Water: An Underrecognized Vector for the Transmission of Equine Herpesviruses

Life on Earth depends on water, which is not available in sufficient quantities everywhere. In the world’s arid regions, animals gather around scarce water sources to survive. However, these gatherings can become epicenters of viral transmission that trigger epidemics.   When water becomes scarce, the high density of animal species gathering to drink at these rare water sources can quickly turn these locations into hotspots for viral transmission—both within and between species. An international team of scientists led by the German Leibniz Institute for Wildlife and Zoo Research (Leibniz-IZW) investigated freshwater sources as vectors of viral transmission within mammal populations. They sought to determine whether mammalian viruses could remain stable enough in water to spread effectively. This is particularly true of equine herpesviruses. While it is well established that they remain stable and infectious in water for weeks in the laboratory, what about in a natural aquatic environment? Culturing a virus in nature poses a real challenge. Other microorganisms thrive there and can interfere with viral spread. However, the researchers were able to demonstrate that equine herpesviruses can indeed replicate and remain infectious in water samples collected in Africa and Mongolia. But equine herpesviruses have also adapted to the living conditions of animals that depend on water sources. When zebras gather at watering holes during the dry season, their stress levels rise, which has an unexpected consequence. In these equines, stress is in fact associated with the shedding of the herpesviruses they carry into the environment, precisely when they gather to drink. Stress acts as a signal for the virus, which infects the water to spread to other equines—but that’s not all. Animals that share the watering hole with zebras are also exposed, which explains, for example, the cross-contamination of rhinos with equine herpesvirus. The study’s findings also reveal a rather strange characteristic of equine herpesviruses. Their evolution remains limited, and they change little over time, whereas most viruses tend to mutate rapidly. The equine herpesviruses found in Mongolia and Africa are nearly identical to those circulating among domestic horses. According to the scientists, the stability of these viruses over time is likely the result of a long evolutionary process that allowed them to adapt to using water as a vector. Even the slightest mutation could have disrupted this state of equilibrium, leading them to an evolutionary dead end. Equine herpesviruses do not shed or spread through water alone. Further research should reveal the existence of other viruses capable of using water as a vector to infect animals, but this will certainly require moving beyond a simple study of viral dynamics and host-pathogen interactions to include factors related to adaptation to the environment, more specifically to aquatic life.