Infectious Diseases

Repeat EHV-1 Immunization May Prevent Infection

By Paul Basilio

Equine herpesvirus 1 (EHV-1) is a highly contagious, highly prevalent virus that causes disease worldwide. Although the virus causes mostly mild disease, severe disease is possible, and ongoing outbreaks have the potential to have a high economic impact.

New research presented by Bettina Wagner, DVM, Dr med Habil, a professor of immunology at Cornell University, shows that repeated intramuscular (IM) administration of an EHV-1 vaccine can induce EHV-1–specific mucosal antibodies to effectively prevent infection and disease, as well as reduce nasal shedding and cell-associated viremia.

Background
EHV-1 enters the horse through the upper respiratory tract, where it spreads to regional lymph nodes. From there, it establishes leukocyte-associated viremia in the peripheral blood.

“It can stop there and only cause fever and respiratory signs,” Dr. Wagner said during a presentation at the 2024 ACVIM Forum in Minneapolis. “But in some cases, the virus can spread to the vascular endothelial cells, where it can then go to the pregnant uterus or to the CNS to establish more severe forms of EHV-1.”

Control of the leukocyte-associated viremia is therefore seen as 1 of the key parameters for protecting horses against the virus. In the past, this has been established mainly by peripheral T-cell immunity, but recent research has shown that local mucosal immunity in the upper respiratory tract can provide protective immunity against severe outcomes.

Dr. Wagner said that mucosal antibodies (mucAbs) can neutralize EHV-1 and prevent replication in the upper respiratory tract. When horses are protected, they only show low or no viral replication in the upper respiratory tract, and they show low or no cell-associated viremia, which prevents severe outcomes. In addition, recent research has shown that an intranasal modified-live vaccine resulted in no mucosal antibodies.

The Study
Dr. Wagner and her colleagues designed a longitudinal vaccination study to test 2 hypotheses: IM EHV-1 vaccination will result in both systemic and mucAbs, and frequent vaccination will not increase antibodies beyond certain concentrations.

A total of 14 adult Icelandic research horses (10 mares and 5 geldings) with prior EVH-1 exposure were enrolled. They were divided into 2 groups, depending on whether they had been experimentally infected or whether they had been vaccinated. On average, each horse had their last exposure between 1 and 2 years before the study.

Each horse received a dose of the EHV-1 vaccine (Calvenza, Boehringer Ingelheim Animal Health) at days 0 and 22, and at 2, 3, 6, and 8 months. Baseline blood samples showed that each horse had an existing protective antibody response. Nasal swab and serum samples were obtained at different times and tested for EHV-1–specific antibodies as part of the EHV-1 risk evaluation assay from Cornell. They also focused on immunoglobulin G (IgG) 4/7, which has been shown to neutralize EHV-1.

EHV-1 antibodies increased in all horses after the second vaccination at 22 days, after which they hit a plateau.

“In horses with pre-existing immunity, you can only do so much,” Dr. Wagner said.

For mucAbs, however, the results were a bit different. All horses had existing mucosal immunity, and mucAbs increased after each vaccination, with the exception of 1. Significant antibody increases were seen at 6, 9 and 10 months compared with baseline, which confirmed that IM vaccination provokes
mucAbs at the upper respiratory tract.

“If you vaccinate a horse intramuscularly, that provokes an activation of the B-cell memory response at the regional lymph nodes,” Dr. Wagner explained. “They have memory cells that are activated, and within 3 to 5 days we see antibody-secreting cells in the peripheral blood.”

Those are called plasmablasts, and they typically travel to the bone marrow, where they reside as plasma cells.

“One hypothesis is that a large number of antibodies circulate in the periphery and go on to the respiratory mucosa via some mechanism that we don’t know about,” she said. “The second hypothesis is that those plasmablasts not only reside in the bone marrow but that they can also reside in the respiratory mucosa and provide the local antibodies that we see in our study.”

Dr. Wagner noted that the dosing schedule used in the study is not recommended by the manufacturer.

“It is likely that [intramuscular] vaccines that induce EHV-1–specific mucosal antibodies effectively prevent infection and disease, and also reduce nasal shedding and cell-associated viremia,” she said. “Of course, individual vaccine formulations may differ in their ability to induce mucosal antibodies, and especially IgG 4/7.” MeV

Dr. Wagner is the director of Serology/Immunology at the Animal Health Diagnostic Center at Cornell University. This study was funded by the Harry M. Zweig Memorial Fund for Equine Research.