Summer 2026 Equine Disease Update: Screwworm, Vesicular Stomatitis & New Tracking Tools

It's a busy and unusual summer for equine disease in the United States. Two outbreaks that most horse owners have never had to think about — New World screwworm and Vesicular Stomatitis — are both active right now, even as the familiar mosquito-borne season begins its annual climb. Here's where things stand, and a look at the new tools we've built to help you keep track.

New: two dedicated outbreak trackers

Some outbreaks move fast enough, and matter enough, to deserve their own page. We've added two, reachable from the new Outbreaks menu at the top of the site:

• New World Screwworm Tracker — USDA-confirmed U.S. detections by county, plus the outbreak's northward spread through Central America and Mexico.

• Vesicular Stomatitis Tracker — affected and quarantined premises across the Southwest, updated from USDA APHIS situation reports.

Both update automatically, show a live map, and list every affected county so you can see at a glance whether activity is near you.

New World Screwworm: a threat moving north

For the first time in nearly 60 years, New World screwworm has been confirmed in U.S. livestock. The first domestic animal case was confirmed on June 3, 2026 in Zavala County, Texas, and as of early July there are 31 confirmed animal cases across 13 counties in Texas and New Mexico.

Screwworm is not a typical "disease" — it's a parasitic fly whose larvae feed on the living tissue of any warm-blooded animal, including horses. A female fly lays eggs at the edge of even a minor wound — a scratch, a tick bite, a fresh gelding site, or a newborn's navel — and the hatching larvae burrow inward, creating a wound that enlarges instead of healing.

Behind the U.S. cases is a much larger wave: more than 128,000 animal cases have been reported across Panama, Costa Rica, Nicaragua, Honduras, Guatemala, and southern Mexico as the outbreak has pushed north. Our screwworm tracker maps that spread alongside the confirmed U.S. detections.

What to watch for: any wound that grows, smells foul, or shows maggots feeding in living tissue. Screwworm is a reportable disease — call your state veterinarian or USDA APHIS (1-800-940-6524) immediately, and inspect animals daily during fly season.

Vesicular Stomatitis: an active Southwest outbreak

The 2025–26 Vesicular Stomatitis (VSV) outbreak began in October 2025 and remains active. It has affected premises across Arizona, New Mexico, and Colorado, and — notably — every affected premises in this outbreak has been equine. Arizona's cases have been released from quarantine, but the outbreak has moved north, with counties in New Mexico (Rio Arriba, Valencia) and Colorado (Montrose) still under quarantine as of the latest reports.

VSV causes painful, blister-like sores on the mouth, tongue, muzzle, and coronary bands. It's rarely fatal, but it's serious for two reasons: its lesions look nearly identical to foot-and-mouth disease, so every suspected case triggers a regulatory response, and confirmed premises are quarantined until the sores heal — which can halt shows, sales, and movement.

VSV spreads through biting insects (black flies, midges) and contact with fluid from the sores. Fly control, isolating affected animals, and not sharing water troughs or equipment are your best defenses. Track current quarantines on the Vesicular Stomatitis tracker.

Arbovirus season is beginning — vaccinate now

While screwworm and VSV are the unusual stories, the predictable one is just as important: West Nile Virus and Eastern Equine Encephalitis season is starting. In our data, these mosquito-borne diseases are nearly silent through the winter and spring, then climb sharply beginning in August and peak in September. So a quiet map right now is normal — it is not a reason to relax.

We're already seeing early activity, including a notable rise in EEE cases in the Gulf states. Both WNV and EEE are vaccine-preventable, and EEE in particular is often fatal in unvaccinated horses. If your horses aren't current on their arbovirus vaccines, now — before the season peaks — is the time. Talk to your veterinarian, and remember that in warm climates a booster partway through the season may be recommended.

More complete data

Behind the scenes, we've also expanded and cleaned up our data. Florida's state reporting is now fully integrated, adding county-level Strangles, EEE, and West Nile detections to the map, and we've tightened how outbreaks are de-duplicated so each real event shows up once. The result is a more complete and more accurate picture across the interactive map, state pages, and disease trends.

What you can do

• Vaccinate for West Nile and EEE now, ahead of the seasonal peak.

• Inspect wounds daily during fly season and report anything suspicious of screwworm.

• Practice fly control and isolation to reduce Vesicular Stomatitis and arbovirus risk.

• Sign up for free alerts for your state, so you hear about a new outbreak near you the moment it's reported.

Stay vigilant, keep your vaccines current, and we'll keep watching the map so you don't have to.