Montana Department of Livestock and MSU Extension issue guidance after New World screwworm detected in Texas

The Montana Department of Livestock and Montana State University Extension have issued guidance for the Montana livestock industry after a destructive parasitic fly called New World screwworm was detected in Texas.

For the first time since 1966, the New World screwworm – also known as NWS, or Cochliomyia hominivorax – has been detected in the United States. A confirmed case was found in Zavala County, Texas, on June 3. Since then, an additional five cases have been confirmed in south Texas.

According to scientists with the Montana Department of Livestock and MSU Extension, New World screwworm is a concern for the U.S. livestock industry due to the flies laying eggs in the wound of a living animal and the larvae feeding on the living tissue and causing severe tissue damage. After feeding, the larvae drop to the ground and will hatch into adult flies. The primary mitigation effort of releasing sterile male flies into the population has been ongoing, since female flies only reproduce once in a lifetime.

In response, on June 4 the Montana Department of Livestock released an animal health order for livestock traveling into Montana from affected locations. According to the order, animals from an infested state must have an import permit from the Department of Livestock and a certificate of veterinary inspection – or CVI – documenting that the animal has met the state’s requirements to leave an infested zone. The CVI cannot be issued before the initial New World screwworm detection in the infected state. Because animal health authorities in Texas do not have authority over small animals, the order has different requirements for small animals and livestock species.

The CVI must indicate that all animals listed on the document are free from wounds and evidence of New World screwworm larvae infestation, and all animals must have official identification. The CVI can only be issued within five days prior to movement to Montana. These requirements apply statewide for small animals. For livestock, these requirements are applied at the county level. Impacted counties are listed in the order. Animals under this order include all warm-blooded animals, including cattle, horses, mules, other equids, sheep, llamas, alpacas, bison, swine, ostriches, rheas, emus, goats, alternative livestock species, dogs, cats, rabbits, rodents, game animals, fur-bearing and wild animals, and poultry and other birds. More detailed information can be found in the order from the Montana Department of Livestock.

Animals entering the state from any location in and around the infested zone should be monitored for signs and symptoms of New World screwworm infestation. Individuals should contact the Montana Department of Livestock at 406-444-2976 or mtstatevet@mt.gov for more information regarding animal travel within and around the infected zones.

Additional tools and information related to the response to New World screwworm can also be found on the USDA Animal and Plant Health Inspection Service website at https://www.aphis.usda.gov/animal-emergencies/nws.

To learn more about the Montana Department of Livestock animal health order, visit https://news.mt.gov/Department-of-Livestock/Animal-Health-Order-No-2026-01.