CFIA Press Release: New World and Old World screwworm

Current situation

Screwworm is not present in Canada.

On June 3, 2026, the United States confirmed a detection of a New World screwworm (NWS) infestation in a calf in Texas. Additional detections have since been reported in Texas and New Mexico.

The CFIA is monitoring the situation and taking precautionary measures to prevent the introduction of New World screwworm into Canada.

CFIA New World screwworm news release – June 5, 2026
United States Department of Agriculture current status
Import restrictions for live animals entering Canada from the United States

What is screwworm

New World screwworm (NWS, Cochliomyia hominivorax) and Old World screwworm (OWS, Chrysomya bezziana) are parasitic flies whose larvae (maggots) feed on the living tissue of warm-blooded animals.

When screwworm larvae burrow into the flesh of a living animal, they cause a painful condition known as a screwworm infestation (myiasis). If left untreated, screwworm infestations can result in serious wounds, infection and death if left untreated.

Screwworm can affect livestock, pets, wildlife, occasionally birds, and in rare cases, people.

While cases in humans have been reported, screwworm infestations are primarily an animal health concern and are not a food safety risk.

NWS and OWS flies are not present in Canada. Canada's climate is generally not suitable for the long-term establishment of these flies, particularly during the winter months. The flies prefer hot, humid environments with an air temperature between 25-30 degrees Celsius. However, they may survive for a period of time at temperatures as low as 8 degrees Celsius.

In Canada, NWS and OWS are immediately notifiable diseases under the Health of Animals Act. Laboratories are required to contact the CFIA of any suspected or confirmed infestation.

Difference between New World screwworm and Old World screwworm

The main differences between NWS and OWS are the fly species and geographic location.

Range of New World screwworm
NWS is present in parts of the Caribbean, Central and South America and Mexico.

In the United States, NWS was eradicated in 1966. A localized outbreak in Florida was successfully eliminated in 2017. In June 2026, the United States confirmed the re-emergence of NWS, with detections reported in Texas and New Mexico.

Range of Old World screwworm
OWS can be found in parts of Asia, Africa and the Middle East. OWS has never become permanently established in the Western Hemisphere.

Human health risks

Although uncommon, screwworm can affect people.

Infestations occur when a female screwworm fly lays eggs in an open wound or mucous membrane. Screwworm infestations are painful and can cause wounds that worsen over time, develop secondary bacterial infections, have a foul-smelling odour, or contain maggots.

The risk is generally low for most Canadians.

People who travel or spend time with animals in areas where screwworm is present and have open wounds may be at increased risk.

Canadians returning from travel destinations should seek medical attention if they develop symptoms consistent with screwworm infestation.

The Public Health Agency of Canada recommends that all travellers consult a health care provider at least six weeks before travel to understand the risks and appropriate precautions.

Food safety

People cannot become infested with screwworm by eating meat or other animal products. The primary concern is the health and welfare of live animals.

Canada's precautionary measures are focused on preventing the introduction and spread of screwworm in live animals. They do not affect food safety.

Import restrictions related to New World screwworm apply to certain live animals, including live cattle and horses, and are separate from food safety measures.

Signs and symptoms of screwworm in animals

Animals that may be affected by screwworm
Screwworm can affect warm blooded animals including livestock (cattle, pigs, sheep, goats and horses), wildlife, dogs, cats and less commonly, birds.

Newborn animals may be particularly vulnerable because female screwworm flies are attracted to wounds and body openings, including the navel of newborn animals.

Pets and companion animals
Screwworm can affect pets and companion animals, such as dogs and cats.

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.

Signs of screwworm infestation in animals
Adult screwworm flies can be difficult to distinguish from some fly species. Signs of infestation are usually not visible until eggs hatch and larvae begin feeding.

Signs of screwworm may 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
• Animal owners should regularly inspect their animals for wounds and signs of illness as part of routine animal care. Early detection and treatment are important to reduce the risk of severe outcomes and spreading screwworm.

New World screwworm and Old World screwworm are immediately notifiable disease in Canada under the Health of Animals Act. Animal owners must contact their veterinarian immediately if they suspect a screwworm infestation.

How screwworm spreads in animals

Female screwworm flies lay their eggs in wounds or open skin or mucous membranes such as the nose, mouth, eyes, ears, genitals. This can include the navel of newborn animals and surgical incisions.

After hatching, the larvae burrow into the animal's skin and feed on flesh. After approximately one week the larvae leave the wound, fall to the ground and can burrow into the soil to mature into adult flies to continue the life cycle.

Wounds infested by screwworms often attract other fly species, so it is common to have multiple fly species infestations in the same animal.

Diagnosing animals

Owners should contact a veterinarian immediately if they suspect screwworm.

Other fly species present in Canada can cause infestations that appear similar to screwworm. Examples include the green bottle fly (Lucilia sericata), black blow fly (Phormia regina), and blue blow flies (Calliphora species). Some of these flies can also infest wounds in live animals or may be found in existing screwworm lesions. Unlike screwworm, whose larvae feed on the living tissue of animals, many other fly species feed on dead tissue, decaying organic material, or both.

An owner or veterinarian may suspect screwworm based on clinical signs; however, laboratory identification is required to confirm diagnosis.

Treating animals with screwworm

If screwworm is detected in an animal in Canada, the CFIA will work with the veterinary community to provide guidance on the management and treatment of affected animals. This will include isolating infested animals in a location where any larvae leaving the wound can be found and destroyed. Preventing larvae from entering the environment or another host helps reduce the risk of further spread. Wounds must be cleaned, eggs and larvae removed, disposed of properly, and treatments applied to prevent reinfestation.

A veterinarian can provide advice on treatment and wound care for livestock and pets.

For complete and up to date information, please visit the CFIA website for article details https://inspection.canada.ca/en/animal-health/terrestrial-animals/diseases/immediately-notifiable/screwworm