Now that the weather’s cooler, don’t be lulled into a false sense of security regarding mosquito-borne equine diseases like West Nile virus. In mild areas, mosquitos can remain a threat throughout the fall and winter. And with warmer temperatures everywhere, there’s no telling when a virus-laden mosquito might show up.

West Nile virus can cause severe neurologic disease in horses. That’s why the American Association of Equine Practitioners, or AAEP, now considers West Nile an essential vaccine for horses in North America. They suggest it’s never too late to start your horse on a West Nile vaccination series. And horses at higher risk—like those less than 5 or over 15 years of age—should receive boosters more often.

So, don’t trust the weather to protect your horse against West Nile virus. Just one mosquito can wreak havoc.