How flesh-eating screwworms in cattle could raise US beef prices

CHICAGO – New World screwworm, a devastating parasite that eats cattle and other wild animals alive, travelled north from Central America to Mexico before being confirmed in a Texas calf on June 3, creeping past biological barriers that kept the pest contained for decades, experts said.

Washington has halted cattle imports from Mexico for the past year, citing the insect’s spread further into Mexico.

With the US cattle herd already at a multi-decade low, the closure elevated record-high beef prices by keeping more calves out of the US supply chain.

The first confirmed case in the US during the latest outbreak represents a serious challenge for ranchers and could cause beef prices to rise further.

What is New World screwworm?

Screwworms are parasitic flies whose females lay eggs in wounds on any warm-blooded animal. Livestock and wild animals are usually the victims.

Once the eggs hatch, hundreds of screwworm larvae use their sharp mouths to burrow through living flesh – feeding, enlarging the wound and eventually killing their host if left untreated.

When screwworms infect a cow, a tiny scrape, a recent brand or a healing ear tag can quickly become a gaping wound, carpeted with wriggling maggots that put the entire herd at risk of infestation.

Screwworms were eradicated from the US in the 1960s when researchers began releasing massive numbers of sterilised male screwworm flies that mate with wild female screwworms to produce infertile eggs.

Why does this matter to US consumers?

The US typically imports over a million cattle from Mexico every year. The import suspension contributed to rising beef prices by tightening the supply of beef cattle, which dwindled after drought forced ranchers to slash herds.

Mexican cattle are usually fed and fattened on US farms for five to six months before slaughter, and a diminished slaughter rate can elevate beef prices.

An expansive US outbreak would further tighten the cattle supply and put other livestock and household pets at risk.

Screwworms will even feed on humans if they can, said Dr Timothy Goldsmith, a veterinary medicine professor at the University of Minnesota.

Homeless people would be especially vulnerable to infestation because they sleep outside and have less access to hygiene products and medical care, Goldsmith said.

What is being done to fight the parasite?

A factory designed to breed and sterilise screwworms in Panama is releasing 100 million sterile flies every week, but experts say more factories would have needed to come online quickly to choke off the fly’s spread north.

A production facility for sterile flies in Texas is not expected to open until late 2027. USDA completed a dispersal facility for sterile flies in the state in February.

Screwworms cannot fly more than 19km on their own, but they can cover large distances while burrowed inside their hosts, said Sonja Swiger, entomologist at Texas A&M University.

The flies have already passed through the narrowest stretches of land in Panama and Mexico, meaning exponentially more sterile flies need to be released to control the outbreak.

In 2025, the US Department of Agriculture announced it would invest US$21 million (S$26.9 million) to convert a fruit fly factory in Mexico to produce sterile screwworms.

How could this impact American cattle ranchers?

The USDA estimated a screwworm outbreak would cost the Texas economy US$1.8 billion in livestock deaths, labour costs and medication expenses.

After decades of eradication, most cattle ranchers no longer have the experience or tools to diagnose and treat screwworm.

Infestations can be cured, but treatment involves removing hundreds of larvae and thoroughly disinfecting wounds, a time-consuming, pricey and labour-intensive process.

“This is a pest we don’t want back. This is a bad thing,” said David Anderson, livestock economist at Texas A&M University. “I can’t imagine having to deal with that. It’s gross.” REUTERS