To date, scientists have identified 14 genetically different elephant herpesviruses (EEHV), most of which are known to cause hemorrhagic disease. The viruses found in symptomatic elephants at different zoos and other institutions are genetically distinct, which means that they are not all the same strain spread by the transfers of elephants between and among zoos. The most common cause of acute EEHV cases and deaths in elephants is the EEHV 1A strain.
Herpesviruses are widespread in all vertebrate taxa, including humans. While herpesviruses are usually species specific, they can affect closely related species. (EEHV does not pose a health risk to humans, though humans are host to their own strains of herpesviruses). All herpesviruses do share common some features. Once inside a host, the virus can go into a latent (hidden) phase after causing only mild symptoms or no signs of disease at all. Scientists do not yet know where in the body EEHV resides during the latent phase.
For reasons unknown, an elephant herpesvirus can come out of latency and circulate throughout the bloodstream, causing disease. This is the only time when a herpesvirus can be readily detected in blood samples. Reliable tests are not yet available to detect a latent infection. Most elephants are able to fight the virus and survive when it comes out of latency. Calves appear to be most susceptible to EEHV disease after they have been weaned, at a time when they are not protected by their mother’s antibodies.