Wild lion populations can generally tolerate a certain level of
parasites and disease. But new research shows that extreme climate
conditions — such as severe droughts — can cause infection rates to
skyrocket, resulting in mass die-offs. Véronique LaCapra reports.
The
savannas of Tanzania are home to as many as half of the world's lions.
University of Minnesota ecology professor Craig Packer has been
studying lions in Tanzania's Serengeti National Park for the past 30
years. He was there in 1994, when inexplicably, the animals started
dying.
He estimates that about 1,000 lions died over a
relatively short period of time. Packer says it was unprecedented.
"Over the previous 30 years nothing had been seen anything like it."
Then,
in 2001, it happened again. "In the nearby Ngorogoro crater we saw
another die-off," says Packer, "just like the one in the Serengeti
seven years earlier."
By analyzing the lions' blood, Packer and
his colleagues found that the two die-offs seemed to be triggered by
outbreaks of canine distemper virus, a disease that usually affects
domestic dogs.
Packer describes distemper as the "carnivore
equivalent of measles." When a person gets measles, their body produces
an immunological response to the virus, so that if the same person is
infected again, the disease will not develop.
"Distemper has the
same effect in the bodies of carnivores," Packer explains. "Whenever an
animal is exposed to distemper, the animal — if it survives the
infection — has antibodies and is therefore immune to further cases."
According
to Packer, it's relatively common for lions to get the disease. In the
Serengeti, he has seen distemper outbreaks about every six or seven
years, usually with no symptoms. "Most of the time it's harmless, you
just find out that the animals were exposed because you see their
antibodies in their blood."
But in 1994 and 2001, about a third
of the animals that got infected, died. What had turned a normally
benign outbreak into a devastating epidemic?
Packer says that in
1993, in the Serengeti, "we had the first in a series of pretty serious
droughts that we've been having out in East Africa."
The drought
was especially brutal for a common prey of lions, the Cape buffalo. For
the lions, the drought-weakened buffalo made an easy catch: they ate
almost nothing else. But the unexpected feast came at a high price.
"The buffalo," Packer explains, "were infested with ticks."
And
with them, came a tick-borne parasite, called babesia. Animals in the
Serengeti are always exposed to ticks and babesia, which at its worst
can cause malaria-like symptoms, anemia, and hemorrhaging. In a normal
year, Packer says, lions can tolerate the parasite. "But […] the
drought of 1993 led to an unprecedented increase in the lions' exposure
to babesia."
And then, as soon as the drought ended, distemper
struck the babesia-ridden lions. The combination proved fatal. As
Packer puts it, "getting canine distemper is like having a short sharp
bout of AIDS."
Like AIDS, the distemper virus attacks the
immune system, weakening the body's defense against other infections.
"So the virus kind of liberated the tick-borne disease so that it could
be far more destructive than it would have otherwise been," explains
Packer.
Although the lion populations were able to recover from
the die-offs within a few years, Packer thinks deadly epidemics will
become more frequent if climate change continues.
"What we're
seeing now is much greater variability in the weather than we'd ever
seen in the past." Packer says that most climate change models predict
a greater variance in weather, with a greater chance of flood or
drought in any given year. "That's definitely been the experience in my
36 years in working out here."
Packer adds that extreme droughts
and floods are likely to continue unleashing new, potentially
synergistic combinations of diseases, which could be more deadly
together than they would be on their own. His research is published in
the journal PLoS ONE.
News
