New World Screwworm in Texas, New Mexico and Arizona: A Practical Guide for US Ranchers

Short answer: As of November 5, 2026, USDA had confirmed 116 New World screwworm infestations in the US, 56 of them active. Cases are in Texas, New Mexico and Arizona, and movement restrictions cover parts of 16 Texas counties. If you run cattle in the Southwest, check every wound, especially after processing. Report suspect cases to your state animal health agency right away, and keep clear records of treatments and animal movements.

Screwworm hadn't been seen in US livestock for about 50 years until June 2026. Most ranchers working today have never dealt with it. Here's a practical guide.

What is New World screwworm?

New World screwworm (Cochliomyia hominivorax) is a fly that lays eggs in open wounds and on mucous membranes of warm-blooded animals. The larvae burrow into living tissue and feed on it. As the wound grows, it attracts more flies. Without treatment, an infested animal can die. It affects cattle, horses, dogs, wildlife and, in rare cases, people.

The US got rid of screwworm decades ago by releasing sterile male flies. When wild females mate with them, their eggs don't hatch. The fly has spread back north through Central America and Mexico over the past few years.

Where are screwworm cases in the US?

Date	Status
Early June 2026	First US livestock case in about five decades, found in Texas
September 2026	Fewer than 50 US cases so far; USDA releases its billionth sterile fly in Texas (KFYR)
November 4, 2026	113 confirmed cases, including 36 in October and 24 in the first days of November (KBTX)
November 5, 2026	116 confirmed cases, 56 active and 60 inactive (Texas Public Radio)

Recent cases include cattle in Hudspeth and Mitchell counties in Texas, a horse in Greenlee County, Arizona, and horses, cattle and a dog in New Mexico. Two elk in Pecos and Jeff Davis counties were infested too, and a wild fly was trapped in Otero County, New Mexico. Of the 116 cases, 114 were in domestic animals.

What are the signs of screwworm in cattle?

Watch for:

• Wounds that get worse instead of healing
• Bloody, thick or foul-smelling discharge
• Swelling around an injury
• Larvae (maggots) visible in a wound
• Pain, restlessness, head shaking or unusual behaviour

The riskiest spots are fresh wounds from branding, castration, dehorning and ear tagging, plus newborn navels and wounds from calving, fighting or wire.

What should I do if I suspect screwworm?

1. Don't move the animal until a vet has examined it.
2. Call your veterinarian and your state animal health agency. In Texas, call the Texas Animal Health Commission at 1-800-550-8242.
3. Collect larvae if your vet asks you to, following their instructions for identification.
4. Check the rest of the herd, especially animals processed in the same group.
5. Write down what you found: the animal ID, location, date and treatment.

Screwworm is a reportable animal disease. Early reports help officials contain it and protect your neighbours' cattle.

How can I lower the risk on my operation?

• Time processing for cooler weather when you can. Fly numbers drop in cold weather. Texas A&M entomologist Aaron Tarone said fall weather is "likely having an impact on fly numbers."
• Treat wounds right away with a wound spray labeled for screwworm or fly control, and check them again within a few days.
• Inspect newborn calves daily during calving season, starting with navels.
• Inspect cattle you buy and keep them apart from the herd before mixing, especially cattle from affected counties or from Mexico.
• Watch for movement restrictions from your state and USDA APHIS before shipping cattle.

How is USDA fighting the outbreak?

The main tool is still sterile fly releases. Historically it took about 500 million sterile flies a week to control screwworm in the US. A Panama plant produces about 100 million a week and a Mexico plant about 40 million. A new plant at Moore Air Base in South Texas is due to finish construction next April, with a goal of 100 million flies a week about a year from now. USDA Under Secretary Dudley Hoskins said USDA will stay in response mode until production reaches full output.

How does screwworm affect cattle prices?

Screwworm drives US-Mexico border decisions, and that trade normally moves about 1 million feeder cattle a year. When Mexican cattle are shut out, US feeder supplies get tighter. When ports reopen, more feeders come in. USDA started reopening ports in phases in 2026 (Farm Policy News). Canada has also restricted livestock imports from Texas. See our November 2026 US cattle market update for current prices.

Why do treatment and movement records matter?

If screwworm turns up near your place, officials will ask which animals were treated, where they came from, and when they moved. Ranchers who can answer quickly get through restrictions faster. Keep:

• Treatment and wound records with dates and animal IDs. See cattle health tracking.
• Purchase and source records for every animal you bring in.
• Movement dates between pastures, sale barns and buyers. Our guide to EID tags and USDA traceability rules covers the basics.

The takeaway

Screwworm is spreading in the Southwest, and it's most active in warm weather. Check wounds closely, report suspect cases right away, and keep clean treatment and movement records. CattleOS lets your crew record treatments, wound checks and moves by voice at the chute, so you have a dated record for every animal if you ever need one. See how USDA treatment records work in CattleOS.

Hashim Farooq is the CTO and co-founder of CattleOS. Case counts come from USDA data as reported November 4 and 5, 2026. Check USDA APHIS and your state animal health agency for the latest information. This article isn't veterinary advice.