How is the hepatitis C-like hepacivirus of horses transmitted?

Posted on October 11, 2016   by Benjamin Thompson

In 2011, a new virus was identified in dogs that were suffering from respiratory disease. Analysis revealed that this virus – at the time named canine hepacivirus (CHV) – was the closest known relative of the human virus hepatitis C (HCV), which infects millions of people across the world and can cause serious liver problems.

Previously, HCV and its relatives were believed to be limited to humans and chimpanzees, so the discovery of a virus that infected another species was exciting for researchers. It opened up the possibility that canine hepacivirus could be used as a model to understand how HCV infects humans.

However, further searches in other dogs failed to detect any more of the virus, suggesting that canines weren’t CHV’s natural host after all. CHV-like viruses were subsequently identified in horses, while an almost identical match was found in horse blood serum in New Zealand, raising the possibility that this virus is one that can infect not just dogs, but a variety of non-primate species.

The virus has since been named nonprimate hepacivirus (NPHV) to reflect this.

Estimates suggest that NPHV is widespread in horses, infecting between 2–7% of animals, depending on location and breed. Not much is known about what the virus actually does – it may be involved in the development of hepatitis, but the majority of infections appear to be mild, identified only by an increase in liver enzyme levels.

Another unknown is the route by which the virus is naturally transmitted between horses. New research, published in the Microbiology Society’s Journal of General Virology, investigated ‘vertical transmission’ of NPHV – from mother to child (mare to foal, in horse terms).

The research team, led by Dr Jessika Cavalleri and Professor Eike Steinmann, followed 20 Thoroughbred mares in late pregnancy at a stud farm in Germany. To find evidence of when the virus is transmitted, the team took samples from the horses pre- and post-birth, looking for either viral genetic material or antibodies against the virus.

Results showed that, of the 20 mares, 16 contained antibodies in their blood against NPHV, while four tested positive for the presence of NPHV RNA. Of these four infected mares, only one gave birth to a foal that tested positive for the virus, both in its umbilical blood and its blood serum 14 hours after birth. Umbilical blood was used as a substitute for foetal blood, as horse placentas are different from human ones­ – the mare and foal’s blood are completely separate.

This result suggests that the virus was transmitted during pregnancy, at an as yet undefined time. In humans, estimates suggest that 5% of infants that are born to hepatitis C-infected mothers become infected themselves, although again the exact time and mechanism are poorly understood.

To complicate matters further, the research team showed that seven other foals became infected with NPHV within six months of being born. While one received a blood plasma transfusion, which may be the route of infection, the way that the infection was passed on to the other foals will require further research. It could be that nasal secretions are responsible, or potentially an insect vector could pass the virus between horses. The mystery of this horizontal viral transmission is something that the research group are currently working to solve.

While there remains much that we don’t know about the new virus, this work represents the first evidence that it can be passed down from mare to foal. Only further research will help to establish where it’s found, what it’s actually doing, and how it’s passed between horses and, potentially, species.

Brown, R., Cavalleri, J., Todt, D., Pfaender, S., Steinmann, E., Walter, S., Feige, K., Gather, T., Baumgaertner, W., Becher, P., Hansmann, F., Moritz, A., & Postel, A. (2016). Vertical transmission of hepatitis C virus-like nonprimate hepacivirus in horses Journal of General Virology DOI: 10.1099/jgv.0.000561