New World Screwworm Reemerges in US, Spreading Northward

New World screwworm, a concerning pest that mainly impacts livestock and wildlife, was detected in South Texas in June 2026. It has since expanded northward in Texas, and there was one case in New Mexico as of July 23, 2026.

The cases mark the return of a pest that was eradicated from the U.S. in 1966.

As of July 23, there have not been any screwworm cases in Colorado. However, the state has a multiagency response plan in place in case screwworms show up.

Screwworms’ return has heightened concern among the livestock industry, animal health officials and pet owners. In livestock, screwworm infestation can lead to production losses and negative economic impacts. In pets, screwworms can lead to extensive and potentially life-threatening tissue damage if not treated.

I am a veterinarian at Colorado State University Veterinary Diagnostic Laboratories. I have over 20 years of regulatory veterinary medicine experience with the U.S. Department of Agriculture and expertise in leading federal animal health programs, laboratory network coordination, surveillance, epidemiology, disease outbreak response and diagnostics. I work closely with Ashley McGrew, a veterinarian who oversees the lab’s parasitology section and as a board-certified parasitologist is trained to identify specific fly larvae.

New World screwworms are the parasitic larvae of a species of fly that lays eggs in the living tissue of warm-blooded animals. They get their name from the larvae’s screwlike appearance. Screwworm flies often lay eggs in open wounds or warm, sensitive areas, such as in the nose, ears, genitalia or the navel of newborn animals.

After the eggs hatch, the larvae feed on the host animal’s tissue. The feeding causes a deepening of the wound, damage to the tissue and sometimes death if not detected in a timely manner. Screwworms can infest livestock, wildlife, pets and birds. People are rarely infested.

The risk for humans and most animals in the U.S. is low. Screwworms are not contagious and do not spread directly from animals to people or from person to person. However, a person with suspicious lesions should seek immediate medical attention.

The USDA has been closely monitoring New World screwworms since the 1960s. Since then, the USDA, in partnership with Panama, has been utilizing the sterile insect technique to control screwworm populations.

This method works by exposing screwworm larvae to gamma radiation, a specific type of electromagnetic radiation, which sterilizes the larvae. When they grow into adult flies, they are released by the millions in and around areas where screwworms have been found as well as barrier zones to curb their movement. The released sterile male flies mate with wild flies who lay infertile eggs that do not hatch. Over time, screwworm populations gradually decrease.

After many decades of control, the flies began progressing northward from Central America in 2024. That prompted the USDA to close the Mexican border in July 2025 to live cattle and horses as a precautionary measure. The closure has had negative impacts on the U.S. beef industry. Since then, the USDA, wildlife agencies and southern border states have conducted active surveillance for screwworms.

The USDA has also developed a robust New World screwworm response playbook to coordinate federal, state and public health efforts in the U.S.

Experts do not know exactly why the New World screwworm has reemerged in the United States. Similar to what scientists have seen among diseases carried by ticks and mosquitoes, pest habitat ranges are expanding globally due to changes in environment, ecology and populations.

In Colorado, the likelihood of the screwworm fly establishing a permanent population is relatively low across much of the state due to higher altitudes and freezing temperatures. However, the Colorado Department of Agriculture has been preparing an emergency response plan.

The plan ensures coordinated action between Colorado’s Department of Agriculture, Colorado Parks and Wildlife, the Colorado Department of Public Health and Environment and the USDA. Animals entering Colorado from areas with reported screwworm cases now require certificates for movement issued by an animal health official in the affected state and must meet enhanced health inspection requirements.

Colorado State University Veterinary Diagnostic Laboratories’ full-service parasitology laboratory routinely identifies animal parasites in livestock, pets and wildlife. As a member of the National Animal Health Laboratory Network, which conducts surveillance and testing for animal disease outbreaks, CSU will provide support to the state if screwworm is detected here.

Livestock and pet owners should use pest control strategies, such as maintaining clean spaces for their animals and using insecticides, barn treatments and fly traps, to try to keep flies away from their animals.

Livestock and pets should also be monitored for any wounds with evidence of larvae. That includes the wound draining fluid, having a foul odor or getting bigger. Animals may show signs of discomfort if they have screwworms, such irritation and restlessness, head-shaking, biting or licking at wounds, decreased appetite, depression and separation from the herd.

Livestock and pet owners should contact their veterinarian with any concerns or suspicion of screwworms, especially if the animal has unusual wounds or signs of larval infestation. If detected early, animals can be treated and fully recover from infestation.

Any suspected case must be reported to the Colorado State Veterinarian’s Office or the USDA area veterinarian in charge immediately. For suspicious cases involving wildlife, contact Colorado Parks and Wildlife.

Screwworm is detected by finding larvae present on an animal. The larvae are then collected by a veterinarian and submitted to a laboratory for identification. Currently, all screwworm suspect samples are required to be submitted to USDA’s National Veterinary Services Laboratories for official confirmation.

If New World screwworm is identified in an animal in Colorado, state or federal officials will conduct an investigation to understand how the animal became infested.

The state’s Department of Agriculture will work closely with livestock owners to help eliminate screwworm within the herd and prevent further spread. Culling groups of animals will not be used for general control and eradication of screwworm. However, separating out affected animals and temporary control of their movement will be important components of a herd-management plan.