Two California horses test positive for equine infectious anemia

CURRENT FULL VERSION: California has identified two new equine infectious anemia cases in Stanislaus County, with a 7-year-old and an 8-year-old Quarter Horse gelding testing positive on March 19. The 25 potentially exposed horses on the premises have been tested and quarantined while officials await results and complete the standard 60-day retest window. Epidemiologic tracing is ongoing, and early indications point to suspected iatrogenic transmission. (thehorse.com)

The cases fit a pattern California regulators have been tracking for the past two years. CDFA’s EIA updates show multiple confirmed cases in 2024 and 2025, including clusters in Orange, San Joaquin, San Bernardino, Madera, and Riverside counties. Several of those investigations involved Quarter Horses, and CDFA has said that in California, most EIA cases are caused by iatrogenic transmission, with many positive detections found in the Quarter Horse racing population, including sanctioned and unsanctioned settings. (cdfa.ca.gov)

That context matters because EIA control depends less on clinical recognition than on testing and biosecurity. Many infected equids are asymptomatic carriers, and both CDFA and USDA note there is no treatment or vaccine. The virus is spread through blood and other body fluids, most commonly by blood-feeding insects or contaminated needles, syringes, IV equipment, blood products, or other instruments. When signs do appear, they can include fever, depression, anemia, poor stamina, muscle weakness, and progressive loss of body condition. USDA advises annual testing, fly control, and never reusing needles, syringes, or IV sets. (aphis.usda.gov)

The testing sequence is also important for practitioners managing exposed groups. CDFA says ELISA can detect antibodies earlier but may produce false positives, so AGID, the traditional Coggins test, is the confirmatory assay. AAEP notes horses may remain seronegative on AGID for the first two to three weeks after infection and, in rare cases, may not develop detectable antibodies until 60 days post-exposure or longer. That’s why exposed horses remain under quarantine through the retest period even after initial negative results. A negative Coggins test is also central to movement control, with most U.S. states requiring proof of a recent negative test for interstate travel. (cdfa.ca.gov)

Industry messaging around recent western U.S. cases has stressed the same point. An American Horse Publications item tied prior California, Texas, and New Mexico cases to the importance of sterilized medical equipment, biosecure injection technique, and routine Coggins testing, noting USDA determined one multistate string of Quarter Horse racehorses had contracted EIA through iatrogenic transmission. Separately, Texas recently reported positive horses in Harris and Milam counties that were euthanized, while state officials worked with owners and local veterinarians to monitor exposed horses and implement biosecurity measures. That doesn’t establish the source in this Stanislaus County event, but it does reinforce why regulators are focusing on trace-back work and exposure management. (americanhorsepubs.org)

Why it matters: For equine veterinarians, this is a practical biosecurity story as much as a surveillance story. A positive EIA result triggers quarantine, confirmatory testing, regulatory oversight, and potentially euthanasia or lifelong isolation of the infected equid under USDA program rules cited by CDFA. If not euthanized, infected horses must be maintained under extremely strict lifelong quarantine, typically at least 200 yards from unaffected equids. For ambulatory and racetrack practitioners, the case is a timely prompt to review injection practices, inventory control for single-use supplies, blood product sourcing, and client education around current Coggins status before movement, sale, or event entry. It also underscores the need to think beyond symptomatic horses, since inapparent carriers can sustain transmission risk. (cdfa.ca.gov)

What to watch: The next milestones are the exposed horses’ test results, the 60-day retest outcome, and any CDFA or EDCC updates that clarify whether this was an isolated premises event or part of a broader epidemiologic link. If additional positives emerge, veterinarians should expect renewed scrutiny on movement records, treatment practices, and testing compliance in connected horse populations, similar to the exposure monitoring and biosecurity response now underway in recent Texas cases. (thehorse.com)