Canada implements temporary import restrictions on livestock after New World screwworm found in Texas
- Jun 29
- 5 min read
By Tamara Botting
The New World screwworm (NWS) isn’t currently present in Canada, and the Canadian Food Inspection Agency (CFIA) is working to keep it that way.
At the start of June, CFIA announced that it was implementing a temporary import restriction on livestock from parts of the United States after an NWS was found in a calf in Texas. As of time of publication, additional infestations have been found in other parts of Texas, and in New Mexico.
The NWS is a parasitic fly. The female fly lays its eggs in any openings it can find on living, warm-blooded animals, including livestock, wildlife, pets, occasionally birds, and humans, albeit rarely. The openings the female fly is looking for includes wounds from things like fighting, insect bites, shearing and docking; open skin as a result of surgical incisions or the navel of a newborn animal; and mucous membranes, such as the eyes, ears, mouth, nose and genitals.
Once the eggs hatch, the larvae (maggots) burrow into the host animal’s skin and feed on its flesh for approximately one week.
“If left untreated, screwworm infestations can result in serious wounds, infection and death,” the Government of Canada webpage on New World and Old World screwworm said.
After that, the larvae leave the wound and fall to the ground, where they burrow into the soil until they mature into adult flies, and so continue their life cycle.
The flies prefer hot, humid environments with an air temperature between 25 and 30 C, but can survive for a time at temperatures as low as 8 C.
The news release from CFIA about the new livestock import restrictions said, “While our colder climate is not hospitable for the long-term establishment of the fly in Canada, they can survive shorter periods of time in the summer months.”
The threat of NWS is taken seriously; under Canada’s Health of Animals Act, veterinarians must be contacted immediately if there’s a suspected infestation.
According to CFIA’s import restrictions as stated on its website, the livestock included under the new restriction are: equidae (horses, mules, etc.), suidae (pigs and wild boar), bovinae (cattle, bison and water buffalo), caprinae (sheep and goats), cervidae (deer, elk, moose, caribou and reindeer), and camelidae (llamas, alpacas, etc.)
The restriction applies to live animals entering Canada from affected U.S. states, and those returning to Canada from affected U.S. states. The restrictions don’t apply to meat, animal products or animal byproducts, as the NWS feed on living tissue; this is an animal health concern, not a food safety risk.
Live animals coming from a non-affected state in the U.S. are permitted to enter Canada, provided they didn’t live in or travel through an affected state – even if they didn’t stop in the state – within 21 days before being exported to Canada, and meet all of the other applicable import conditions.
Similarly, live Canadian animals returning to Canada are prohibited if they have lived in or traveled through an affected U.S. state – even if they didn’t stop in the state – within 21 days before returning to Canada. The animal would need to be moved to a non-affected U.S. state and remain there for at least 21 days before returning to Canada; again, they would also be required to meet all other applicable import conditions.
On CFIA’s website, it cautioned that several U.S. states are also taking precautionary measures to reduce the risk of NWS spreading, and “some U.S. states may also prohibit interstate movements without permits, certification and inspections. Canadian animal owners and exporters should check state movement requirements before travelling.”
Furthermore, “Animal owners and exporters cannot bring animals back to Canada using the original Canadian health certificate.”
While this information was up to date as of publication, animal owners and exporters must contact their local CFIA Animal Health District Office to confirm the current status of the NWS and the requirements for returning their animals to Canada.
Because NWS can affect any live, warm-blooded animals, the Government of Canada webpage notes, “Pet owners travelling with their animals to areas where screwworm is present should be cautious and inspect their pets for wounds or other signs of infestation. Travellers returning to Canada with pets or service animals should continue to monitor their animals and consult a veterinarian if signs or symptoms arise.”
The webpage gave a list of possible signs of screwworm infestation, which include:
wounds that worsen over time
discharge or foul-smelling odour from a wound
visible maggots in a wound
discomfort or irritation
decreased appetite
reduced milk production
animals separating from the herd
“Wounds infested by screwworms often attract other fly species, so it is common to have multiple fly species infestations in the same animal,” the Government of Canada webpage said.
While NWS feed on living tissue, many other fly species feed on dead tissue and/ or decaying organic material; laboratory identification is needed to confirm a diagnosis.
If an NWS infestation is detected, the management and treatment protocols include isolating the infested animal(s) in a location where any larvae leaving the wound can be found and properly destroyed, to prevent the larvae from entering another host or the environment to start the cycle again. The wounds would also need to be cleaned and the eggs and/ or larvae removed.
The Government of Canada webpage notes that the NWS is found in parts of the Caribbean, Central America, South America and Mexico. (The OWS has never been permanently established in the Western Hemisphere; it’s found in parts of Asia, Africa and the Middle East).
Canada currently doesn’t allow cattle, bison, sheep, goats, cervids or swine to be imported from Mexico; while horses can be imported, there are stringent import conditions that must be met.
The NWS was eradicated in the U.S. in 1966; there was a localized outbreak in Florida in 2017, but that was successfully eliminated. The current outbreak in the U.S., which started in early June 2026, is ongoing as of publication; on June 21, the United States Department of Agriculture (USDA) website was reporting 15 cases in the country (in Texas and New Mexico), 12 active and three inactive.
A July 2025 article titled ‘History of screwworms in the US,’ published on the Washington State University website noted, “There are no vaccines or biological products available for managing NWS, except for the use of sterilized male flies in the sterile insect technique (SIT).”
In this process, male flies are sterilized by gamma or x-ray irradiation and sequentially released into the wild in large numbers. When they mate with wild female flies, it will only result in infertile eggs, which reduces and eventually eradicates the population.
The article notes, “SIT is very expensive because of the cost of continuous production and aerial dispersion of sterile files. Historically, it has been considered cost effective only when used as an eradication strategy in situations where geography would favor such a program.”
To read the entire article, visit bit.ly/4eo1hgX.
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