At least 50 hippos and other large animals have been killed by anthrax poisoning in eastern Congo’s Virunga National Park and have been spotted floating along a major river that feeds one of Africa’s great lakes, the head of the park said on Tuesday.

Tests confirmed anthrax poisoning, said Virunga Park director Emmanuel De Merode, adding that buffalo have been killed too. The exact cause of the poisoning was not yet clear.

De Merode said that a team was on site and that they were trying to get the hippos out of the water and bury them, but that it was difficult because they did not have excavators.

“It’s difficult due to lack of access and logistics,” De Merode told Reuters. “We have the means to limit the spread (of the disease) by…burying them with caustic soda.”