Equine infectious anemia (EIA) continues to spread worldwide, surging especially in Mexico and parts of South America, and infectious disease experts have reported increased infection rates in the southern United States.

Officials have even detected the EIA virus (EIAV) in northwestern Canada, possibly due to climate change—which might be expanding the regions and seasons where flies spread the disease naturally—or factors such as moving infected equids or reusing needles and other veterinary equipment without proper sterilization. With no known cure and no effective vaccines against this sometimes fatal disease (whether due to the disease’s clinical effects or legal requirements for euthanasia), global testing and monitoring remain essential, said Côme Thieulent, PhD, a research assistant professor in the Department of Pathobiological Sciences at Louisiana State University’s School of Veterinary Medicine, in Baton Rouge. Thieulent and his colleagues recently completed a literature review on how common the disease is throughout the world.

“Prevalence of EIA is quite high in some parts of the world,” he said. “So we need to collaborate with other countries.” The goal, he explained, is to improve surveillance and research while developing better management tools, including an effective vaccine.

Equine Infectious Anemia Worldwide

Similar to human immunodeficiency virus (HIV) infection in humans, EIAV lasts a lifetime but often goes unnoticed in some individuals, especially donkeys, which are particularly resistant to disease after infection but can still transmit the virus. Equids that develop EIA experience fever, lethargy, poor condition, and/or poor performance due to a drop in red blood cells that reduces oxygen delivery throughout the body, said Udeni Balasuriya, PhD, professor of virology, also based at Louisiana State University.

Because the virus can lie hidden for years, it’s crucial to identify its carriers, he said. Agar gel immunodiffusion (AGID) tests (Coggins) and rapid ELISA tests—required in most countries when horses are transported, compete, or change owners—offer some insight. The World Organization for Animal Health (WOAH, previously OIE) has declared EIA a notifiable disease worldwide. Researchers studying infection trends have focused on local data only, leaving major gaps in the global picture. “There is an urgent need for more comprehensive scientific studies,” Balasuriya said.

To fill that void, Thieulent, Balasuriya, and their colleagues examined the past 24 years of scientific literature worldwide, up to Dec. 31, 2024, calculating EIAV prevalence across 42 countries and six continents. They reviewed nearly 170 studies based on serological testing, involving more than 28.5 million equids.

Equine Infectious Anemia Hotspots in the Americas

The team reported EIAV prevalence highest in central and northern Mexico, where more than a quarter of all tested equids were positive. Rates in Guatemala (15.9%) and in some regions of Argentina (20.55%) and Brazil (21.26%) were nearly as high as in Mexico. Columbia, Nicaragua, and Venezuela had prevalence rates of around 7% each.

“We were surprised to see such high prevalence in Mexico … which could pose a potential risk to horses in the U.S.,” Thieulent said, adding that more complete data in Mexican states bordering the U.S. are lacking.

The climates in these regions offer favorable living conditions for flies and likely contributed to those increased rates, Balasuriya explained. Even so, scientists have linked recent outbreaks in the U.S. to contaminated veterinary equipment, making such iatrogenic (caused in the process of medical treatment) spread a possibility as well, he added.

By contrast, the U.S., Canada, Australia, and most of Europe and Asia reported much lower EIAV prevalence—on average less than 0.5%, he said—and specifically less than 0.005% in the U.S. and 0.21% in Canada (based on studies from 2009 and 2012). The Middle East and Japan appear to be essentially free of EIAV, the team reported, and Russia’s rate hovered at 0.09%.

More tropical regions across the globe—including U.S. states California, Texas, and Mississippi—had slightly higher averages, likely due to their climate and possibly to their proximity to countries with greater risk, Balasuriya said. The researchers noted testing in most African countries occurred too infrequently and inconsistently to provide reliable averages. Overall, most countries lacked data within the past five years, which are key to understanding today’s risk.

Global EIA Research Involvement

Overall, the figures in the review might not represent the general population of equids in each area, because some data come from studies focused on horses in specific high-risk settings, such as sales or transport, said Thieulent. But they nonetheless highlight the uneven distribution of EIAV worldwide.

The review also revealed stark differences in how countries respond to the virus. Some, including European countries, the U.S., and Canada, have long-standing surveillance and control programs. Others—particularly parts of Africa and Oceania—have little or no published data on EIAV, suggesting the virus could be circulating unnoticed in some regions, the researchers wrote.

Such inconsistency, combined with climate shifts and international horse movement, probably allows the virus to spread quietly, Balasuriya warned. Without broader, coordinated action, he noted, outbreaks will become harder to prevent.

Managing EIA Outbreaks

Currently, disease management methods are necessary to prevent spread but can be financially and ethically challenging, Balasuriya explained. By law, EIAV-positive horses in many countries, including the U.S., must be euthanized or spend the rest of their lives in permanent quarantine facilities. If they were imported, they can be shipped back to their home country, he added.

Future EIA Research

In their global literature review, researchers revealed EIA spreads unevenly worldwide, with rising cases in the Americas and new outbreaks in Canada, highlighting surveillance gaps and the need for coordinated international control. Collaboration between countries could improve researchers’ understanding of the virus and the fight against this worldwide problem, said Thieulent.

In the meantime, Balasuriya has created an international alliance of scientists willing to collaborate in the fight against EIA.

The study, “Equine infectious anemia virus worldwide prevalence: A 24-year retrospective review of a global equine health concern with far-reaching implications,” appeared in Veterinary Microbiology in May 2025.