We've all heard of the warning signs: frothing at the mouth, unusual behavior. These are symptoms associated with rabies, and there's good reason to avoid animals that exhibit them. Indeed, rabies ranks among the most ruthless viruses, killing the vast majority of infected individualscarnivores in particular. But researchers have found a curious exception to that rule. According to a report in the current issue of the Proceedings of the National Academy of Sciences, the spotted hyena can carry the virus without developing symptoms.

The finding came about as a result of research aimed at determining why clinical cases of rabies appear so rarely in Africa's national parks. Focusing on Serengeti National Park in Tanzania, Marion East of the Institute for Zoo and Wildlife Research in Berlin and her colleagues analyzed brain and saliva samples collected from resident carnivores. To their suprise, whereas rabies proved virulent to bat-eared foxes and white-tailed mongooses, for example, spotted hyenas in many cases eliminated the virus from their bodies or else apparently carried low viral loads and experienced no ill effects.

Intriguingly, when the researchers analyzed the rabies virus harbored by the hyenas, it turned out to be a strain genetically distinct from the one carried by the other carnivores. Exactly why the two strains exist in the same ecosystem is a mystery. Similarly, the question of why the hyena strain should be less virulent remains unclear. But the authors note that frequent exposure to small viral loads as a result of social contact among the hyenas might lead to a high degree of immunity