New World screwworm (NWS, Cochliomyia hominivorax) and Old World screwworm (OWS, Chrysomya bezziana) are parasitic flies whose larvae (maggots) eat the living tissue of warm-blooded animals. When screwworm fly larvae (maggots) burrow into the flesh of a living animal, they cause serious, often deadly damage. Screwworms can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people.
NWS and OWS flies are not found in Canada and cannot survive winter temperatures in Canada. The flies prefer hot, humid environments with an air temperature between 25-30 degrees Celsius.
In Canada, NWS and OWS are immediately notifiable diseases under the Health of Animals Act. Laboratories are required to contact the CFIA regarding the suspicion or confirmation of infestation.
On this page
- Range of screwworm
- Risk to human health
- Signs in animals
- Transmission
- Diagnosing animals
- Treating animals
- Additional information
Range of screwworm
The main differences between NWS and OWS are the geographic location and fly species. NWS is endemic to Cuba, Haiti, the Dominican Republic, and countries in South America with cases spreading north to Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador, and Mexico. NWS was eradicated in the United States in 1966. A small outbreak was successfully eliminated in 2017.
OWS can be found in parts of Asia, Africa and the Middle East. OWS has never become permanently established in the Western Hemisphere.
Risk to human health
Screwworms can infest humans if an adult female screwworm fly lays its eggs in an open wound or mucous membrane. Screwworm infestations are painful and symptoms can include wounds/sores that do not heal or worsen over time and have a foul-smelling odour. In some cases, maggots can be seen around or in the open wound/sores.
The risk is generally low for most Canadian travellers for NWS and OWS. People who travel to or spend time with animals in areas known to have NWS or OWS and have open wounds are at an increased risk.
The Public Health Agency of Canada recommends that all travellers consult a health care provider at least six weeks before travel to understand the risk of travel and how to minimise these risks.
Canadians returning from travel destinations should consult their medical professional if they develop symptoms similar to those caused by screwworm infestation.
Signs in animals
Adult screwworm flies are hard to distinguish from other flies. Signs of infestation are usually not visible until after the eggs hatch into maggots and begin to feed. Signs of screwworm infestations in livestock include discomfort, decreased appetite, lower milk production and separation from the herd. A wound that worsens over time and is accompanied by discharge or foul odor is also usually observed. Death can occur from secondary infections within a week or two.
Transmission
Female screwworm flies lay their eggs in wounds or mucous membranes. The eggs hatch into maggots and burrow into the animal's skin to feed on flesh. After feeding for about a week, the maggots leave the wound, fall to the ground and can burrow into the soil to pupate. This means that the immature maggots develop into mature adult flies capable of continuing to spread the screwworm infestation.
Wounds infested by screwworms often attract other screwworm flies, so it is common to have multiple infestations in the same animal.
Diagnosing animals
An owner or veterinarian may suspect NWS or OWS based on clinical signs, but the diagnosis can be confirmed only with laboratory testing. Other species of flies that are present in Canada can infest wounds or mucous membranes of animals and result in infestations that are similar in appearance.
Treating animals
Infested animals should be isolated in a location where any maggots that leave the wound can be found and destroyed. The maggots should be prevented from entering the environment or another host. Owners should consult a veterinarian for wound care options in pets and livestock.