Two devastating disease outbreaks among African lions can't be blamed just on a virus raging through their populations at the time. The outbreaks were accompanied by explosions in a tick-borne disease fueled by exceptional droughts, according to a new study. The authors argue that climate change could make such complex die-offs more common.
Almost one-third of the lions in Serengeti National Park in Tanzania died in 1994. Researchers discovered that the animals had been infected with canine distemper virus (CDV), which is known primarily as a dog virus but is able to infect many carnivore species. But tests of lion blood samples collected before 1994 showed that CDV infections had occurred before without causing deaths, says ecologist Craig Packer of the University of Minnesota, St. Paul.
An even worse lion die-off in the nearby Ngorongoro Crater area in 2001 revealed a potential second culprit. Some sick lions were also infected with Babesia, a tick-borne parasite. Yet, how often each of these pathogens infects lions, and under what circumstances they kill, remained uncertain. Now, a retrospective study has given Packer and colleagues a clearer picture of what's going on.
The team first showed that the 1994 and 2001 CDV episodes weren't the only ones: Antibody tests on almost 600 blood samples taken since 1983 revealed that five other outbreaks had occurred without causing a mortality spike. Tests also showed that most lions had low levels of Babesia in their blood through the years. Only during the 1994 and 2001 die-offs did parasite levels in their blood soar. CDV suppresses immunity, which gives parasites such as Babesia a window of opportunity. But that's obviously not the whole story, because parasite levels did not jump during the five other CDV outbreaks.
The missing ingredient is the extreme drought that occurred in 1994 and 2001, the team concludes today in PloS ONE. Herbivores were starving at the time, which made them vulnerable to ticks. Cape buffaloes--a key lion prey in Tanzania--were "covered with ticks and barely able to even scratch themselves," Packer recalls. As lions feasted on the dying animals or their carcasses, ticks "jumped ship" and latched onto the lions, he says. That exposed the lions to a Babesia onslaught that, as a result of CDV immune suppression, they couldn't handle.
The team doesn't directly attribute the die-offs to climate change, but if extreme drought episodes become more frequent in the tropics--as climate models predict they will--the lions could suffer, Packer says.
Many infectious disease outbreaks may result from interactions between multiple pathogens and environmental conditions, says veterinary scientist Ard Nijhof of Utrecht University in the Netherlands, but nailing down those interactions can be difficult. Thanks to its large sample collection, the Packer team was able to show "very nicely" that drought and Babesia helped seal the lions' fate, Nijhof says.