In Virunga National Park, which is in the DRC, 50 hippos were killed by anthrax.

At least 50 hippos and other large animals have been killed by anthraxpoisoning in eastern Democratic Republic of 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.

Images shared by the park show the hippopotamuses motionless on their sides and backs in the Ishasha River, or caught among foliage on the river’s muddy banks.

The deaths represent a major loss for the park, which has been working to increase the number of hippos in recent decades after poaching and war reduced the population from over 20,000 to a few hundred by 2006. The park now holds about 1,200 hippos.

Park guards noticed there was a problem when the dead animals started to appear about five days ago along the river, which forms Congo’s border with Uganda and runs through an area under the control of rebel fighters.

Anthrax is a serious disease usually caused by bacteria found naturally in soil. Wild animals can become infected if they inhale anthrax spores in contaminated soil, plants, or water.

In a statement on Tuesday, the Congolese Institute for Nature Conservation warned residents to avoid wildlife in the area and to boil water from local sources before drinking.

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.”

The river runs north to Lake Edward, where locals spotted more corpses.

“There are more than 25 hippopotamus bodies floating in the waters of the lake, from Kagezi to Nyakakoma,” Thomas Kambale, a civil society leader in Nyakakoma, told Reuters.

Virunga is a vast expanse of deep forests, glaciers and volcanos, with more species of birds, reptiles, and mammals than any other protected area in the world.

It has been caught in the middle of militia activity since civil wars fought around the turn of the century.