Why don't animals get sick from filthy, shrinking water holes in Africa?

This week's question comes from Michel Dubuc in Ottawa, Ontario. Dubuc asks: 

Elephants, crocodiles and many other wild animals will congregate around reduced watering holes during the dry season. They all drink from what seems like these bacteria- and feces-infested ponds, yet they don't seem to get ill. Why is that?

Dr. Charlene Berkvens, associate veterinarian at the Assiniboine Park Zoo in Winnipeg, said particularly in an African Serengeti setting, wild animals are often driven to reduced watering holes when their larger water sources dry up amid seasonal drought periods. This can mean they end up slurping from what looks like dirty water because it's their only option.

They don't always get sick, though, and that could be due to a few things.

For one, Berkvens said that what looks like a bacteria-ridden or feces-infested pond may in reality be primarily composed of dirt, mineral and non-disease causing organisms. Wild animals may get exposed to low levels of these organisms through the course of their lives and build up immunity.

But watering holes can be a significant source of disease in wildlife in these places, said Berkvens. The congregation of a large number of animals from different locations at these watering holes, especially during droughts, can lead to the spread of infectious diseases such as viruses.

Some diseases have been linked to these locations in some instance, including anthrax. It's one of the classical bacterial diseases that can be fatal in a wide variety of species including people. Berkvens said last year, hundreds of elephants died in Botswana as the result of exposure to neurotoxins produced by blue-green algae in watering holes.

Berkvens says in some instances contaminated water can be a serious treat to wildlife, and climate change has the potential to worsen these effects.