Lately, I’ve been reading the book Spillover by David Quammen. It’s a fascinating look at “zoonoses,” the term for contagions from animals that spill over into human populations. Illnesses such as HIV, ebola, and coronaviruses (like the one that caused the COVID pandemic) fall into this category.
As the human population has exploded over the past few centuries and we have encroached ever further into areas formerly uninhabited by humans, the crossover of sicknesses from animals to people has become more common. Disease-causing organisms which have long circulated in animal populations can make a foray into people and cause human suffering and economic losses whenever humans encroach onto formerly wild spaces. No one knows how many yet-undiscovered germs lurk in animal populations which could cause new human diseases, and as such, it’s hard to be prepared for these threats.
The idea of these novel bugs from far-flung corners of the world at the interface between human settlements and wilderness may capture your imagination (as it has mine throughout Quammen’s book), but biological threats do not have to be novel or exotic to catch us unprepared. Frequently, the biological threats governments struggle to contain are not actually mysterious or new at all. Some are familiar, well understood, and quite preventable. The comeback story of the New World screwworm offers a clear example.
New World Screwworm’s Eradication and Resurgence
In June, the US Department of Agriculture confirmed that the New World screwworm had returned to the United States. It was first found in a calf in Texas 50 miles from the US-Mexico border, then reported in more cattle and a dog in the weeks that followed.
Screwworms are parasites which can take humans, livestock, pets, and even birds as their host. Screwworm flies can lay eggs in skin scrapes as small as tick bites or in the mucous membranes of warm-blooded animals or people. Once hatched, the larvae burrow into the skin and feed on living tissue before leaving their host to transform into adult flies. Screwworm infection can prove deadly if left untreated.
Screwworms also impose substantial costs on agriculture. Infested livestock may die or require veterinary treatment, while ranchers must spend more time and money inspecting animals, treating wounds, and controlling outbreaks. Infestations can also reduce meat and milk production. The US Department of Agriculture estimates that a widespread outbreak in Texas alone could cost $1.8 billion annually in livestock deaths and treatment costs.
The screwworm detections in Texas were noteworthy because the US successfully eradicated the New World screwworm from its soil in the 1960s by releasing millions of sterile male flies over the course of years. These sterile flies mated with females and crowded out fertile males in such large numbers that future generations became successively smaller until eventually, local populations were eradicated.
The US continued the effort after local eradication, releasing sterile flies south of our border in partnership with central American countries. By the early 2000s, the campaign had pushed screwworm south to Panama, and then even Panama was formally declared screwworm-free in 2006. Governments working together had rid the entire North American continent of the parasite.
The program was costly. The production and release of sterile flies cost about $1.3 billion across North America over roughly 55 years. But the economic benefits of eradication were much larger. An estimate updated in 2021 places annual benefits to livestock producers in the United States at $896 million, with an additional benefit of $329 million in Mexico and $88 million in Central America. That study identifies livestock producers as the primary direct beneficiaries and describes additional benefits including greater availability of livestock and dairy products, fewer meat and milk shortages, and reduced reliance on food imports. It notes that public health benefits may also exist, but those are separate from the producer benefit estimates. The US Department of Agriculture estimates that screwworm eradication also resulted in $2.8 billion in annual benefits to the wider economy, although the agency does not detail how that broader estimate is calculated or divided among different types of benefits.
Even so, as the eradication effort moved south, sterile screwworm production and release facilities in the US consolidated. Domestic facilities stopped producing sterile male flies and production of the flies moved to a Mexican facility. Given that the problem had retreated, this made sense; Mexico was a better location to invest the funds and efforts. Eventually, though, the last fly production plant in Chiapas, Mexico, closed in 2012, after the US transferred its ownership to the Mexican government. This closure left a facility in Pacora, Panama as North America's only producer of sterile flies.
For a decade afterward, all seemed well. Then, in 2023, a screwworm outbreak in Panama prompted a local state of emergency. The screwworm began to march north again, and by the following year, it was spotted in Mexico. The US Department of Agriculture formulated a response plan in 2025, then announced plans for a new facility in Texas to produce sterile flies in March 2026.
By June 2026, the screwworm had infected over 170,000 animals and 2,000 people in Central America and Mexico, causing 10 human deaths. That same month, the screwworm was detected in Texas.
The Price of Rebuilding Defenses
The old adage is that an ounce of prevention is worth a pound of cure. Although the US continued funding containment efforts in Panama long after screwworms were eradicated on US territory, regional sterile fly production became concentrated in a single facility without enough capacity to respond to the large outbreak which emerged. Now that the barrier has been breached, eradicating the screwworm in the US again will be costly.
The remaining facility in Panama can currently produce approximately 100 million sterile flies per week. This is nowhere near enough capacity for the present problem, so the US Department of Agriculture is building a $750 million facility to produce up to 300 million sterile flies per week in Texas. They are also spending $25 million to release sterile flies in Arizona, and spending $21 million on sterile fly production at a facility in Mexico which should churn out 100 million flies per week once active. These represent initial investments; once operational, the Texas facility will cost additional operating dollars each year, and fly releases will likely be needed again in Arizona in future years, too.
In addition to these efforts, the US Department of Agriculture awarded $105 million to 40 other projects meant to strengthen the US defense against screwworms. The US will also fund projects to provide Mexico traps and lures, enhance live animal inspections at ports of entry, and to audit Mexico’s animal health controls.
Preventing screwworm’s resurgence will certainly be expensive, but the potential for agricultural losses if we do not stop it extends well beyond Texas. Before eradication, the screwworm was endemic to Florida, California, Arizona, New Mexico, Oklahoma, Louisiana, Arkansas, Missouri, and Kansas.
There’s substantial crossover between that list and the list of the largest cattle inventories among US states today. Texas had 12.5 million heads of beef or dairy cattle in 2022, by far the largest inventory. Kansas ranked #3, with 6.1 million, Oklahoma #5, with 4.5 million, and Missouri #6, with 4 million.
If the U.S. federal government allows screwworm to continue claiming ground, ranchers owning a sizable share of the US cattle supply would need to spend money surveilling their cattle. The infection is generally treatable so long as it is caught early, so ranchers would be incentivized to keep a close eye on their herds. Once spotted, veterinary care and quarantining are needed, all of which raise operations costs for farms and would be likely to raise the final price of beef and dairy for consumers.
Why Governments Underinvest in Prevention
After the COVID pandemic, it might seem obvious that governments should spend more to detect and contain biological threats before they become emergencies. Screwworm shows us why that remains difficult, though. The US did not wholly abandon its prevention efforts once the problem disappeared from its soil; it maintained a sterile fly barrier south of our border for decades. But as the parasite disappeared from public view and faded from recent memory, production facilities eventually closed, leaving the system dependent on one Panamanian plant with limited capacity. This approach left the US with little margin for error and no slack capacity in the case of a reappearance of the flies.
The choice to consolidate sterile fly production to one facility may have seemed appropriate at the time. Maintaining multiple facilities would have cost more during normal years and may have been hard to justify. But as soon as the problem reemerged, we saw why capacity was needed: biological production cannot be turned on overnight. The facility the US Department of Agriculture is funding in Texas won’t open until the end of 2027.
The lesson is perhaps that successful prevention often paradoxically makes itself appear unnecessary. When prevention programs work as intended, their benefits manifest as bad outcomes that people never have to experience. In the case of screwworm, it’s livestock that don’t die, veterinary treatments not needed, cattle trade that isn’t interrupted, and beef prices that don’t rise. It’s all hypothetical and invisible. The facilities, expertise, surveillance, and institutional capacity that prevent those outcomes can start to look like wasteful public spending when their benefits are so intangible. These kinds of programs could be an easy cut for politicians looking to scale back budgets.
While as a policy analyst, I understand that prevention programs frequently have benefits which outweigh their costs, I also understand the impulse to undervalue prevention. If you recall, my introduction to this topic was the tangentially related book, Spillover. That book is filled with unfamiliar pathogens emerging from strange animals in exotic forests and caves. These frightening biological threats captured my attention readily; they appeal to a sense of discovery and danger. The book inspired me to set out looking for the next potential strange disease emerging from some distant corner of the world. Instead, I found this story about an old agricultural parasite that governments had already spent decades beating back. Though I hope this post is interesting, I know it won’t be in the same way that Spillover is. Managing old problems is not as inherently interesting as the mystery and intrigue of fighting a new one.
Because of this, a novel threat can easily generate headlines, emergency declarations, and new funding. Screwworm was an old problem we believed we’d already solved. Its long absence made continued preparedness harder to justify investment in, and now that screwworms have returned, the US must rebuild its defenses while also responding to the incursion the screwworms have already made.
The New World screwworm’s resurgence is a reminder that discovering and defeating biological threats are only one aspect of the public health challenges we face. Governments must also stay vigilant at the task of sustaining the institutions that keep old threats from becoming new emergencies, long after their original danger has faded from public memory.
