where females lay their eggs. Upon hatching, the larvae burrow into the host’s flesh, consuming living tissue and fluids, leading to progressively larger wounds and attracting more flies. Without intervention, secondary infections are common, and the host often perishes. After feeding, the larvae drop to the ground to pupate and emerge as adult flies, perpetuating the cycle.

The history of screwworm eradication in the United States is a testament to scientific innovation and large-scale cooperative efforts. Records from the U.S. Department of Agriculture detail extensive programs in the mid-20th century, particularly focusing on the southeastern United States from 1932 to 1959. During this period, the screwworm spread throughout the Southeast, leading to significant livestock losses. Initial control methods included topical treatments like benzol and pine tar oil, later replaced by a more effective paste called Smear 62. However, these methods required intensive application and animal confinement.

A pivotal breakthrough came with the development of the Sterile Insect Technique (SIT), pioneered by scientists Edward Fred Knipling and Raymond C. Bushland. This method involves rearing vast quantities of screwworm flies and sterilizing them, typically through radiation. The sterile males are then released into the wild in overwhelming numbers to mate with wild females. Because the females that mate with sterile males produce no viable offspring, the overall screwworm population gradually declines and can eventually be eradicated.

Early trials of the SIT were conducted on Sanibel Island, Florida, in the early 1950s, proving successful. Further tests on Curacao, an island off the coast of Venezuela, in 1953 demonstrated the technique’s efficacy on a larger scale. Following these successes, large-scale fly production facilities were established in Florida, culminating in a full-scale eradication program that effectively eliminated the screwworm from the southeastern United States by 1959. Subsequent efforts focused on the southwestern United States.

The current authorization by HHS allows the FDA to utilize its Emergency Use Authorization mechanism to expedite the availability of animal drugs. This means that drugs already approved for other purposes or available in other countries, but not formally approved in the U.S. for screwworm treatment, can be made accessible to veterinarians, farmers, and animal health officials. This proactive measure is crucial for equipping the nation with the necessary tools to manage and potentially contain any screwworm infestations that may emerge. The FDA plans to issue further guidance on the appropriate use of these authorized products.

The resurgence of the New World screwworm highlights the ongoing importance of biosecurity and preparedness in the agricultural sector. For livestock owners and those involved in animal care, staying informed about the latest guidance from regulatory agencies like the FDA and USDA will be essential. The potential economic impact on the agricultural industry and the broader food supply chain underscores the national security implications of this persistent parasitic threat.

Article by Ken Buckler, based upon information from the U.S. Department of Health and Human Services, the US Department of Agriculture, and Congress.gov.

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