New World Screwworm Cases to Date from the USDA-APHIS website. Click to enlarge.

New World screwworm (NWS) is a fly that produces a flesh-eating maggot that has devastating consequences for the mammals it encounters. The last time this pest had a self-sustaining population in the US was 1966. Since then, we have seen small import outbreaks, but this current outbreak indicates a more serious threat, with multiple species affected across Texas and New Mexico, including cattle, sheep, dogs, and goats.

The outbreak started with the first case reported on June 3, 2026. Today, July 13, 2026, the USDA has reported 35 total cases. Nineteen of those cases are active.

There is no approved preventative treatment that can deter the screwworm, but there are emergency treatments that are effective, and if caught early enough, the affected animals can be saved. The best precaution that producers have is to keep their eyes on their animals daily, looking for wounds or soft tissue orifices (ears, eyes, nostrils) where maggots appear to be eating living flesh. Since the time from egg to larvae is only 2 days, the infestation can take hold quickly. While it's unlikely that NWS will naturally migrate up to NYS before it is eradicated from the US again, it could move up this way quickly and sporadically on imported animals. Experts believe that it will die in a typical upstate NY winter.

Eradication measures are ramping up as laboratories in the US, Mexico, and Panama increase production of sterile male flies, which are released into areas with known outbreaks. Because the female screwworm flies will only mate once, if the population of males is overrun by sterile males, it will eventually self-limit to a point of eradicating the pest. These laboratories are remnants of the first eradication effort that began in 1958, and since the eradication in 1966 have worked at low production levels to keep the NWS out of the US and to fight smaller import outbreaks. It's estimated that these labs will be back at full production capacity in early 2027.

If you think you may have NWS on your farm, please reach out to your veterinarian. They can work with you to get the USDA out to test your animals. Unlike some foreign animal diseases, a confirmed positive of NWS does not mean eradication of a herd or flock as a containment method. Individual animals are instead isolated, treated, and euthanized if there is no chance of recovery.

For more information about the outbreak, you can visit the USDA's Animal Plant Health Inspection Service Website at https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/current-status/us-confirmed-cases-new-world

To review the PRO-DAIRY webinar on NWS, visit this link. Panelists included Rob Lynch, DVM, dairy herd health and management specialist, PRO-DAIRY Animal Science; Joy Bennett, DVM, NYS Department of Ag & Markets; Michael Capel, DVM, Perry Veterinary Clinic; and Carrie Telgen, DVM, Diamond V.