Is New World Screwworm a Risk in Washington or Oregon?

By Pest Stop | Olympia, Washington | June 2026

On June 3, 2026, the USDA confirmed the first New World screwworm infestation in U.S. livestock since the pest was eradicated in the 1960s. The cases are in Zavala and La Salle Counties in deep South Texas, within 150 miles of the Mexican border. As of this writing, three calves have been confirmed positive.

Washington and Oregon have never had a confirmed case of New World screwworm. The current outbreak poses a very low risk to animals and residents in the Pacific Northwest based on geography, climate, and the biology of the pest itself.

What Is New World Screwworm?

New World screwworm (Cochliomyia hominivorax) is a parasitic fly native to South America and the Caribbean. The larvae infest open wounds on living warm-blooded animals, including cattle, horses, sheep, goats, dogs, cats, wildlife, and in rare cases, people. The maggots burrow into living tissue and cause rapidly expanding wounds that can be fatal within 10 days if untreated.

Any open wound can be a target, including wounds as minor as a tick bite. A female fly can lay 200 to 300 eggs at a time, and eggs hatch in as little as six to eight hours.

New World screwworm is not the same as common blowflies or bottle flies found in the Pacific Northwest. Those species feed on dead tissue. Screwworm targets living animals exclusively.

Has Washington or Oregon Ever Had a Screwworm Case?

No. Neither Washington nor Oregon has ever had a confirmed New World screwworm detection, either before the 1960s eradication campaign or in any subsequent outbreak or isolated detection.

Historically, even when screwworm was endemic across the southern and western United States, the Pacific Northwest fell outside its year-round survival range. The CDC notes that winter freezes historically restricted the fly's permanent range to southern Florida, southern Texas, and Puerto Rico. Seasonal livestock transport occasionally brought cases further north in summer months, but the fly could not establish itself in colder climates.

That biological reality has not changed.

Why Is Screwworm Unlikely to Reach Washington or Oregon?

There are three reasons the current outbreak is unlikely to affect the Pacific Northwest.

Distance. The confirmed cases are approximately 1,800 miles from Olympia and 2,000 miles from Portland. The screwworm fly has a self-propelled flight range of only 6 to 12 miles. It cannot travel here on its own.

Climate. New World screwworm requires warm, subtropical conditions to survive year-round. Cold Pacific Northwest winters kill the pupal stage in soil and suppress adult fly populations. Climate suitability research identifies Texas and Florida as the highest-risk states. Washington and Oregon do not appear in risk modeling for potential establishment.

Livestock import restrictions. USDA suspended live animal imports from Mexico through southern border ports of entry in May 2025. This addresses the most realistic pathway for northward spread through the supply chain.

What Is Being Done at the National Level?

The federal response is aggressive and well-resourced. USDA APHIS has deployed more than 8,000 detection traps across the border region and tested nearly 80,000 samples. The U.S. is releasing 100 million sterile flies per week in Mexico and along the southern border as part of the Sterile Insect Technique, the same eradication method that eliminated screwworm from the U.S. in the 1960s.

The federal government has committed $750 million to build a domestic sterile fly production facility in Edinburg, Texas. Texas Governor Greg Abbott issued a disaster proclamation in January 2026 and has renewed it every 30 days. Quarantine and movement restriction zones are active in the affected South Texas counties.

What Should Washington and Oregon Residents Watch For?

The risk is low, but awareness is appropriate for anyone with livestock, horses, or farm animals.

Signs of screwworm infestation in animals include:

• Wounds that are growing larger rather than healing
• Foul odor coming from a wound
• Visible maggots in or around a wound
• Maggots or eggs near body openings such as the nose, ears, or genitals
• Unusual agitation or distress in an animal

If you observe these signs and have recently transported animals from affected regions, contact your veterinarian immediately. In Washington, report concerns to the Washington State Department of Agriculture. In Oregon, contact the Oregon Department of Agriculture. USDA APHIS also accepts reports directly at aphis.usda.gov.

Frequently Asked Questions

Can my dog or cat get screwworm in Washington or Oregon? The current risk to pets in Washington and Oregon is extremely low. The pest is not present here, and the climate does not support its survival year-round. If your pet has traveled with you to South Texas or Mexico, inspect any wounds carefully and contact a veterinarian if you notice rapid wound growth or maggot activity.

Is the food supply affected? No. The USDA Food Safety and Inspection Service has confirmed that screwworm does not infest meat, produce, or food products. Any affected animal would be identified through standard inspection before entering commerce.

Is screwworm the same as regular maggots? No. Common flies in Washington and Oregon, including blowflies and bottle flies, lay eggs in dead or decaying tissue. New World screwworm specifically targets living animals. If you see fly larvae in a wound on a living animal, contact a veterinarian regardless of the cause.

Should I cancel livestock purchases or transport from Texas? Use caution and verify the origin of any animals. Consult your veterinarian and review current USDA movement guidance at aphis.usda.gov before purchasing or transporting livestock from affected counties in South Texas.

About Pest Stop

Pest Stop is a licensed pest control company based in Tumwater, Washington, serving 15 counties along the I-5 corridor in Washington and Oregon since 1988. We are members of the Washington State Pest Management Association and monitor state and federal pest alerts that affect our customers and the region.

WSDA License #70870 | ODA# AG-L1046902CPO

For pest concerns in your home, property, or outbuildings, contact Pest Stop at peststop.com.

For official New World screwworm information and updates, visit screwworm.gov or aphis.usda.gov.