The U.S. suspended Mexican beef imports because of screwworm. Could it spread to Canada?

Jacob Shelley, health sciences professor at Western University in Paris, Ont., says Canada should be concerned about the possible spread of screwworm to Canada.

After the United States suspended imports of Mexican live beef cattle on Sunday, one Canadian public health expert says Canada should be concerned about the flesh-eating screwworm and the potential for infections that could have “far-reaching implications.”

“I think anytime we have a pest that’s spreading between animals, and can threaten an entire kind of system with risk, we ought to be concerned,” Jacob Shelley, associate professor at the School of Health Studies at Western University, said Wednesday during a video interview with CTVNews.ca from Paris, Ont.

The U.S. government first detected the pest in Mexican cattle shipments and limited the imports in late November, but it lifted the ban in February when it created protocols to assess incoming animals, The Associated Press reported.

The U.S. then announced it paused the shipments again Sunday, prompting Mexico to say Tuesday it will strictly limit cattle shipments from Central America.

The parasite is commonly found in Cuba, Haiti, the Dominican Republic and South American countries, but has spread north to Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador and Mexico.

In Canada, the parasite infected a Canadian traveller who returned from Costa Rica in recent months. Costa Rica declared a screwworm national emergency in February.

How is Canada responding?

The Canadian Food Inspection Agency (CFIA) said in an email to CTVNews.ca on Thursday that it is “monitoring” the screwworm’s spread in South America, Central America and Mexico.

The CFIA said the screwworm parasite is an “immediately notifiable disease” in Canada, and Canadian laboratories are required to contact it about any possible cases. The CFIA also said it will put in place “risk mitigation measures,” to prevent screwworm from entering the country, and “control measures” if necessary.

Canada bans the importation of cattle, bison, sheep, goats, cervids and swine from Mexico, it said. Additionally, Canada has “stringent import conditions” on importing horses from Mexico to prevent animal diseases from entering the country.

It also said Canada’s winter conditions don’t support the “over-wintering and establishment” of the parasitic fly, which is used to tropical and sub-tropical climates.

Dr. Isaac Bogoch, an infectious diseases specialist at Toronto General Hospital, said the screwworm case of the Canadian traveller shows how “better control efforts” are needed.

He said screwworm is a fly infestation rather than an infectious disease, bacteria, virus or fungal infection so there’s no vaccine. It’s dangerous because the fly can lay eggs in a wound or scrape in a human or animal that can then turn into larvae, he added.

“The reason why this is so dangerous is because the larvae eat live tissue, not dead tissue, so they can cause significant harm to any animal ... that they’re infesting,” Bogoch said in a video interview with CTVNews.ca on Wednesday.

Bogoch said a “sterile insect technique” had previously prevented the infection from spreading by breeding and sterilizing millions of flies over Panama.

“That was pretty successful in keeping flies that can cause this very, very damaging infestation from coming north into the northern part of Central America, into Mexico, and of course into the United States and beyond,” Bogoch said. “Unfortunately, over the last year, that barrier has been breached and there’s been an outbreak of New World screwworm flies.”

Now the U.S. is releasing the sterile flies in southern Mexico to help prevent the infected flies from moving north, Bogoch said.

“From a Canadian standpoint, I think it’s just important to have surveillance and ensure that we don’t see these flies introduced into Canada, but really the border is going to be south and the control efforts are really going to be south, focused on the southern part of Mexico and of course Central America, where most of the control efforts are in place,” he said.

Preventing screwworm from entering Canada could be difficult because flies are prolific breeders and Canada’s current agricultural practices allow humans and animals to frequently come into contact in large slaughterhouses and farms, Shelley said.

“The preventative kind of measures may not be in place in Canada to address this without simply restricting the import of cattle at this point,” Shelley said. “The screwworm did cause considerable havoc in the U.S. in the past ... both to the farmers and the industry associated with cattle, but also for consumers who would face an increased cost for beef.”

U.S. Agriculture Secretary Brooke Rollins said on the X social platform that it took the livestock industry 30 years to recover after the “devastating pest” invaded the United States.

While the infrastructure is in place in Canada to monitor for pests, Shelley says the “system is really ill-equipped” to prevent the spread, with the U.S. reducing inspections to monitor and identify animals at risk.

“It’s more that once this is introduced into our environment, these types of pests can be very damaging,” he said, noting the same problem occurred with the pine beetle in Canada.

What could happen if screwworm spreads to Canada?

If the screwworm spreads to Canada, Shelley says cattle could be culled, which may lead to an increase in beef prices.

“If this were to spread, it could be more catastrophic than simply a few cattle or a few humans becoming infected,” Shelley said. “(It) could affect the entire industry in Canada, so caution is warranted here.”

The U.S. pause in Mexican cattle imports already risks raising U.S. beef prices amid limited supply, Reuters reported.

The screwworm can seriously damage the tissues of any warm-blooded animal, including humans, and even cause death.

With files from The Associated Press