New World Screwworm Cases in US Animals Jump to 113 as Outbreak Spreads

Confirmed New World screwworm cases in animals across the United States have jumped to 113, the US Department of Agriculture (USDA) reported on Wednesday, November 4, 2026 — 42 higher than a week earlier and 34 higher than three days before, when 79 cases had been confirmed on Sunday, November 2, a rise of roughly 43 percent in three days.

Of the 113 cases, 55 are active and 58 are inactive, across 32 counties in Texas, New Mexico and Arizona. Sixty-one came in the 30 days ending November 4 — more than half of all cases since the outbreak began June 3, 2026.

Key Points

• New World screwworm cases in US animals reached 113 as of November 4, 2026, up 42 in a week.
• 55 active and 58 inactive, across 32 counties in Texas, New Mexico and Arizona.
• 61 cases came in the 30 days ending November 4 — more than half of all cases since June 3, 2026.
• 52 cattle, 21 sheep, 16 dogs, 13 horses, 7 goats, 2 elk, 1 chicken and 1 cat.
• The FDA issued an emergency use authorisation for Bravecto to treat screwworm in dogs and puppies.

What Happened

The USDA's latest figures show an outbreak that has accelerated sharply after months of slow spread. Texas and New Mexico reported their first infestations in early June, and Arizona confirmed its first case on October 28, in a dog in Cochise County. Arizona has since added cases, including a horse in Greenlee County under a 12.4-mile containment zone.

The latest confirmed case is a second animal from the wild: an elk in Jeff Davis County, Texas, confirmed on November 4. The first wildlife case was an elk in Pecos County, Texas, confirmed on October 21. The two elk matter because wildlife can carry the pest beyond fenced pastures, making the outbreak harder to contain. USDA traps have caught wild screwworm flies in five locations, four in October, and most infestations are clustered along the border with Mexico.

Background and Context

New World screwworm is the larva of a parasitic fly that lays its eggs in the wounds of warm-blooded animals. The larvae feed on living tissue, and an untreated infestation can cause severe wounds, suffering and death. The pest can infest livestock, pets and wildlife, and less commonly people, and causes serious economic damage if not detected and treated quickly.

The United States eradicated New World screwworm in the 1960s with sterile male flies that collapsed the breeding population. The pest survived in Central America and the Caribbean, and its return to Mexico two years ago put US officials on alert. Texas is the largest cattle state in the country, and a pest that wounds cattle and forces quarantines can cost ranchers heavily.

New World Screwworm: The Pest the US Beat in the 1960s

The sterile insect technique that eradicated New World screwworm in the 1960s is still the main weapon against it: factories rear millions of sterile male flies, which are released to mate with wild females, producing no offspring and collapsing the population over time. The USDA has been expanding releases and surveillance as the pest has moved north through Mexico.

Detection relies on looking: ranchers, veterinarians and pet owners are being told to check every wound, because the female fly lays her eggs at the edge of an open wound and the larvae burrow into living flesh within hours. Early treatment can kill the larvae. This week the FDA issued an emergency use authorisation for Bravecto, a chewable tablet whose active ingredient is fluralaner, to treat screwworm in dogs and puppies. The FDA has authorised several pet drugs against screwworm since June, including Simparica Trio and nitenpyram. The agency notes that isoxazolines, the drug class that includes fluralaner, have been linked to neurological problems in some animals, including tremors, loss of coordination and seizures.

Reactions and Response

The USDA has not announced new restrictions alongside the latest figures, but its surveillance data tells the story of a response that is widening as the numbers rise. Trapping has intensified, and the 12.4-mile containment zone in Greenlee County, Arizona, shows state officials will restrict animal movement around a confirmed case.

State animal health officials in the three states urge ranchers and pet owners to inspect animals daily and report suspicious wounds immediately: any wound with maggots should be treated as a possible screwworm case until a veterinarian rules it out. The FDA's authorisation for Bravecto gives veterinarians another tool for dogs and puppies.

Why It Matters

The jump to 113 cases matters because of the speed of the increase. A rise of 42 cases in a week and 34 in three days, after a summer of single-digit monthly counts, suggests the outbreak is entering a faster phase. If that pace continues, the case count could double again within weeks.

It matters for the beef industry, for pet owners whose dogs and cats are now among the confirmed cases, and for wildlife. The two elk cases show the pest is no longer confined to animals that people handle and inspect. The United States has beaten this pest before and knows what works — sterile fly releases, surveillance, rapid treatment and movement controls — but all of those tools depend on finding cases early. Health officials are also watching other fast-moving stories: the Florida dengue outbreak, the largest in decades, the measles death in Wisconsin, the state's first in more than 35 years, and the cancelled hepatitis B vaccine trial in Guinea-Bissau.

What Happens Next

The USDA will publish its next case count in the coming days: the figure to watch is whether the weekly increase stays near 40 or falls back as containment zones and trapping take effect. A third elk or a case in another wild species would confirm the pest is established in wildlife, not just passing through it.

Arizona is the state to watch most closely: its first case came only on October 28, and the Greenlee County horse case shows the pest is already moving within the state. For ranchers and pet owners in Texas, New Mexico and Arizona, the advice is unchanged: inspect every animal, treat every wound, and report anything suspicious at once.

Frequently Asked Questions

What is New World screwworm? The larva of a parasitic fly that lays its eggs in the wounds of warm-blooded animals. The larvae feed on living tissue and can cause severe wounds, suffering and death if not treated.

How many cases have been confirmed in the United States? The USDA reported 113 confirmed cases as of November 4, 2026, up 42 in a week — 55 active and 58 inactive, across 32 counties in Texas, New Mexico and Arizona.

Which animals are affected? 52 cattle, 21 sheep, 16 dogs, 13 horses, 7 goats, 2 elk, 1 chicken and 1 domestic cat. The two elk are the only wildlife cases so far.

Can New World screwworm affect people? Yes, less commonly than animals. Anyone with a wound in an affected area should seek medical advice promptly.

What should animal owners do? Inspect animals daily, treat every wound, and report any wound containing maggots to a veterinarian or state animal health official immediately.