Two young Asian elephants at the Cincinnati Zoo & Botanical Garden have successfully overcome a potentially fatal viral infection after receiving a groundbreaking mRNA vaccine, marking a significant milestone in elephant conservation efforts. The elephants, Sanjay and Kabir, tested positive for Elephant Endotheliotropic Herpesvirus (EEHV) in February 2025 but cleared the infection without developing symptoms, demonstrating the vaccine's protective efficacy in real-world conditions.
First Successful Field Test of EEHV Vaccine
The mRNA vaccine, developed through a collaboration between Houston Zoo and Baylor College of Medicine, represents the first documented success against natural EEHV exposure following vaccination. Both elephants received their vaccine doses in Fall 2024 and developed strong antibody responses approaching levels seen in elephants that had survived natural infections.
"This vaccine, created by Baylor College of Medicine and Houston Zoo, is a tremendous source of hope for the future of Asian elephants, both in zoos and in the wild," said Dr. Mike Wenninger, director of animal health at the Cincinnati Zoo. "We are incredibly grateful for this collaboration and that it helped save Sanjay and Kabir's lives."
Critical Need for EEHV Protection
EEHV poses a severe threat to Asian elephant populations, serving as the leading cause of death for juvenile Asian elephants in North America and Europe with mortality rates between 60% and 80%. The virus has also killed hundreds of elephants in their native range countries, affecting both captive and wild populations.
Sanjay and Kabir were among four elephants transferred from Dublin Zoo to Cincinnati in 2023. Testing by the BCM EEHV serology laboratory revealed that both young elephants lacked maternal antibodies against EEHV, making them especially vulnerable to infection. The Cincinnati Zoo established an in-house EEHV PCR lab to monitor the elephants and administered plasma transfusions from virus-exposed elephants, though antibody levels from transfusions didn't reach those seen in naturally infected survivors.
Vaccine Performance Under Natural Exposure
The vaccine's effectiveness was tested when a herd mate, Sabu, began shedding EEHV in his trunk secretions, creating a natural exposure scenario. In February 2025, low levels of EEHV were detected in Sanjay's blood, indicating early infection. However, Sanjay never developed symptoms and the virus cleared from his system without requiring aggressive treatment. Kabir also experienced a mild EEHV infection and recovered quickly without intervention.
By February 2025, both elephants continued to maintain detectable antibody levels that were sustained longer than those provided by plasma transfusions. Sanjay's viral levels remained low and eventually cleared while his antibody levels continued to rise, suggesting the vaccine helped his immune system control the virus.
Collaborative Development Effort
The vaccine development involved multiple partners, including Dr. Jeroen Pollet of the National School for Tropical Medicine, the International Elephant Foundation, Colossal Biosciences, and the Houston Methodist Center for RNA Therapeutics. The collaboration was led by virologist Dr. Paul Ling, whose work has already advanced EEHV detection and treatment protocols used worldwide.
"This is a powerful example of what can be achieved through collaboration among AZA-accredited zoos," officials from the Cincinnati and Houston Zoos stated. "The success of this vaccine offers real hope for the future of Asian elephant conservation and opens the door for national and international distribution to protect this endangered species."
The successful protection of Sanjay and Kabir represents the first documented cases of natural EEHV exposure following vaccination, providing crucial evidence that the vaccine can prevent severe disease in this endangered species.
