Foals of mares vaccinated for Hendra virus have a suboptimal response to HeV vaccination.

Overview

• This study examined how baby horses (foals) born to mares vaccinated against Hendra virus (HeV) respond to their own HeV vaccinations.
• It found that foals initially receive protective antibodies from their mothers but respond poorly to their own vaccinations, likely due to interference from maternal antibodies.

Background on Hendra Virus and Vaccination

• Hendra virus (HeV) is a highly lethal virus affecting horses and can be transmitted to humans, causing serious neurological and respiratory illness with high death rates in Australia.
• An equine vaccine against HeV has been available since 2012 to protect horses from infection.
• Foals receive maternal antibodies by ingesting colostrum (first milk) shortly after birth, providing early passive immunity during the first months of life.
• It was assumed foals of vaccinated mares have sufficient circulating antibodies to protect them until they can be actively vaccinated.
• Prior to this study, no research had been conducted on passive immunity (maternal antibody levels) or active immunity (foals’ own response) against HeV in foals.

Objectives and Methods of the Study

• The study aimed to investigate anti-HeV antibody levels both in vaccinated mares at parturition and in their foals over the first six months of life.
• Testing for HeV neutralising antibodies usually requires Biosafety Level 4 (BSL-4) facilities due to the infectious virus used in virus neutralisation tests (VNT), which are complex and high containment.
• The researchers used two methods to measure HeV antibodies:
• Authentic Virus Neutralisation Test (VNT) using infectious HeV at BSL-4.
• Microsphere-Based Immunoassay (MIA) detecting antibodies against the HeV glycoprotein (G protein), which is safer and easier to perform.
• A subset of samples was tested using both methods to validate the MIA by demonstrating a strong correlation with VNT results.
• This correlation allowed researchers to use MIA results as an indicator of neutralising antibody levels in a larger sample set.

Key Findings

• Mares at the time of parturition had high levels of neutralising antibodies against HeV due to vaccination.
• Foals acquired high levels of maternal antibodies shortly after birth through colostrum.
• These maternal antibodies declined over time, dropping below the levels predictive of protection in most foals by the age of 6 months, which is when foal vaccination typically begins.
• Foals demonstrated a suboptimal immune response to their own HeV vaccinations.
• This reduced response is likely because maternal antibodies interfere with the foals’ ability to mount an effective active immunity when vaccinated.

Implications of the Study

• The interference by maternal antibodies with foal vaccination suggests current vaccination protocols may not induce sufficient protection in young foals.
• The validated microsphere-based immunoassay (MIA) provides a reliable, high-throughput method to assess HeV antibody levels without requiring BSL-4 containment.
• This MIA tool can facilitate further serological studies on larger populations of horses to optimize timing and strategies for vaccinating both pregnant mares and their foals.
• Optimizing vaccine protocols could improve passive and active immunity, potentially reducing risk of HeV infection in foals and breaking a window of vulnerability.