What is New World screwworm?
New World screwworm (Cochliomyia hominivorax) is a parasitic fly that lays eggs in the open wounds or natural body openings of warm-blooded animals. Unlike most flies that feed on dead tissue, screwworm larvae feed on living flesh — burrowing into tissue with sharp mouth hooks, causing wounds to expand rapidly. Left untreated, an infestation can kill a large animal within days.
The United States eradicated screwworm through a decades-long sterile insect program, releasing billions of sterile male flies to prevent reproduction. The last established U.S. infestation was eliminated in 1966. The fly's reappearance in northern Mexico — just south of the Texas border — is why TAHC and USDA are taking preventive action now rather than waiting for a confirmed Texas case.
Active quarantine zone — 22 counties
TAHC has established a 22-county quarantine zone. All warm-blooded animals in these counties require TAHC authorization and a movement certificate before leaving. Confirmed cases have been found in 13 of the 22 counties:
Confirmed Cases — 13 Counties
26 confirmed premises as of July 12, 2026
Quarantine Zone Only — 9 Additional Counties
Zone may expand. Check www.tahc.texas.gov for the latest Executive Director Orders.
If you own land or move livestock in any quarantine-zone county, call TAHC at 1-800-550-8242 before transporting animals. Violations of the quarantine order can result in fines.
Which animals are at risk
All warm-blooded animals are susceptible — there is no species immunity. For Texas landowners the most relevant are:
- Livestock — cattle, sheep, goats, horses, pigs. Newborns are especially vulnerable at the navel.
- White-tailed deer — particularly fawns during June–July fawning season and bucks with rut wounds in fall.
- Other wildlife — javelina, rabbits, turkeys, and small mammals are all susceptible.
- Dogs and cats — outdoor pets with wounds are at risk.
What to watch for
Early detection is the key to saving an animal. Signs of screwworm infestation:
- Live maggots in a wound. Screwworm larvae are cream-colored, about 1–2 cm long, and move in a screwing motion. If you see live maggots in a wound on a living animal, treat it as a suspected case immediately.
- Foul odor from a wound. Rotting tissue produces a distinctive, strong smell. Investigate any unusual odor coming from an animal.
- Wound that is getting larger. Normal healing wounds shrink. A screwworm infestation causes the wound to enlarge and deepen.
- Behavioral changes. Repeated head shaking, separation from the herd, loss of appetite, or signs of pain and distress.
- Unusual fly clustering. Large numbers of flies concentrated around a specific animal or wound.
If you find suspected screwworm on wildlife: Do not attempt to treat wild animals yourself. Contact your local TPWD wildlife biologist immediately. Find yours at tpwd.texas.gov. Reporting is critical — it's how the state tracks potential spread.
What to do now — prevention steps
Inspect livestock more frequently
During warm months when flies are active, check all animals daily — especially newborns (navel area), any existing wounds, and animals that appear lethargic or isolated. The difference between catching it at 24 hours vs. 72 hours is significant.
Treat all wounds immediately
Any cut, scrape, castration site, dehorning wound, or skin abrasion should be treated with an approved wound spray containing an insect repellent immediately. Keep wound spray on hand at all times.
Minimize elective wound-causing procedures
Avoid dehorning, castrating, and branding during peak fly season if possible. If procedures are necessary, treat wounds immediately and monitor closely.
Monitor your deer herd
Check game cameras regularly during fawning season and the fall rut. A deer isolating from others or showing unusual behavior may be an early sign. Report anything suspicious to TPWD.
Know your county's status
If you're in or near a high-risk county, contact TAHC before moving livestock. Requirements can change quickly as the situation develops.
Where to get current information and report
Texas Animal Health Commission
1-800-550-8242 · www.tahc.texas.gov
High-risk county list, livestock movement requirements, current status
USDA APHIS
1-800-414-8731 · www.aphis.usda.gov
Federal screwworm information, confirmed case map, national status
TPWD — Wildlife reports
tpwd.texas.gov → find your regional biologist
Report suspected screwworm in deer, turkey, or other wildlife
Your local veterinarian
Call immediately if livestock show signs
Diagnosis and treatment for your animals
Bottom line
Screwworm is active in Texas and the outbreak is still expanding. With 34 confirmed cases across 13 counties and a 22-county quarantine zone, this is the most serious livestock disease threat Texas has faced in decades.
The actions required aren't complicated: inspect your animals daily, treat every wound immediately, know what to look for, and report anything suspicious. Early detection is the difference between a treatable case and a dead animal.
For the latest case counts and county-by-county status, see the live screwworm tracker — updated twice daily.