The Return of the New World Screwworm: A Threat to Cattle and Beyond

A cowboy herding cattle on a ranch in eastern Oregon. Picture: Shutterstock

In a concerning development, a flesh-eating parasitic fly known as the New World screwworm (Cochliomyia hominivorax) is making a significant comeback in North and Central America. This parasite not only poses a severe risk to the cattle industry but also targets a variety of hosts, including humans and pets.

History of Eradication

This insidious fly had been eradicated from regions in Central and North America after decades of effort. But what factors are contributing to its resurgence? The New World screwworm is particularly malicious; the female fly is attracted to the scent of open wounds to lay her eggs, resulting in larvae that consume living tissue and cause extreme suffering, potentially leading to death.

During the 1960s, cattle farmers in Texas were reportedly dealing with around 1 million infestations annually. Collaborative efforts between scientists and government entities during the 1960s and 1990s utilized the fly’s biology against it, eradicating it from the US and Mexico using the sterile insect technique (SIT).

How the SIT Worked

Female screwworms mate only once before beginning to lay eggs, while males are more promiscuous. To combat the population, billions of sterile males were released from planes to prevent females from producing viable offspring. By 1982, due to a combination of this technique and favorable cool weather, the screwworm was eradicated from the US, a campaign reportedly costing around US$750 million. This successful eradication allowed for significant growth in cattle production.

The Resurgence

For many years, a facility in Panama released millions of sterile flies, acting as a barrier against the Northward spread of the New World screwworm. However, in recent times—particularly since 2022—the screwworm has begun its migration northward through Central America, with a sharp increase in cases reported in Panama in 2023. By November 2024, it had reached Mexico.

Experts hypothesize several reasons for this resurgence, including fly transport via cattle movements, warmer temperatures increasing fly survival rates, and potential adaptations in mating behavior that help females avoid sterile males.

Impact and Response

Currently, approximately 17 million cattle in Central America are at risk. The threat escalates with Mexico having a cattle population double that of Central America. As the screwworm spreads towards the US, around 14 million cattle in states like Texas and Florida are now vulnerable.

Humans are also at risk; at least eight cases of human infestations have already been documented in Mexico since April.

Governmental Response

In response to this alarming situation, the US government has temporarily restricted live animal imports from Mexico. Collaborative efforts among the governments of the US, Central America, and Mexico are underway to enhance surveillance and work towards eradicating the screwworm through increased sterile fly releases.

Currently, over 100 million sterile pupae are produced and safely sterilized weekly in Panama—a joint venture funded by the US Department of Agriculture (USDA) and Panama’s Ministry of Agriculture Development. Nonetheless, experts suggest that a successful eradication campaign may require several times this number of sterile flies.

Challenges Ahead

Production, sterilization, and release processes are time-consuming and may not yield immediate reductions in wild populations. Historical evidence indicates that integrated controls, including anti-parasitic veterinary medicines, are essential for managing infestations as they arise.

Surveillance also remains crucial but is challenging due to the current generation of veterinarians and farmers lacking direct experience with screwworm infestations.

Finally, ongoing climate change raises questions regarding future weather conditions that previously facilitated successful eradication efforts. Further research is critical to adapt current strategies to these shifting environmental factors.

Written by Hannah Rose Vineer, Senior Lecturer at the Institute of Infection, Veterinary & Ecological Sciences, University of Liverpool, and Livio Martins Costa Junior, Professor of Parasitology