John Roach, National Geographic News 25 Jun 08;
Droughts and downpours exacerbated by climate change allowed two diseases to converge and wipe out large numbers of African lions in 1994 and 2001, according to a new study.
Lions regularly survive outbreaks of canine distemper virus (CDV) and infestations by a tick-borne blood parasite called Babesia. But both normally occur in isolation.
In 1994 and 2001, however, a "perfect storm" of extreme drought followed by heavy seasonal rains set up the conditions for the two diseases to converge, the study said.
The effect was lethal: The synchronized infections wiped out about a third of the Serengeti lion population in 1994. The nearby Ngorongoro Crater lion population experienced similar losses in 2001.
(Read a National Geographic magazine online extra about Serengeti and other lions.)
"It was already well known that die offs can be triggered by droughts and floods," Craig Packer, an ecologist at the University of Minnesota in St. Paul, explained in an email from his research site in Tanzania.
"We were able to identify the interacting components of a lethal co-infection that had not previously been considered," he said.
The research is published in today's issue of the journal PLoS ONE.
"Lethal One-Two Punch"
Packer and his colleagues combed through more than 30 years of data on the lion populations to determine the complex combination of factors that caused the mass die offs.
They found that at least five CDV outbreaks swept through the lion populations with no ill effect. The two die offs, which are also tied to CDV outbreaks, were preceded by extreme droughts.
Probing further, the researchers discovered the droughts weakened lion prey, including the Cape buffalo.
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.
Conservation Implications
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.
