INDIANAPOLIS (WISH) — Two elephants at the Indianapolis Zoo that tested positive for a deadly virus in May are now healthy, the zoo said Tuesday.

Kedar and Zahara were the third and fourth elephants at Indianapolis Zoo to test positive for elephant endotheliotropic herpesvirus this year. The other two, 6-year-old Nyah and 8-year-old Kalina, died in March a week after contracting the aggressive herpes virus. 

The two elephants “have beaten the flare up of the virus” and will continue to be monitored through frequent blood testing, along with the rest of the zoo’s elephants, a spokesperson for the zoo said.

Zahara, a 12-year-old female, tested positive on May 21. Kedar, a 13-year-old male African elephant, tested positive on May 9. In mid-May, the zoo said Kedar was showing improved health after receiving antiviral medicine.

According to the zoo, it’s still not known what causes the virus to move out of the dormant stage, and the virus exists in all elephant populations: in the sanctuaries, zoos and the wild.

There is no vaccine to fight the virus, according to experts. The virus cannot be contracted by humans or other animals. It attacks endothelial cells in elephants, rupturing capillaries and causing hemorrhaging. 

The hemorrhagic disease is the leading cause of death in juvenile Asian elephants born in North America, said Paul Ling, an associate professor at Baylor College of Medicine and member of the International Elephant Foundation’s EEHV Advisory Committee.