An anthrax outbreak has killed at least 180 hippos in a national park in south-western Uganda, reports say.
It was initially unclear what was killing the park's hippos
The first hippo deaths in the Queen Elizabeth National Park occurred in July, but it has taken months to determine the cause.
Scientists say they are struggling to develop a way to diagnose the disease quickly and contain it.
The rest of the park's cattle is being vaccinated and people have been warned to stop eating hippo meat.
"We are vaccinating the livestock around the park or those in close proximity," John Bosco Nuwe, the park's chief warden, told the Associated Press news agency.
"We're telling people not to panic and stop eating hippo meat."
German scientists have identified the disease as anthrax, a soil-borne disease caused by the bacillus anthracis bacterium.
The bacteria produces toxic pores that remain in the soil for years. Symptoms include high fever and bleeding.
A German task force has been set up to fight the epidemic, which Mr Nuwe says has killed 14 buffalos.
The task force will identify anthrax strains, determine contamination levels in the park's soil and water and train Ugandan scientists on new methods of dealing with the bacteria.
While anthrax has been developed into a biological weapon, it exists naturally in most countries around the world.