New World Screwworm: What Canadian Livestock Producers and Horse Owners Need to Know

The New World screwworm (NWS) has become one of the most closely watched animal health issues in North America. While the parasite is currently confined to Mexico and portions of the southern United States, recent detections in Texas and New Mexico have prompted increased surveillance and border measures in both Canada and the United States. Canadian livestock producers and equine owners should understand what screwworm is, why it is receiving so much attention, and how to reduce the risk of introduction and spread.

What is New World Screwworm?

New World screwworm (Cochliomyia hominivorax) is not a typical maggot. Unlike most fly larvae that feed on dead or decaying tissue, screwworm larvae feed on the living tissue of warm-blooded animals. This makes the parasite particularly damaging and potentially fatal if infestations are not identified and treated quickly.

Adult female flies are attracted to wounds, surgical sites, navels of newborn animals, tick bites, branding wounds, dehorning sites, and even natural body openings. The female lays eggs at the wound site, and within hours the larvae hatch and begin burrowing into living tissue. As they feed, the wound enlarges, attracting additional flies and causing severe tissue damage.

All warm-blooded animals are susceptible, including:

·       Cattle

·       Horses

·       Sheep and goats

·       Swine

·       Wildlife

·       Pets

·       Humans (rarely)

Why Is Screwworm Back in the Headlines?

For decades, New World screwworm was considered eradicated from the United States through one of the most successful insect-control programs ever implemented. The program relied on the release of millions of sterile male flies, preventing reproduction and eventually eliminating the parasite from North America.

However, since 2023, screwworm has spread northward through Central America and Mexico. The ongoing outbreak has resulted in thousands of cases across affected countries and has recently crossed into the southern United States. In June 2026, USDA officials confirmed the first U.S. cases associated with the current outbreak in Texas, followed by additional detections in Texas and New Mexico.

In response, U.S. authorities have launched an aggressive eradication effort that includes animal movement controls, surveillance programs, and large-scale releases of sterile flies.

Flesh-eating screwworm detected 25 miles from US border in Mexico, USDA says | Reuters

What Is Canada Doing?

The Canadian Food Inspection Agency (CFIA) has implemented temporary import restrictions and enhanced safeguards to reduce the risk of New World screwworm entering Canada. These measures include restrictions affecting livestock and horses from affected areas of the United States, as well as existing controls on susceptible animals imported from Mexico.

Canada remains free of New World screwworm, and there have been no reported cases in Canadian livestock. Nevertheless, the CFIA considers vigilance and early detection critical to maintaining Canada’s status.

The Canadian Food Inspection Agency implements disease control measures to prevent spread of New World screwworm - Canada.ca

What Should Producers and Horse Owners Watch For?

Early detection is the most important defence against screwworm.

Animals with screwworm infestations may show:

·       Wounds that fail to heal

·       Rapidly enlarging wounds

·       Bloody or foul-smelling discharge

·       Visible maggots deep within tissue

·       Irritation and excessive licking or rubbing

·       Depression, weight loss, or reduced performance

·       Reduced nursing in young animals

Unlike ordinary fly strike, screwworm larvae typically penetrate deeply into healthy tissue and can cause significant damage in a short period of time.

Practical Steps to Reduce Risk

Although the current risk to Canadian producers remains low, good biosecurity and wound management practices are essential.

1. Inspect Animals Regularly

During fly season, routinely inspect animals for wounds, especially:

·       Calving and foaling sites

·       Castration wounds

·       Dehorning sites

·       Branding locations

·       Ear tags

·       Tick-infested areas

Pay particular attention to newborn animals, which may have vulnerable navels.

2. Treat Wounds Promptly

Clean and treat wounds quickly to reduce fly attraction. Delayed treatment creates ideal conditions for many fly species, including screwworm if it were introduced.

3. Strengthen Fly Control Programs

Maintain effective fly management through:

·       Manure management

·       Fly traps and baits

·       Appropriate insecticide use

·       Good sanitation around barns and handling facilities

While these measures alone will not stop screwworm, they help reduce overall fly pressure and improve herd health.

4. Be Cautious with Imported Animals

Producers purchasing livestock or horses that have recently travelled internationally or through affected regions should carefully review health documentation and monitor animals closely following arrival. Current CFIA import requirements are designed to minimize this risk.

5. Contact Your Veterinarian Immediately if You Suspect a Case

Any unusual wound containing maggots should be examined promptly by a veterinarian. Early reporting and diagnosis are essential components of Canada’s preparedness strategy.

New World Screwworm Outbreak | New World Screwworm | CDC

What Does This Mean for Canadian Producers?

The appearance of New World screwworm in the southern United States serves as an important reminder that animal health threats can move quickly across borders. The good news is that Canada remains free of the parasite, and strong surveillance, import controls, and international cooperation are helping reduce the likelihood of introduction.

For now, Canadian producers do not need to be alarmed, but they should be informed. Consistent wound management, strong biosecurity practices, effective fly control, and prompt veterinary attention for suspicious lesions remain the best tools for protecting livestock and horses.

As the situation evolves, producers should monitor updates from the CFIA, provincial veterinary authorities, and their herd veterinarians for the latest guidance.

Bottom Line

New World screwworm remains a serious livestock pest capable of causing significant economic and animal welfare impacts. While current cases are confined to Mexico and the southern United States, Canadian livestock producers and horse owners should use this opportunity to review wound management and biosecurity practices. Awareness and early detection are the keys to keeping this destructive parasite out of Canada.