A "perfect storm" of diseases can get unleashed by the kind of extreme swings in weather expected with global warming, triggering mass die-offs of wildlife or livestock, research now reveals.
Now the first clear example of such a perfect storm of diseases has been discovered by an international team of scientists.
Global warming is predicted to lead to extreme swings in weather events such as droughts and floods. These could theoretically lead normally tolerable diseases to converge and trigger multiple outbreaks of epidemics with catastrophic mortality.
The clear example of a disease cascade came when researchers investigated outbreaks of canine distemper virus that killed an unusually high number of lions in East Africa, at Tanzania at Serengeti National Park in 1994 and Ngorongoro Crater in 2001.
These infections can have awful effects, such as "a grand mal seizure — the animal is unable to control its movements, starts thrashing about helplessly with every muscle in its body flexed to the maximum, grinding its teeth and foaming at the mouth," said researcher Craig Packer, an ecologist at the University of Minnesota. "Then the seizure stops for a few minutes before starting all over again."
Numerous epidemics of this virus have occurred within these ecosystems over the past 30 years that had proved essentially harmless to the lions, however the lions that survived the 1994 and 2001 distemper epidemics were in unusually poor condition.
"The lions were lethargic, thin, anemic, and had enlarged lymph nodes, physical changes that do not usually occur after recovery from canine distemper virus," said researcher Linda Munson, a veterinary pathologist at the University of California, Davis.
The reason could be that the virus outbreaks in 1994 and 2001 were both preceded by severe droughts, one of the types of weather events predicted to occur more frequently as Earth's climate continues to warm. This debilitated populations of Cape buffalo, a major prey of lions.
After the rains returned, the weakened, starving buffalo suffered heavy tick infestations, resulting in high levels of a tick-borne blood parasite in the lions. These parasites are normally present in the felines at harmless levels.
The canine distemper virus had suppressed the immune systems of the lions, which was already challenged by the high level of blood parasites — a sort of one-two punch. The tick-borne disease thus reached fatally high levels, leading to mass die-offs of lions. The poor condition of survivors of the 1994 and 2001 epidemics also turned out to be due to very high levels of blood parasites.
It was known that global warming and climate change can alter or expand the range of germs, but now we also know that it could "dramatically alter the normal balance between hosts, their parasites and the pathogens those ticks transmit in the same ecosystem where these relationships have been in balance for years," Munson said.
The number of lions analyzed in the Serengeti in 1994 dropped by more than a third after the double infection. Similar losses occurred in Ngorongoro Crater in 2001.
"This is a good example of how extreme variations in climate can lead to disease outbreaks," said Princeton University ecologist Andrew Dobson, who did not participate in this study. "We'll have to look for more and more examples of this as climate gets more variable."
Co-infections may lie at the heart of many of the most serious die-offs in nature, Packer said. Dobson added, "It's likely going on all the time — there just aren't enough people doing this kind of long-term study to see it."
Another place to look for the potential impact of co-infections would be colony collapse disorder in honeybees, he added. This mysterious ailment is claiming the lives of an alarming amount of the bees that help pollinate dozens of key flowering crops, such as apples and citrus fruit.
"There is a strong suspicion that colony collapse disorder is caused by co-infection of multiple disease agents, but more research is needed to nail it down," Packer told LiveScience.
The lion populations recovered quickly, within years of each of the two big die-offs. However, most climate change models predict an increase in droughts in East Africa, so the lions' ability to rebound might increasingly get challenged.
"The next step would be to try to minimize ticks on the lions during the next drought to see if tick removal protected the lions from mortality in case of a co-incident outbreak of distemper," Packer said.
Munson, Packer and colleagues detailed their findings in the June 25 issue of the journal PLoS ONE.