New World screwworm spreads in Texas livestock as US cases climb to 16

State and federal officials are responding to a new case in Texas amid the ongoing threat of the New World screwworm to the United States and its cattle industry.

The U.S. Department of Agriculture said on social media that the latest case was identified in a goat in Terrell County, a Texas-Mexico border county about 150 miles south of Midland.

The confirmed case brings the total to 15 in Texas and 16 nationwide. Mexico is reporting about 1,800 active cases, according to the USDA.

"The case was identified in a castration site wound. NWS only infests open wounds — it does not affect healthy skin," read USDA's social media post.

The detection is about 180 miles west of the initial Texas case reported June 3 in Zavala County and 74 miles west of the most recent case confirmed June 20 in Crockett County.

What is a screwworm?

According to the USDA, the New World screwworm (NWS) is a parasitic fly that impacts livestock, pets, wildlife, and, less commonly, people and birds.

The fly's larvae feed on the living tissue of warm-blooded animals. USDA reports that the current risk to animals and people in the United States remains very low and that NWS is not contagious.

Rather, screwworm infestations begin when a female fly lays eggs on a wound or body opening. The eggs hatch into larvae that burrow into the wound and feed on living tissue; then, after about seven days of feeding, the larvae drop to the ground, burrow into the soil and pupate.

The adult screwworm fly emerges from the soil after seven to 54 days.

What does a New World screwworm look like?

Adult NWS flies have orange eyes, a metallic blue or green body and three dark stripes across the back, according to the USDA.

The telltale signs of a screwworm infection are the growing wounds or lesions and the presence of feeding maggots on still-alive animals.

Some other species of maggot can be found on live animals, said Dr. Jennifer Bloodgood, assistant professor of practice and wildlife veterinarian at Cornell University, but the difference is what they're eating. NWS feeds on healthy, living flesh, while most other maggot species feed on dead tissue.

It can be difficult to discern the difference, especially with wild animals you cannot observe closely. Because of this, public health officials encourage the public to report any suspected cases.

Where U.S. screwworm cases have been confirmed

Here is the breakdown of the screwworm case locations in the U.S. according to the USDA's screwworm dashboard and if they are considered active or inactive.

USDA classifies cases as inactive when the animal has fully recovered or, in cases where treatment was not performed, appropriate measures have been taken to prevent the spread of NWS, such as the appropriate disposal of the dead infested animal.

Texas screwworm cases

There are currently 15 cases — 13 active and two inactive — in Texas by the date they were confirmed:

• June 22: One active domestic goat in Terrell County.
• June 21: Two active domestic cattle cases in Edwards County.
• June 20: One active domestic sheep case in Crockett County.
• June 12: One inactive domestic sheep case in Sutton County.
• June 11:
  • Two active domestic goat and cattle cases in Edwards County.
  • One active domestic cattle case in Tom Green County.
  • One inactive domestic cattle case in Zavala County.
• June 9: One active domestic cattle case in Edwards County.
• June 8:
  • One active domestic goat case in Gillespie County.
  • One active domestic cattle case in La Salle County.
• June 7: One active domestic cattle case in La Salle County.
• June 5: One active domestic cattle case in Zavala County.
• June 3: One active domestic cattle case in Zavala County.

New Mexico screwworm cases

A case has also been confirmed in a dog in New Mexico. It was first reported on June 7 by a veterinarian in Andrews County, Texas, but the U.S. Department of Agriculture classifies it as a case in Lea County, New Mexico, because the dog lives there.

This case has now been classified as inactive.

Texas screwworm map

Mateo Rosiles is the Texas Connect reporter for USA TODAY and its regional papers in Texas. Got a news tip for him? Email him at [email protected].