Dr. Jay K. Varma is a special contributor to Healthbeat. Public health, explained: Sign up to receive Healthbeat’s free national newsletter here.
In 2025, the United States faced a series of reminders that the security of our food supply depends on public health systems that are largely invisible to the general population. Poultry and dairy production were disrupted by H5N1 avian influenza and infant formula by botulism contamination. Less visible was a parasite that once terrified American ranchers: the New World screwworm.
The resurgence of screwworm highlights the fragility of infectious disease control and the importance of sustained government cooperation across different sectors and across international borders.
What screwworm is and why it matters
The New World screwworm is caused by the larvae of a blowfly called Cochliomyia hominivorax. The adult fly targets warm-blooded animals, most often cattle, and lays its eggs in open wounds. These wounds can be large, such as those caused by branding or dehorning, or barely visible, such as a tick bite or an abrasion.
Within hours, the eggs hatch, and the larvae burrow into the flesh and feed aggressively on living tissue. (This is what distinguishes screwworm from other maggots, which eat decaying or dead tissue.) As the larvae grow, the wound expands, attracting more flies, which deposit even more eggs. The larvae cause severe pain, tissue destruction, secondary infections, and often death of the animal.
After several days, mature larvae drop to the ground, pupate in the soil, and emerge as adult flies ready to repeat the cycle on another cow. Under warm conditions, the entire process can take only a few weeks. In a herd setting, infestations can spread rapidly, devastating entire herds of cattle.
Before eradication, screwworm caused hundreds of millions of dollars in financial losses each year in the United States. Ranchers spent enormous time and money on treatment and prevention. The parasite also occasionally infected wildlife, pets, and some humans.
How screwworm was controlled in the 20th century
Beginning in the mid 20th century, U.S. scientists developed a special technique to stop screwworm reproduction. The “sterile insect technique” involved breeding millions of male screwworm flies in specialized facilities, sterilizing them with radiation, and releasing them into the wild. Because female screwworm flies mate only once, they produce no offspring after they mate with a sterile male fly. Over time, the screwworm population dies off.
Using this approach, screwworm was eliminated from the United States by the mid 1960s. The program then moved south, in collaboration with Mexico and Central American countries. By the early 2000s, screwworm had been pushed as far south of the United States as Panama. Every week, sterile flies were released to prevent northward spread from endemic regions in South America, creating a biological barrier to screwworm making its way into the United States.
This screwworm control program remains one of the most successful examples of international disease control in the Americas. To maintain it, however, requires constant effort to produce sterile flies and conduct surveillance for breakthrough infections.
Why the screwworm threat has returned
For decades, screwworm was largely absent from the United States and Mexico. That changed in 2023, when outbreaks began spreading northward through Central America and into southern Mexico. The reasons are still being investigated, but likely include interruptions in sterile fly production during the Covid pandemic, increased movement of cattle and people, and the sheer difficulty of maintaining surveillance across remote terrain.
In August, the United States confirmed its first human screwworm case in decades. The patient, a Maryland resident, had recently traveled to El Salvador and returned with larvae embedded in living tissue. The infection was treated successfully, and there was no evidence that infection was either acquired locally or transmitted onward.
While the risk to Americans of this case was low, it was considered an important warning sign. Because they are so rare, even one human screwworm infection suggests widespread prevalence in animals.
The economic stakes are particularly worrisome. Cattle herd sizes in the United States are near historic lows due to drought and rising costs. In response to the screwworm threat, the United States halted cattle imports from Mexico in 2025. Prolonged disruption could drive prices higher, an important problem because the average American consumes 60 pounds of beef each year.
What is being done to stop the New World Screwworm outbreak?
The United States and Mexico have begun to rebuild the capacity that previously stopped screwworm transmission in both countries. Sterile insect production in Panama has been expanded to release roughly 100 million flies per week. Facilities in southern Mexico are being renovated to produce tens of millions more, and the U.S. Department of Agriculture is working on additional dispersal capacity in Texas.
These efforts are expensive and slow. Experts estimate that several hundred million sterile flies per week may be needed to re-establish the barrier that once existed. At the same time, workforce reductions and loss of institutional memory are making it harder for the U.S. government to respond quickly.
New tools are also emerging. Last month, the Food and Drug Administration conditionally approved a topical antiparasitic drug for cattle that can help prevent or treat screwworm infestations. While this drug gives cattle farmers another option, there are challenges with using it; it is prescription only, has withdrawal periods, and cannot be used in all cattle.
Why this matters beyond screwworm
The return of screwworm illustrates broader lessons about public health and food security. When they work, public health protections are invisible, and, when they fail, the consequences are difficult and expensive.
Particularly with food security, these protections must embrace a One Health model, in which human, animal, and environmental sectors work in partnership. And, finally, at a time when the United States has dramatically cut its global health workforce, screwworm offers an important example about why such work remains essential.
Dr. Jay K. Varma is a physician and epidemiologist. An expert in the prevention and control of infectious diseases, he has led epidemic responses, developed global and national policies, and implemented large-scale programs that saved hundreds of thousands of lives in Asia, Africa, and the United States.
