Rangers have suspended safari rides in a popular nature reserve in eastern India after five one-horned female rhinoceroses died from a suspected infectious disease, officials said Monday.  

The animals were found dead over four days last week in Jaldapara National Park, nearly 700 kilometres (434 miles) north of West Bengal state’s capital Kolkata.

India is home to two-thirds of the world’s remaining one-horned rhinos, a vulnerable species on the IUCN red list

“Blood smears from carcasses have been sent to a laboratory in Kolkata,” the reserve’s chief conservator Ujjal Ghosh told AFP.

“All the five dead rhinos were adult females. We have put our staff on alert.”

The park—spread over 200 square kilometres (77 square miles) in the foothills of the eastern Himalayas—is home to 204 rhinos according to the last official count in 2015.

More than 70 captive elephants used for safaris and patrolling also live in the reserve. The safari rides are carried out on elephants.

Activists said the animals may have died from anthrax, a communicable disease that attacks herbivores.

Humans can contract anthrax directly or indirectly from animals or animal products.

“We suspect that the animals died from a communicable disease like anthrax. Jaldapara forest has the odd case of anthrax which killed animals earlier,” wildlife activist Animesh Bose told AFP.

Rangers were riding on elephants to reach the rhinos and vaccinate them using dart guns, the Hindustan Times reported.

Drones would try to find out if other animals have died or fallen ill, the newspaper said.