The researchers said their findings mark the first clear example of how climate extremes can create conditions in which diseases normally tolerated singly might converge, bringing about mass die-offs in wildlife.

The study -- led by the University of Illinois, the University of California-Davis and the University of Minnesota -- examined outbreaks of canine distemper virus, or CDV, in 1994 and 2001 that caused unusually high mortality among Serengeti lions. CDV cycles periodically within the Serengeti ecosystem, and epidemics have occurred without effects on lion populations.

The study determined both outbreaks were preceded by extreme drought, which led to debilitated populations of Cape buffalo, the lion's prey. After rains resumed, the buffalo suffered heavy tick infestations, resulting in high levels of a tick-borne blood parasite among the lions, which in combination with CDV infections led to the mass deaths.

The researchers said their findings suggest more frequent droughts and floods expected with global warming can alter normal host-pathogen relationships, potentially triggering epidemics with catastrophic mortality.

The study appeared in the June 25 issue of the online journal PLoS One.