Climate extremes can intensify the impact of common infections
Models of the planet’s changing climate predict more frequent episodes of extreme weather like storms and droughts. Those are bad enough, but now an international research team has shown the first clear example of how such events can magnify the impact of disease organisms by causing their effects to converge, killing off masses of wildlife or livestock.
In this case, it was African lions that paid the price. The study, led by University lion researcher Craig Packer and colleagues at the University of California, Davis, and the University of Illinois, uncovered evidence that drought set the stage for a combination of distemper and tick-borne parasites to kill unusually large numbers of lions. It was published June 25 in the Public Library of Science.
The researchers followed lions in Tanzania’s Serengeti National Park and Ngorongoro Crater, two sites where Packer and other researchers have studied populations of the big cats for decades. In both places, lions are periodically infected by the canine distemper virus (CDV), which suppresses the immune system but ordinarily causes little or no harm.
But CDV outbreaks in 1994 in the Serengeti and in 2001 in Ngorongoro Crater led to heavy losses of lions. What was different this time was that both outbreaks followed a severe drought, which weakened the large herbivores on which lions depend for food. Most notably affected were the Cape buffalo.
“The study illustrates how ecological factors can produce unprecedented mortality events and suggests that co-infections may lie at the heart of many of the most serious die-offs in nature.”
When the rains resumed, they brought a flood of ticks, which attacked the starving buffalo en masse. Lions found the dead or dying buffalo easy pickings, but by eating them they also consumed much larger than normal quantities of a tick-borne parasite. With the lions’ immune systems already suppressed by CDV, the parasites devastated them.
“The study illustrates how ecological factors can produce unprecedented mortality events and suggests that co-infections may lie at the heart of many of the most serious die-offs in nature,” says Packer, a Distinguished McKnight University Professor of Ecology, Evolution and Behavior at the University.
The lions of the Crater suffered from higher levels of parasites than those of the Serengeti. For one thing, that area is full of both large herbivorous mammals and carnivores that can carry the parasites. But human behavior may also have been a factor in the heavy infestation of ticks.
“Grass fire on the Crater floor was actively suppressed from the 1970s until the high tick infestations of 2001 (and associated deaths of several rhinoceros) led to a new policy of controlled burning,” say the researchers.
Also, the Crater lions are highly inbred compared to their Serengeti counterparts, and this could leave them more susceptible to parasitic and viral infections. Although about one in three lions died in both the Serengeti and the Crater, their numbers recovered. However, this scenario is likely to recur, since climate change models predict more frequent drought in East Africa.
“If extreme weather events become increasingly frequent owing to global climate change,” the researchers concluded, “the consequent synchronization of proliferating pathogens or their vectors [such as ticks] may cause disease to become a major threat to historically stable populations that had previously coexisted with multiple viral and parasitic pathogens.”