US: New World Screwworm Threat Grows in US as Cases Reach 34

The spread of New World screwworm in the United States is raising fresh concern among cattle farmers, veterinarians and animal health officials.

New reporting says the flesh-eating parasite has now been confirmed in 34 animals in the US, mainly in Texas. The parasite was first confirmed in a Texas calf on June 3, marking its return to the state after decades.

Why New World screwworm is dangerous

New World screwworm is a parasitic fly. Female flies lay eggs in open wounds. After the eggs hatch, the larvae feed on the animal’s living tissue.

Without quick treatment, the wounds can become severe and the animal may die. Cattle, goats, sheep, pets and wildlife can be affected. USDA says infections can cause serious animal suffering and major economic losses.

US releases 100 million sterile flies

The main weapon against screwworm is the release of sterile flies.

About 100 million sterile flies are currently being released across affected areas of the US Southwest and Mexico, according to the latest report. The sterile flies mate with wild females, but no offspring are produced.

Experts cited in the July 12 report warn that around 500 million sterile flies may be needed to push the parasite back.

The US is also expanding sterile-fly production capacity. A new Texas production facility is planned as part of a federal investment of up to $750 million.

Illegal cattle movement may be helping the parasite spread

Researchers are also examining the role of illegal cattle movement through Central America.

Animals moved outside official systems may avoid veterinary checks and disease controls. This can allow infected animals to carry screwworm into new areas, according to experts interviewed in the latest investigation.

A serious threat to US livestock

New World screwworm was successfully pushed out of the United States decades ago. Its return has created a major new animal health challenge.

The increase from the first confirmed Texas case in early June to 34 affected animals by July shows how quickly the situation has changed.

For the US cattle industry, the risk is clear. If New World screwworm becomes firmly established, farmers could face higher animal deaths, treatment costs and tighter livestock movement controls.

The race to stop New World screwworm is now becoming one of the most important US livestock health stories of 2026.