California investigates two new equine infectious anemia cases

CURRENT FULL VERSION: California animal health officials are investigating two new equine infectious anemia cases in Stanislaus County after a 7-year-old and an 8-year-old Quarter Horse gelding tested positive on March 19. According to the EDCC Health Watch report, 25 potentially exposed horses on the premises have been tested and quarantined pending laboratory results and 60-day retesting, while epidemiologic tracing continues. Investigators currently suspect iatrogenic transmission. EDCC Health Watch is a marketing and distribution program that uses verified information from the Equine Disease Communication Center, an independent nonprofit supported by industry donations to provide open-access equine infectious disease reporting. (equusmagazine.com)

The finding fits a pattern regulators and equine health groups have been tracking for several years. EIA is an untreatable, blood-borne viral disease that attacks the horse’s immune system, and infected horses remain carriers for life. While horseflies and deerflies can spread the virus naturally, both USDA and California guidance emphasize that contaminated needles, syringes, instruments, IV equipment, or blood products can also transmit infection. California’s own fact sheet says most EIA cases in the state are linked to iatrogenic transmission, and most positive detections are found in the Quarter Horse racing population, including sanctioned and unsanctioned settings. (aphis.usda.gov)

That broader backdrop matters because EIA is now uncommon nationally, which means each case tends to trigger a substantial disease-control response. USDA says prevalence in the U.S. equine population has fallen from nearly 4% in 1972 to about 0.004% today because of long-running testing and control programs built around official EIA assays, including the Coggins test, which screens for antibodies to the virus and is commonly required for interstate movement. In California, however, case counts have fluctuated in recent years: the state fact sheet lists 6 cases in 2023 and 15 in 2024, after a spike to 40 in 2022. USDA’s 2024 national summary also found that most confirmed U.S. cases last year were associated with current or former Quarter Horse racehorses and that iatrogenic transmission accounted for a large share of infections. (aphis.usda.gov)

The current Stanislaus County event also arrives just days after other March 2026 EIA reports surfaced in North America, including two positive horses in Texas and eight in Alberta, according to EDCC-linked coverage cited in your source set. In Texas, one horse in Harris County and one in Milam County were euthanized after testing positive, and state officials said they were working with owners and local veterinarians to monitor exposed horses and implement biosecurity measures. That doesn’t necessarily indicate a connected outbreak, but it does show how surveillance systems are continuing to pick up sporadic cases that require rapid containment. Based on the available reporting, the California cluster appears to be premises-specific at this stage, with no public indication yet of broader interstate exposure. This is an inference from the limited case information now available. (equusmagazine.com)

Expert guidance has been consistent on the main risk factors. In AAEP materials announcing its EIA guidelines, co-author Angela Pelzel-McCluskey, DVM, MS, then the national equine epidemiologist for USDA APHIS Veterinary Services, said a newer high-risk group had emerged in which spread is primarily iatrogenic, “most commonly through needle, syringe and IV set reuse,” with possible contamination of multidose vials and illegal blood or serum products also cited as concerns. That framing aligns closely with California’s description of in-state transmission patterns. (aaep.org)

Clinical disease can be variable, which is one reason infected horses may still pose a transmission risk. Not all horses show signs, but when illness develops it can include progressive loss of body condition, muscle weakness, poor stamina, fever, depression, and anemia. There is no vaccine and no cure. Once diagnosed, a horse may die, be euthanized, or be kept under extremely strict lifelong quarantine, including separation of at least 200 yards from unaffected equids. (aphis.usda.gov)

Why it matters: For equine veterinarians, the practical takeaway is less about the rarity of EIA and more about the consequences of a single lapse in biosecurity. Positive horses trigger regulatory involvement, quarantine, confirmatory testing, trace investigations, and follow-up testing of exposed animals. In day-to-day practice, this case reinforces the need to avoid needle or syringe reuse, protect multidose medications from contamination, maintain strict dental and surgical instrument sterilization, and counsel trainers and pet parents about official testing requirements before movement, sale, racing, or exhibition. California also encourages use of electronic EIA laboratory submission platforms, which may help streamline compliance and traceability. (cdfa.ca.gov)

What to watch: The next key developments are the pending results from the 25 exposed horses, completion of the required 60-day retesting window, and any state or EDCC updates clarifying whether investigators can confirm the suspected iatrogenic source or identify linked premises or movement history. Broader surveillance is also worth watching, given the recent Texas detections and euthanasias and other March reports elsewhere in North America. (equusmagazine.com)