Epidemiological, Serological, and Virological Analysis of an Outbreak of Elephant Hemorrhagic Disease in Switzerland

Elephant hemorrhagic disease (EHD), caused by several Elephant endotheliotropic herpesviruses (EEHV), represents a frequently lethal syndrome, affecting both captive and free-living elephants. In summer 2022, three young Asian elephants (Elephas maximus) succumbed to EHD in a zoo in Switzerland, despite of considerable preventive efforts and early detection of EEHV1A viremia. In this communication, we describe the extent of preventive measures in terms of prior virus detection, active survey of viremia, and antibody status. In the course of the outbreak, the causative virus was concomitantly analyzed and eventually fully sequenced and compared to other EEHV types and strains. The conclusions from these analyses may be summarized in three points: (1) A previously undetected EEHV1A strain had remained unrecognized among these elephants. Probably, the new virus re-emerged after almost 40 years of latency from one of the oldest elephants in the zoo. (2) While two of the three affected animals had prior immune responses against EEHV1, their strain-specific immunity proved insufficient to prevent EHD. (3) There is an urgent need to develop efficient antiviral drugs and protective vaccines. In particular, ways need to be found to circumvent the present unavailability of appropriate cell cultures and animal models.