Infectious Diseases

A 4-year look at 2 viral causes of hepatitis

By Paul Basilio

Equine parvovirus-hepatitis (EqPV-H) and equine hepacivirus (EqHV) are 2 highly seroprevalent viruses that have attracted significant interest in recent years. These viral infections are significant causes of chronic (EqHV) and acute (EqPV-H) hepatitis in horses, but researchers are still trying to gain a clearer picture regarding their transmission.

As part of this clinical clarification, a team of researchers led by Joy Tomlinson, DVM, PhD, DACVIM, created a prospective longitudinal cohort study to determine transmission rates, clearance rates and the seasonality of viral transmission. Dr. Tomlinson presented their findings at the 2026 ACVIM Forum, in Seattle.

The study
“I enrolled all of the horses that I could find in the area at Cornell and ‘roped’ in someone in Florida,” said Dr. Tomlinson, an assistant professor of Large Animal Medicine at the New Bolton Center, University of Pennsylvania, in Kennett Square, Pa. “We also enrolled 1 breeding farm that had had a previous outbreak of parvovirus.”

The researchers performed quarterly serologic testing using a luciferase immunoprecipitation system, which she described as a “crude” enzyme-linked immunosorbent assay (ELISA), and they evaluated viremia using real-time quantitative polymerase chain reaction (PCR) for EqHV and EqPV-H. The outcomes of interest were new infections and infection chronicity.

At the breeding farm, the team followed the mares and foals for 1 breeding season. They obtained samples from both the mare at foaling and the foal either at birth or at 1 day of age, and then at 1 month and 2 months of age.

“On this particular farm, they also all received Rhodococcus equi plasma at birth and at 1 month of age,” said Dr. Tomlinson, who also directs the Tomlinson Equine Hepatitis Laboratory. “This was after the USDA regulations [requiring PCR-negative plasma] were put in place. We had done a similar approach on this same farm before the USDA regulations were in place, so this was a little comparison.”

A total of 7 herds were enrolled, including teaching and research herds, riding herds, a polo team and the breeding farm. In all, 343 horses were tested at least once during the 4-year study period.

“There was a surprising amount of horse turnover, so I only had 23 horses that were followed for the entire time,” Dr. Tomlinson said. “That was a lot less than I would have thought, but we got decent amounts of follow-up for varying periods.”

EqPV-H
Initial results showed a steadily increasing prevalence during the course of the study. Dr. Tomlinson was excited initially, but a closer look revealed that the increase in prevalence was due to horses moving in and out of the herd, not to actual transmission. The final analysis showed just 1 new infection over the entire 4-year period.

“That’s great news for those of us trying to manage the disease, but pretty sad for meeting the study objectives,” she joked.
As for that 1 new infection, the team does not know where it came from. The horse was residing with chronically infected horses, but there were no known acute infections nearby. Since the virus is quite stable, tracing the infection through sequencing in herd mates is difficult.

Interestingly, once the horses seroconverted, they stayed seropositive and infected. “We had 12 horses that were seropositive from the beginning through the end of the study—that was 3.5 years of follow-up,”
Dr. Tomlinson explained. “Two of them were positive on PCR at every sampling time point, but the rest were intermittently positive and negative.”

As a result, once a horse is positive, Dr. Tomlinson considers it always positive. “We don’t currently have a way to tell if they’re truly negative,” she added.

All foals were given 2 lots of PCR-negative R. equi plasma, but 47 (60%) received seropositive plasma. Overall, one-third of the foals that received seropositive plasma became infected.

“The current USDA regulation has helped, but it has not prevented biologic transmission,” Dr. Tomlinson said. “PCR is insensitive to detect chronic infection, and serology is more sensitive. I do think we need to develop a serologic assay. If you are dealing with blood donor herds or any other sort of allogeneic product herd, consider once positive, always positive. Remove them from your herd or use them with the warning to the client that they may have a transmissible disease.”

EqHV
In contrast to parvovirus—which is a DNA virus—EqHV is an RNA virus and the closest currently recognized genetic relative of hepatitis C virus (HCV). It is also clinically close to HCV, in that EqHV recapitulates a lot of the natural disease phenomena associated with HCV infection, including the dual infection outcome.

“About 80% of horses are able to clear the EqHV infection while experiencing a mild subclinical hepatitis,” Dr. Tomlinson explained. “You may or may not be able to detect that mild rise in liver enzymes.”

However, about 20% of horses will develop persistent infections. Of the persistently infected horses, Dr. Tomlinson said that some will have no detectable disease, some will have mild disease, and some will progress to severe fibrosis, cirrhosis, hepatic failure and death.

“A proportion of cases will present with clinical signs including weight loss or signs of liver failure,” she said. “The initial injury is a hepatocellular injury, so you’ll see variable rises in your hepatocellular enzymes, but they’re usually fairly mild. This is more of a chronic smoldering disease with progressive fibrosis over time rather than an acute overwhelming hepatocellular death.”

The virus is also everywhere—it’s been found in every country that has looked for it so far. There are lower viremia rates, but it seems more transmissible than EqPV-H. In some areas, seroprevalence rates are as high as 80%. Current research shows a global average PCR-positive rate of about 7%, with about half of all horses showing seropositivity.

“We know it can be transmitted iatrogenically, but that cannot account for this high infection rate,”
Dr. Tomlinson said.

In her study, Dr. Tomlinson did observe transmission of EqHV. Overall, the team documented 26 horses with resolving infections and 17 with persistent infections, for a prevalence rate of 2% to 8%, which closely tracks the global prevalence of 7%.

“I suspect that those Coggins sample surveys were mostly capturing the persistently infected horses, which would mean that about 7% of horses in the US are persistently infected,” she said. “That means that 7% of all horses in the US are at risk of getting this chronic hepatitis, which is quite a lot.”

The timing of new infections observed in the study overlapped with the mosquito activity season, suggesting that an insect vector for EqHV is plausible but not definitive. “We also saw quite a number of horses with a seasonal summer and autumn rise in their antibody titers, which potentially would indicate re-exposure,”
Dr. Tomlinson said. “Preliminary epidemiological models do indicate that the persistently infected horses are required for transmission, so it doesn’t seem likely that there is another reservoir species.”

Takeaways
For EqPV-H, Dr. Tomlinson said that she does not believe that chronically infected horses need to be isolated, which is good news. However, equine biologic products sold under current regulatory guidelines can still transmit parvovirus.

“If you have a Theiler’s case and you suspect parvovirus, you should still be asking about biologic product history,” she said. “You should still be getting those lot numbers and reporting them to the USDA and to the company.”

For EqHV, she said that keeping a persistently infected horse in a herd of adult horses should not cause concern, but keeping foals on that farm may not be a reasonable option, as there are some data indicating foals are more likely to develop persistent infections. MeV