Updated October 28, 2025
NCCID Disease Debriefs provide Canadian public health practitioners and clinicians with up-to-date reviews of essential information on prominent infectious diseases for Canadian public health practice. While not a formal literature review, information is gathered from key sources including the Public Health Agency of Canada (PHAC), the USA Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO) and peer reviewed literature.
This disease debrief was prepared by Kevin Yie. Questions, comments, and suggestions regarding this disease brief are most welcome and can be sent to nccid@manitoba.ca.
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Questions Addressed in this debrief:
- What are important characteristics of New World Screwworm?
- What is happening with current outbreaks of New World Screwworm?
- What is the current risk for Canadians from New World Screwworm?
- What measures should be taken for a suspected New World Screwworm case or contact?
What are important characteristics of New World Screwworm (NWS)?
Cause
New World Screwworm Myiasis (the parasitic infestation of a living animal by fly larvae) is an infection caused by the New World Screwworm (NWS) fly (Cochliomyia hominivorax). The disease is transmitted when a female fly lays its eggs on an open wound. Its larvae will then hatch and infest the wound, undertaking obligate myiasis to complete its parasitic lifecycle. The lifespan of the New World Screwworm fly is around 20 days. The incubation period following a female New World Screwworm laying its fertilized eggs is short at 12-24 hours. When hatched, the larvae will feed on its host for 5-7 days before dropping to the ground to pupate.
While primarily a zoonotic disease, NWS can also infect humans, given the right circumstances. The first recorded case of NWS was discovered in 1858 from a human sample collected on Devil’s Island near French Guiana. Since then, case reports have indicated the parasite to be endemic to South and Central America.
In North America, through the use of Sterile Insect Technique (SIT), the New World Screwworm species was thought to have been eradicated in wildlife and livestock in the USA by the 1960s and in Mexico by the 1970s. However, in 2025 there was a resurgence of NWS in Mexican cattle, and the first-ever native human case of NWS in North America was reported. This discovery indicates that the parasite has migrated North due to climate change. Both the American Food and Drug Administration (FDA) and the Canadian Food Inspection Agency (CFIA) have warned about the importation of livestock products infested with NWS, with the former having already banned animal imports from Mexico in 2025 to limit resurgence.
USDA: Southeastern United States Collection: Screwworm Eradication Program Records
CDC: Resurgence of New World Screwworm in the Americas: What Healthcare Providers Need to Know
Signs and Symptoms
New World Screwworm is a very painful disease, characterized by the presence of maggots/larvae around or in a wounded area. If the wound is proximal to a patient’s nose, eyes, or mouth, larvae can also migrate into these orifices.
The severity of NWS symptoms can vary based on the stage of the parasite’s lifecycle. Symptoms include pain, bleeding from the wound(s), a lack of natural healing in the wound(s), and a foul-smelling odor around and in the site(s) of infection. A patient is also likely to feel the movement of larvae in the wounded area(s) and may be able to see NWS larvae, depending on the location of the wound. Secondary bacterial infections may also occur and should be ruled out. Symptoms will vary depending on the etiology and pathogenesis of the secondary infection but commonly include fever and chills.
CDC: About New World Screwworm Myiasis
Severity and Complications
New World Screwworm Myasis is non-fatal if the infestation is discovered early and treated. The disease can cause long lasting severe pain, even after treatment, due to nerve damage. Further complications can also occur if the larvae burrow into vital organ areas, particularly when the site of infection is proximal to an organ – for example, from the scalp to the brain, in one reported case. Complications can also occur due to secondary opportunistic infections. There have been two reported human deaths due to NWS infection, both in Costa Rica, in elderly individuals.
The BMJ: New World screwworm: Nicaragua confirms 30 human cases of flesh-eating parasite
Epidemiology: general
The primary vectors of New World Screwworm are livestock, pets, and wildlife. Human cases represent less than 1% of all NWS case reports.
In Canada, there have been zero reported cases of native New World Screwworm infections, due to the country’s colder and non-tropical environment, which is unsuitable for the flies’ lifecycle. NWS still poses a potential risk to the country’s agricultural system through the importation of livestock and animals infected with the parasite. There have been no reported animal cases of NWS from the importation of livestock from high-prevalence countries into Canada. There has been one case of a Canadian citizen contacting the NWS while traveling aboard.
The BMJ: New World screwworm: Nicaragua confirms 30 human cases of flesh-eating parasite
Laboratory Diagnosis
Diagnosis of New World Screwworm requires differentiating its larvae from other parasitic species. Larvae removed from the infected area(s) of a patient should be submerged into a 70% ethanol solution to preserve the specimen for analysis. Other preservative solutions, such as 5-10% formalin, may be used as an alternative, but are not preferred.
NWS maggots are relatively large compared to other larvae species, growing up to 6.5 mm to 17 mm in length following the feeding cycle. Like many other larvae species, they possess a muscidiform shape that tapers anteriorly and truncates posteriorly. NWS larvae also have encircling bands of short 1- 2- and 3-pointed spines on each body segment.
Unlike other larvae, the tracheal trunks of the NWS larvae lead from its posterior spiracles. Its pigment color ranges from dark brown to black, with the pigmented portion extending anteriorly across at least two full-body segments (on average 3-4). Secondly, the peritreme of the posterior spiracle of the NWS larvae is not closed at the bottom.
American College of Laboratory Animal Medicine: MYIASIS (Muscoidea, Oestroidea)
CDC: Lab Identification of New World Screwworm
Transmission
Female New World Screwworm flies are attracted to the wounds and orifices of live warm-blooded animals. NWS infestation begins when a female fly lays its fertilized eggs on the susceptible site(s) of infection. These eggs eventually hatch into larvae and begin to feed on the nearby flesh. After feeding, the larvae drop to the ground, pupate, and emerge as New World Screwworm flies, completing the parasitic cycle.
CDC: Clinical Overview of New World Screwworm Myiasis
Prevention and Control
Individual prevention of New World Screwworm infection is dependent on the protection of open wounds. When traveling outdoors in regions of high NWS prevalence, it is essential to keep open wounds clean and covered (bandages, gauze). Travelers should wear loose-fitting, long-sleeved shirts and pants, socks as additional coverage, and use insect repellent for general protection. Those visiting rural livestock areas should check for any larvae or eggs on their bodies after every visit, and should protect sleeping quarters with screens or bed nets.
Large-scale control of New World Screwworm requires interventions to arrest the mating process of NWS flies. The controlled release of sterilized male flies (Sterile Insect Technique) prevents eggs from hatching after being deposited. The use of SIT in the United States was effective in eradicating outbreaks of NWS in the past and is being used as a preventive measure for the current resurgence of the parasite.
CDC: About New World Screwworm Myiasis
AVMA: Mexico screwworm case triggers US emergency response
Vaccination
There is no vaccine available against New World Screwworm disease.
Treatment
The primary treatment of New World Screwworm involves the physical removal of larvae from the site(s) of infection. Removal is typically done with tweezers, but surgery may be needed for deeper infestations. Afterward, antiseptics and antiparasitic drugs should be applied to the site(s) of infection to prevent secondary infections or reinfection. Examination of the site(s) of infection should be done periodically (24 hrs) to confirm that no live larvae are present.
CDC: Clinical Overview of New World Screwworm Myiasis
AVMA: New World Screwworm (NWS)
What is happening with current outbreaks of New World Screwworm?
There have been no recorded cases of New World Screwworm Myiasis occurring natively in Canada. There has been one imported case, involving an elderly Canadian national who contracted NWS in a wound on his shin while on vacation in Costa Rica.
In the United States of America, no native human cases of NWS have been reported since the eradication of the parasitic fly species in 1966. An outbreak of NWS in 2017 in the state of Florida, which primarily affected domestic animals and the Florida Key deer, was eradicated using the same method.
In Central America, several countries have declared national emergencies due to the resurgence of New World Screwworms in the past two years, affecting both animals and humans. In Costa Rica, 28 human cases of NWS have been reported. In Nicaragua, 30 human cases of NWS have been reported.
The BMJ: New World screwworm: Nicaragua confirms 30 human cases of flesh-eating parasite
What is the current risk for Canadians from New World Screwworm?
The risk of New World Screwworm Myiasis is low for humans, even in endemic areas. Canadians traveling to areas of high NWS prevalence should, however, follow current prevention and control guidelines for the disease as set by public health and animal health agencies.
What measures should be taken for a suspected New World Screwworm case or contact?
There is no protocol for suspected human cases in provinces and territories. NWS is classified as an “immediately notifiable disease” by the Canadian Food Inspection Agency (CFIA) for any suspected livestock cases/contact.