Infectious Diseases

Study finds gaps in Lepto vaccine coverage

By Adam Marcus

Vaccines against leptospirosis appear to leave many horses inadequately defended against the bacteria, according to a study that underscores the need for broader-spectrum inoculations.

The study, which was published in BMC Veterinary Research, tested the blood of nearly 400 horses in Connecticut for antibodies to species of Leptospira, a corkscrew-shaped microbe that spreads among animals through contact with contaminated urine or the environment. Of those tested, 61.5% were seropositive, with 40 strains of Leptospira covering 26 distinct serogroups and 4 undetermined isolates.

“The main issue with lepto is that it’s a zoonotic disease. You have to be concerned not only about the animal that’s infected by their environment, but about the people working with those animals, and other species they might come into contact with,” said Elsio Augusto Wunder Jr., DVM, PhD, a professor in the Department of Pathobiology and Veterinary Science in the College of Agriculture, Health and Natural Resources at the University of Connecticut, who helped conduct the study. “Our research shows there are more ‘leptos’ out there than we think.”

Leptospirosis is rarely deadly in horses, but it can lead to spontaneous abortions, uveitis and other serious and costly health problems.

The analysis showed that Djasiman, Grippotyphosa, Australis, Pomona, Icterohaemorrhagiae and serovar Room22 strain GWTS1 were the most frequently detected serogroups, and nearly 33% of infected animals tested positive for multiple serogroups.

Like all Leptospira vaccines, the one that’s given to horses contains inactivated cells from a specific serogroup—in this case, Pomona. The microscopic agglutination test the researchers used in the study does not differentiate between natural exposure to the bacteria and vaccination against it. But after excluding Pomona from the analysis, roughly half (51.4%) of horses in the study had antibodies to other forms of the bacteria. That finding was “striking,” Dr. Wunder said, as was the high number of isolates of the Djasiman serovar.

“We recently isolated that same serogroup from an animal that had clinical signs” of infection, he added, suggesting the serovar is not benign and indicates “how much exposure you see for a serovar that’s not in the vaccine.”

The lack of a broad-spectrum, or multivalent, equine vaccine against leptospirosis is a barnyard anomaly. Dogs, pigs and cows all have such vaccines, dating back as far as 50 years, Dr. Wunder said. “What is happening—and not just in equines—is that the epidemiology of lepto has been changing. Vaccinating against some strains gives the opportunity for others to take their places,” he said. Climate factors, such as increased rainfall and warming temperatures, are also spurring the spread of serovars not covered by the vaccine, he noted.

Dr. Wunder, who has a public health grant from the National Institutes of Health to develop a universal vaccine against leptospirosis, said he and his colleagues hope the study’s results will encourage animal vaccine producers to push for more broadly effective immunizations, too. Although the current vaccines are good at preventing clinical disease, they are not effective at reducing colonization—meaning exposed animals, including horses, continue excreting bacteria in their urine, according to Dr. Wunder. He told The Modern Equine Vet that he hopes that a universal vaccine could benefit humans and animals because the strains in that cause disease in both are similar.

The barrier to success is steep. The Leptospira genus has 74 species, of which 43 are known to be infectious to humans and animals. Within that genus are more than 300 serovars that can infect and cause disease, Dr. Wunder said. Vaccines protect against a tiny fraction of those serogroups—1 for horses and 5 for cattle, for example.

The current vaccines are also built on an older understanding of the disease, with foundational studies conducted in developed countries as many as 50 years ago. “That doesn’t make sense in China, Brazil, Sri Lanka and other places where lepto is prevalent now,” Dr. Wunder said. “There’s a huge path to improve them.”

Regardless, the bottom line with leptospirosis, as with all vaccines, Dr. Wunder said, is simple: “You should vaccinate.”

Although they might not be perfect or provide the widest coverage, “they’re better than not vaccinating.”

FOR MORE INFORMATION:
Hamond C, Kangas CL, Lee VW, et al. Seroprevalence of equine leptospirosis in Connecticut, US. BMC Vet Res. 2026 Aug 1.