New World Screwworm Is Back: What Livestock Producers Need to Know

The United States spent more than half a century and hundreds of millions of dollars pushing the flesh-eating New World screwworm as far from its borders as possible. Now, it is back.

The parasite, which can feed on the tissue of any warm-blooded animal and is a particular threat to livestock, wildlife and domestic animals, has re-emerged in the United States in a development that has  animal health officials and cattle producers alike in a heightened awareness mode. The U.S. Department of Agriculture Department (USDA) recently confirmed detection of the New World screwworm in Texas cattle, including the first natural incursion in U.S. cattle since 1982. For producers, the return of the pest is a reminder that eradication does not always mean permanent elimination.

As the industry braces for a new phase in the fight, Merck Animal Health is among the companies urging producers to prepare now rather than wait for a local emergency. As part of its proactive response, Merck Animal Health hosted a New World screwworm symposium. The event brought together cattle producers and veterinarians from high-risk regions. Leading researchers, academic experts, government representatives and Merck Animal Health technical team members delivered programming focused on New World screwworm preparedness, risk mitigation and coordinated strategies to help protect U.S. livestock.

The message from the symposium was consistent: New World screwworm is serious, but it is manageable with planning, surveillance and close coordination between producers, veterinarians and animal health officials.

Experts say the U.S. is likely to rely on many of the same principles that guided the original eradication effort decades ago. Because female screwworms mate only once, the strategy centers on releasing sterile males to interrupt reproduction. But success will depend not only on federal response programs. It will also depend on what happens on ranches, in veterinary clinics and across livestock corridors.

Here are seven key tips producers should keep in mind:

1. Prepare, Don’t Panic
New World screwworm is a serious threat, but not a reason for panic. Producers who plan early will be better positioned to protect animals and keep operations functioning if the pest spreads.

Preparation begins with practical steps. Operations should  review calving seasons, processing schedules, and other high-risk activities with the insect’s biology in mind. Whenever possible, producers may want to shift those activities into periods of lower fly pressure.

Wound management is also critical. Any break in the skin can become a target, including navels, castration sites, dehorning wounds, ear tags, and even tick bites. Experts recommend thinking differently about these risks and using preventative or protective products where appropriate to reduce the chance of infestation.

The message from animal health specialists is not to overreact, but to plan carefully and reduce exposure before a case occurs.

2. Eyes on Animals, Surveillance Is Essential
Routine observation remains one of the most important defenses against New World screwworm. Nothing replaces frequent, disciplined visual checks, especially for newborn animals and any animal with a wound.

The parasite is drawn to wounds and to moist, vulnerable areas such as the umbilical cord, the corners of the eye, and the genital tract. A seemingly minor wound can hide significant tissue damage beneath the surface. In some cases, the external opening may be small while the infestation underneath is extensive.

That is why producers need to know the warning signs. Unusual discharge, a foul odor, irritation, and larvae within living tissue are all red flags. In many cases, the smell of an infested animal is the first warning.

Surveillance can also be strengthened with technology. Game cameras, behavior-monitoring tags, and virtual fencing may help producers extend their reach when labor is limited. But experts say technology is a supplement, not a substitute, for close animal inspection.

3. Report First, Don’t Hide It
If New World screwworm is suspected, reporting should come first. The disease is a reportable foreign animal disease, and prompt notification is not optional.

Officials say they would rather investigate a large number of false alarms than miss a single real case. That caution reflects the stakes: if a case goes unreported, the pest can reproduce and spread, creating more infestations and a larger public and private cost.

The burden is not just on the affected producer. Every missed case increases the risk to neighboring ranches, wildlife, and the broader livestock industry. In a situation like this, delay helps the pest, not the producer.

4. Veterinarians Are Central to the Response
A strong relationship with a veterinarian will be essential in any New World screwworm response. Vets are central to wound treatment, parasite removal, pain management, and guidance on movement or quarantine procedures.

Producers should not wait for an outbreak to ask what the plan would be. The time to talk is now: How often will animals be checked? What products will be used? How will affected animals be isolated? What happens if livestock must be moved?
Those questions matter because treatment is not simply about killing maggots. It is about managing the wound, preventing further infestation, and coordinating care with animal health authorities.

5. Wildlife Cannot Be Left Out
Wildlife may also be affected, and experts warn that the consequences could be significant. In infested areas, deer and other species may suffer substantial losses, and surveillance in wildlife is more difficult than in livestock.

That makes wildlife part of the larger containment picture. Because producers and regulators have fewer tools to intervene directly in wildlife populations, protecting livestock becomes even more important. Reducing the environmental burden in domestic animals can help limit spillover into surrounding ecosystems.

6. Quarantines Are Tools, Not Punishments
If the New World screwworm spreads, movement restrictions and quarantine zones may be necessary. But producers are being urged to view those measures as containment tools, not punishments.

The goal is to protect disease-free areas while allowing commerce to continue safely whenever possible. That means structured movement protocols, inspections, treatment, and certification may all be part of the response.

Animal health officials say the intention is to preserve the speed of commerce where possible while putting safeguards in place. Business may not continue as usual, but it can continue.
Producers familiar with other livestock health threats, such as cattle fever ticks, may recognize the logic behind area quarantines and managed movement.

7. Use More Tools in the Toolbox
Today’s producers have more options than previous generations did when New World screwworm was endemic in the United States. Modern parasite control products, better surveillance tools, and stronger education efforts all improve the odds of containment.

But experts also caution against a reaction that creates another problem, such as resistance to existing parasite controls. The goal is a balanced, integrated ectoparasite program that protects animals without undermining future options.

Sterile fly release remains the cornerstone of eradication. But on the ground, success will depend on producers using every appropriate tool available and working closely with veterinarians and regulators.

A Familiar Threat, and a New Test
The return of New World screwworm is a reminder that eradicated threats can reappear. For producers, the immediate task is clear: prepare now, watch animals closely, report quickly, and work with veterinarians on a plan. The earlier the response begins, the more likely it is to protect herds, limit losses, and keep the industry moving.