Is New World Screwworm a threat to Canadian Wildlife?

TL:DR: While occasional travel-related cases in humans or short-term infestations from livestock imports may occur, climate conditions are not suitable for the fly to become established across Canada.

What is screwworm?

There has been a lot of media attention on New World screwworm as the species has potential to cause harm to humans, livestock, and wildlife; truly a “One Health” disease.

The New World screwworm is a parasitic fly whose larvae feed on the living flesh of warm-blooded animals, unlike most maggots that eat only dead tissue. Females lay hundreds of eggs in open wounds, and the hatched larvae burrow into the tissue, enlarging the injury as they grow. After about a week, they drop to the ground, pupate in the soil, and emerge as adult flies ready to repeat the cycle. Left untreated, infestations can be fatal for wildlife, pets, and livestock.

In the early part of the previous century, the distribution of New World screwworm ranged as far north as central California and Georgia on the coasts, and Missouri in the interior, with occasional cases as far north as near the Canadian border. However, the species was eradicated from the United States and Mexico after an (almost) billion dollar effort that took decades.

The eradication approach was genius: female flies only mate once, and males mate multiple times, so millions of sterilized males were released that would “mate” with females and “waste” their one opportunity to breed. It was (and still is) an effective strategy, with the countries being considered free of the fly by 1966 (US) and 1991 (Mexico).

For more information on New World screwworm follow these links:

https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm
https://www.usgs.gov/centers/nwhc/science/preparing-potential-emergence-new-world-screwworm-us
​​https://www.cdc.gov/new-world-screwworm/situation-summary/index.html
https://www.woah.org/en/disease/new-world-screwworm-cochliomyia-hominivorax/

What disease does it cause?

Feeding by the fly larvae causes a disease in mammals (and rarely birds) called myiasis, defined as: “ the parasitic infestation of the body of a live animal by fly larvae (maggots) that grow inside the host while feeding on its tissue. “ (note that this link has nasty images of infections; avoid it if you don’t like gross stuff! ).

Why is there concern now?

There has been a recent re-emergence of the fly throughout the Americas, with the World Organisation for Animal Health reporting more than 20,000 new outbreaks in the Americas. In 2016/17, there was an outbreak in Florida with cases in about 100 key deer (an endangered species), five dogs, two cats, two pet pigs, and a racoon (the fly was eradicated after the outbreak). Further, the re-emergence in livestock in Mexico led to a ban on importation of cattle to the US from Mexico. This past summer the United States announced a major plan to prevent northward expansion of the disease. From a Canadian perspective, a case was reported in a Canadian who traveled to Costa Rica, and there is concern about a potential border closure if the fly is found in US livestock.

Are there susceptible species in Canada?

The New World screwworm is a parasite of mammals and rarely birds (see Chapter 3.1.15 in the WOAH Manual of Diagnostic Tests and Vaccines for Terrestrial Animals) so any mammal in Canada, including humans and wild mammals, is potentially susceptible.

Is it a threat to Canadian wildlife?

The good news is that modeling by Uriel Mauricio Valdez-Espinoza and colleagues demonstrated that while climatic conditions in Florida and Texas put those states at high risk of fly invasion; anything at more northern latitudes had low risk. It’s simply too cold up here for the fly to become established in the wild. Nice to have good news once in a while eh?

That said, just in case it does find its way here, screwworm is an immediately notifiable disease in Canada, which means that a laboratory in Canada must report suspicion or diagnosis of the disease to the Canadian Food Inspection Agency so that control measures can be put in place.

As part of our ongoing efforts to monitor the health of wildlife in Canada, the Canadian Wildlife Health Cooperative will continue to monitor wildlife for the disease.

Photo credit:

Photo credit: Jennifer Koches, USFWS.

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