New World screwworm reaches Texas

New World screwworm (Cochliomyia hominivorax) is a fly whose larvae burrow into the living flesh of warm-blooded animals, entering through wounds as small as a tick bite or through the navel of a newborn.^2,5 It infests livestock, pets and wildlife, and in rare cases people.^1,2 The United States eradicated it in 1966 by releasing sterile male flies, and cleared a small outbreak in the Florida Keys in 2017.^1,5 In recent years it has moved north through Central America and Mexico.^1,2 On 3 June 2026 the USDA confirmed a case in a three-week-old calf in Zavala County, Texas,² and on 5 June a second in a one-month-old calf about 5.6 miles away.³ The response set a 20 km infested zone around each detection with quarantines and movement controls, and added ground releases of sterile flies to the aerial releases already under way.^2,3 After the second case another 4 million sterile flies a week were shipped for 24 ground release chambers.³ The only sterile fly factory now operating in North America is in Panama, run jointly by the USDA and Panama's agriculture ministry.¹ Meat from inspected animals is not at risk; the threat is to the animals and to trade in live ones.^2,5

Who needs an answer

• USDA APHIS and the Texas Animal Health Commission, deciding where to draw zones and where to release sterile flies.^2,3
• Ranchers inside and near the zones, who are advised to put off branding, castration and other wound-causing work.¹
• Neighbouring states, which may halt animal movements from an affected state.⁵
• Wildlife managers, since wild animals can be hosts too.¹
• Trading partners and the livestock markets that depend on moving live animals.^2,5

The domains it chains

Fly biology and how far adults travel; cattle, wildlife and pets as hosts; the supply of sterile flies from a single factory; animal movement, markets and trade rules; veterinary treatment and drug withdrawal times before slaughter.⁵

What a twin should let people try

• Release plans: with a fixed weekly supply of sterile flies, where and how often should they be released to stop the spread?
• Zones: how large must a quarantine zone be, given how far the fly moves, before the cost to ranchers outweighs the gain?
• Spread: if the fly takes hold, how far north could it reach in a warm season, and how much would wildlife hosts slow eradication?

Where to start

APHIS publishes fact sheets on the sterile insect technique and release methods, a standard operating procedure for possible detections, and a page on the unified government response (Stop Screwworm), all linked from its screwworm page.¹