Date: June 12, 2026
Manitoban veterinary professionals are advised of rising cases of New World screwworm (NWS) in Mexico and the United States. As of June 11, 2026, eight cases have been confirmed in Texas (6 cattle, 2 goat cases), and one case in a dog from New Mexico. These mark the first detections of NWS in the United States since 2016.
What is New World Screwworm?
NWS is a parasitic fly (Cochliomyia hominivorax) endemic in South America and many Caribbean countries. During the larval stage, maggots eat the living flesh of their warm-blooded host causing significant tissue damage and creating opportunities for secondary bacterial infections. If left untreated, an infestation of NWS can be fatal.
• Any animal or person with an open wound is at increased risk of contracting NWS.
• Livestock is at highest risk of infestation, especially at the umbilicus and wounds from dehorning and castration. NWS can also infest wildlife, pets, occasionally birds, and people.
• In Canada, NWS is a federal immediately notifiable disease.
• The USA eradicated NWS in 1966, and use of the Sterile Insect Technique created a barrier zone at Panama’s Darién Gap between Central and South America. The barrier failed in 2023, and NWS re-emerged in Mexico in late 2024.
• The Centers for Disease Control and Prevention (CDC) reports more than 170,000 cumulative cases of NWS in animals in Mexico and Central America, and more than 2000 cases in humans. Sustained, year-round transmission of the fly life cycle requires regular daily air temperatures between 25°C and 30°C and a relative humidity of 30% to 70%.
• While most regions in Canada are considered very low risk for the establishment of NWS, localized cases and seasonal establishment are still a risk to animal health and could trigger potential trade impacts if NWS was detected in Canada.
Import Restrictions for Livestock
On June 5, 2026, the Canadian Food Inspection Agency (CFIA) issued a news release announcing temporary import restrictions for livestock, including horses, from the State of Texas.
• Livestock/horses will not be permitted into Canada if they have been in Texas at any time within the previous 21 days.
• Livestock/horses from Texas are required to reside in another state for a minimum of 21 days before entry into Canada would be permitted.
• Strict import restrictions on livestock/horses remain in place for Mexico.
For additional information on livestock import requirements, see the Automated Import Reference System (AIRS) or contact your local CFIA district office.
Companion Animal Context
There are currently no import restrictions for companion animals moving into Canada from areas impacted by NWS.
• There have been over 3000 canine cases reported since the current outbreak in Mexico began in November 2024.
• As of June 8, 2026, there are currently 662 active cases of NWS in dogs in Mexico.
• Dogs present a potential risk factor for entry of NWS into Canada that may lead to temporary transmission pathways, and significant impacts to other animals and people.
• Both Credelio/Credelio CAT and NexGard/NexGard Combo are effective extra-label in cats and dogs at killing the larval stage of NWS.
Recommendations for Veterinary Professionals
• Clinical signs of NWS include swellings, foul odor, unresolving wounds, and visible burrowing maggots. Infestations with NWS (myiasis) can be very deep with secondary infections that can be fatal if untreated.
• In livestock, wounds may be in discrete locations that are not easily observed. It can also be difficult to detect infested wounds in heavy coated dog and cat breeds.
• Report any suspected cases to the CFIA, especially if there is a history of travel, and save maggots for confirmatory testing.
• Treatment includes physical removal of maggots and wound debridement. Remove all larvae and place them in 70% ethanol to kill them (never throw them outside or in the trash). It is critical that the larvae do not drop to the soil where they can complete their life cycle.
• Injectable ivermectin and doramectin may be given extra-label in livestock/horses under veterinary supervision. Topical products including F10 Antiseptic Wound Spray with Insecticide may also be considered extra-label. Other drugs including but not limited to Negasunt Powder may be available under emergency drug release. In the event of a NWS detection, treatment options would be advised by the CFIA in consultation with the Veterinary Drugs Directorate (VDD).
• Isoxazoline-based oral products have efficacy for both prevention and treatment in cats and dogs.
• Recommend and prescribe Credelio or NexGard for cats and dogs travelling from warm climates 24 hours prior to re-entry into Canada with a follow up dose in 30 days.
• Discuss potential implications for clients travelling to Mexico or Texas with livestock, horses, and pets.
Summary
NWS is a serious and life-threatening myiasis-causing parasitic fly that can infest both animals and people. Rising cases of NWS in Mexico and the State of Texas highlight the potential risk factors for disease incursion into Canada. Veterinarians remain a first line of defense to combat animal health and public health threats including but not limited to NWS. Preventative ectoparasitic treatment options remain readily available for companion animals travelling or entering Canada from warm climates.
For further information or guidance, veterinarians may contact their local CFIA district office or the office of the provincial Chief Veterinarian at 204-945-7663.
For more information on NWS and the current situation in the United States visit:
• New World Screwworm Confirmed Detections Dashboard | USDA APHIS
• Emergency Response NWS | USDA Animal and Plant Health Inspection Service
• Screwworm.gov | Unified Government Response To Protect the United State
