By Kathy Simmons, DVM, Chief Veterinarian, National Cattlemen’s Beef Association
The U.S. cattle industry has not faced the threat of New World screwworm (NWS) for over 60 years. Currently, the NWS fly, about the size of a common housefly, has migrated across Central America from Panama and entered Mexico.
Live cattle trade to the U.S. from Mexico was halted on Nov. 22, 2024, after a cow in the southern Mexican state of Chiapas was found to have NWS myiasis. Live cattle trade with Mexico will only resume with established NWS mitigation protocols, the holding of Mexican cattle for preventive treatments, and multiple inspections of Mexican cattle on both sides of the border.
At the time of this article, NWS has not been found in the U.S., but this harmful pest can travel on humans, vehicles, pets, livestock and even on some wildlife species — all of which increase the likelihood it could eventually enter our country.
What is NWS Myiasis?
NWS myiasis is the infestation of NWS larvae or maggots that feed on the living tissues of all warm-blooded animals, including humans, and rarely birds. Adult female flies lay their eggs, often as many as 200 to 300 eggs at a time, at the edges of wounds on animals or at the mucous membranes or body orifices. Within 12 to 24 hours the eggs will hatch, and larvae emerge to feed on living flesh by burrowing into tissue, tearing at the tissue with their hook-like mouthparts, like a screw being driven into wood and hence, their name.
The larvae can be difficult to detect for the first 24 to 48 hours, but as larvae feed on tissue, the wound enlarges and drains a serosanguineous fluid. There is severe inflammation and secondary infection as well as the stench of necrotic tissue.
Screwworm larvae pass through three stages (or instars), and they will reach maturity about five to seven days after the eggs hatch. At maturity, the larvae stop feeding and fall to the ground where they burrow and pupate to become adult flies. Adult flies live for two to three weeks in the field. Females mate only once in their lifetime.
Diagnosis, Treatment and Prevention of NWS Myiasis
Laboratory diagnosis of NWS is usually made by identification of the parasites under the microscope. NWS is a foreign animal disease that is reportable to state animal health authorities and to USDA-APHIS. The U.S. is responsible for reporting NWS to the World Organization for Animal Health and to our trading partners.
Before collecting or sending any samples from animals with a foreign animal disease, the proper authorities should be contacted. Samples should only be sent under secure conditions and to authorized laboratories. NWS can infest humans, so samples should be collected and handled with proper precautions. Larvae should be removed from the wound prior to treatment and placed in 80% ethanol for transport to the lab. Formalin should not be used.
Treatment for NWS myiasis generally includes cleaning and debriding the wounds and applying organophosphate insecticides, which are effective against newly hatched larvae, immature forms and adult flies. Carbamates and pyrethroids are also effective against larvae. Antibiotics are indicated if an infection is present. Livestock can also be protected by regular spraying or dipping with insecticides, or by subcutaneous injections of ivermectin and related compounds.
In endemic areas, animals must be inspected for screwworms every few days. NWS myiasis is often fatal in untreated cattle within 14 days.
Whenever possible, procedures that leave wounds (castration, dehorning, branding, ear tagging) should not be performed during screwworm season, and sharp objects should be removed from livestock pens. No vaccine is currently available for NWS.
Eradication from a Region
Screwworms can be managed by repeatedly releasing sterile male flies that mate with wild NWS female flies to produce unfertilized eggs. This process is called sterile insect technique and leads to a reduction in screwworm numbers and eventual eradication.
In addition, infested animals in a region are treated and their movements are controlled. Currently, the U.S. and Panama operate an NWS sterile male fly production facility in Pacora, Panama, through the Panama-U.S. Commission for the Eradication and Prevention of Screwworm, or COPEG, which produces 100 million sterile male flies per week.
The National Cattlemen’s Beef Association is committed to informing cattle producers and their veterinarians about current animal health risks and advocating for cattle health issues in Washington, D.C., on behalf of the U.S. cattle industry.
Click here for more resources.
More information on Bovine Veterinarian:
Protecting the Herd from New World Screwworm
