New International screwworm, a relating to pest that basically affects cattle and natural world, was once detected in South Texas in June 2026. It has since expanded northward in Texas, and there was once one case in New Mexico as of July 23, 2026.
The instances mark the go back of a pest that was once eliminated from the U.S. in 1966.
As of July 23, there have now not been any screwworm instances in Colorado. On the other hand, the state has a multiagency reaction plan in position in case screwworms display up.
Screwworms’ go back has heightened worry a few of the cattle trade, animal well being officers and puppy homeowners. In cattle, screwworm infestation can result in manufacturing losses and unfavourable financial affects. In pets, screwworms can result in in depth and probably life-threatening tissue injury if now not handled.
I’m a veterinarian at Colorado State College Veterinary Diagnostic Laboratories. I’ve over two decades of regulatory veterinary drugs enjoy with the U.S. Division of Agriculture and experience in main federal animal well being systems, laboratory community coordination, surveillance, epidemiology, illness outbreak reaction and diagnostics. I paintings carefully with Ashley McGrew, a veterinarian who oversees the lab’s parasitology segment and as a board-certified parasitologist is skilled to spot particular fly larvae.
New International screwworm larvae will get its title from its screwlike look. Grownup New International screwworm flies are about the similar dimension or somewhat higher than a not unusual housefly and feature orange eyes, a metal blue or inexperienced frame and 3 darkish stripes alongside their backs.
John Kucharski/USDA
What’s New International screwworm?
New International screwworms are the parasitic larvae of a species of fly that lays eggs within the residing tissue of warm-blooded animals. They get their title from the larvae’s screwlike look. Screwworm flies incessantly lay eggs in open wounds or heat, delicate spaces, similar to within the nostril, ears, genitalia or the navel of new child animals.
After the eggs hatch, the larvae feed at the host animal’s tissue. The feeding reasons a deepening of the wound, injury to the tissue and occasionally demise if now not detected in a well timed approach. Screwworms can infest cattle, natural world, pets and birds. Persons are infrequently infested.
The danger for people and maximum animals within the U.S. is low. Screwworms aren’t contagious and don’t unfold at once from animals to other people or from individual to individual. On the other hand, an individual with suspicious lesions will have to search quick clinical consideration.
What’s being accomplished about screwworms?
The USDA has been carefully tracking New International screwworms because the Sixties. Since then, the USDA, in partnership with Panama, has been using the sterile insect option to regulate screwworm populations.
This technique works via exposing screwworm larvae to gamma radiation, a selected form of electromagnetic radiation, which sterilizes the larvae. Once they develop into grownup flies, they’re launched via the hundreds of thousands in and round spaces the place screwworms had been discovered in addition to barrier zones to curb their motion. The launched sterile male flies mate with wild flies who lay infertile eggs that don’t hatch. Through the years, screwworm populations steadily lower.
After many many years of regulate, the flies started progressing northward from Central The usa in 2024. That caused the USDA to near the Mexican border in July 2025 to reside livestock and horses as a precautionary measure. The closure has had unfavourable affects at the U.S. pork trade. Since then, the USDA, natural world businesses and southern border states have performed energetic surveillance for screwworms.
The USDA has additionally evolved a powerful New International screwworm reaction playbook to coordinate federal, state and public well being efforts within the U.S.
Southern border states are actively tracking screwworms thru cattle checkpoints.
Joel Angel Juarez/Getty Photographs
Mavens have no idea precisely why the New International screwworm has reemerged in the US. Very similar to what scientists have noticed amongst illnesses carried via ticks and mosquitoes, pest habitat levels are increasing globally because of adjustments in setting, ecology and populations.
In Colorado, the possibility of the screwworm fly setting up an everlasting inhabitants is somewhat low throughout a lot of the state because of upper altitudes and freezing temperatures. On the other hand, the Colorado Division of Agriculture has been making ready an emergency reaction plan.
The plan guarantees coordinated motion between Colorado’s Division of Agriculture, Colorado Parks and Flora and fauna, the Colorado Division of Public Well being and Atmosphere and the USDA. Animals getting into Colorado from spaces with reported screwworm instances now require certificate for motion issued via an animal well being legitimate within the affected state and will have to meet enhanced well being inspection necessities.
Colorado State College Veterinary Diagnostic Laboratories’ full-service parasitology laboratory robotically identifies animal parasites in cattle, pets and natural world. As a member of the Nationwide Animal Well being Laboratory Community, which conducts surveillance and checking out for animal illness outbreaks, CSU will supply enhance to the state if screwworm is detected right here.
What can cattle and puppy homeowners do?
Farm animals and puppy homeowners will have to use pest regulate methods, similar to keeping up blank areas for his or her animals and the usage of pesticides, barn remedies and fly traps, to take a look at to stay flies clear of their animals.
Farm animals and pets will have to even be monitored for any wounds with proof of larvae. That incorporates the wound draining fluid, having a nasty scent or getting larger. Animals would possibly display indicators of discomfort if they’ve screwworms, such inflammation and restlessness, head-shaking, biting or licking at wounds, diminished urge for food, melancholy and separation from the herd.
A USDA press convention supplies updates on screwworms in Kerrville, Texas, on June 9, 2026.
USDAgov/Flickr
Farm animals and puppy homeowners will have to touch their veterinarian with any considerations or suspicion of screwworms, particularly if the animal has odd wounds or indicators of larval infestation. If detected early, animals can also be handled and entirely recuperate from infestation.
Any suspected case will have to be reported to the Colorado State Veterinarian’s Place of job or the USDA house veterinarian in fee in an instant. For suspicious instances involving natural world, touch Colorado Parks and Flora and fauna.
Screwworm is detected via discovering larvae provide on an animal. The larvae are then accrued via a veterinarian and submitted to a laboratory for identity. These days, all screwworm suspect samples are required to be submitted to USDA’s Nationwide Veterinary Services and products Laboratories for legitimate affirmation.
If New International screwworm is recognized in an animal in Colorado, state or federal officers will habits an investigation to know the way the animal was infested.
The state’s Division of Agriculture will paintings carefully with cattle homeowners to assist do away with screwworm inside the herd and save you additional unfold. Culling teams of animals is probably not used for common regulate and eradication of screwworm. On the other hand, keeping apart out affected animals and transient regulate in their motion shall be vital elements of a herd-management plan.
