When the rains resumed, Babesia-carrying ticks emerged en masse and proliferated in their buffalo hosts. Many of the buffalo died.
The lions feasted on the weakened, parasite-infested buffalo, but the feast left the hunters with unusually high concentrations of Babesia. The subsequent CDV outbreak proved lethal, according to the study.
"CDV is immunosuppressive—like a short, sharp bout of AIDS—thus greatly intensifying the effects of the Babesia," Packer said.
This co-infection, or synchronization of the diseases, caused the mass die offs, Packer and his colleagues concluded.
Sonia Altizer is an ecologist who studies wildlife diseases at the University of Georgia in Athens. She was not involved with this study, which she said is "at the leading edge" of the field.
"[It] lays out mechanistically how a climate anomaly could allow a combination of pathogens to have a lethal one-two punch," she said.
Study author Packer and his colleagues warn that as global climate change continues to produce more extreme weather anomalies, potentially fatal synchronized infections are likely to become more common.
"Many mysterious maladies [such as] colony collapse disorder in honeybees are likely to result from co-infections," Packer noted.
Altizer said the research adds to a growing body of evidence showing how extreme climate events can have major impacts on the spread of infectious diseases.
Since more deadly co-infections are likely to arise, she said researchers need to reconsider how they treat wildlife and humans.
"Understanding the mechanism by which the animals are actually dying or succumbing to disease then changes how you should go about preventing that," she explained.
In the case of the lions, Packer noted, wildlife managers may be able to better protect populations by reducing their tick loads immediately following a drought rather than controlling for CDV.
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