A virus that originated in dogs jumped to lions — and no longer requires dogs in order to spread among the big cats. The finding provides a case study of how diseases jump between species and has raised concerns about the role viruses play in extinction threats.
Canine distemper virus, a relative of measles, was once thought not to cause disease in cats — until outbreaks among zoo animals in the 1990s suggested otherwise. Once scientists were alerted to the danger, they began studying wild lions for signs of CDV as well. In 1994, approximately 30 percent of Serengeti lions died from an epidemic of CDV, researchers found. Earlier studies suggested that domestic dogs living in areas surrounding the Tanzanian national park were responsible for the continued presence of the disease, but now it appears that CDV has mutated and evolved to thrive in other carnivores, according to a study published in the Proceedings of the National Academy of Sciences.
In 1994, a CDV epidemic killed approximately 30 percent of Serengeti lions
A major way diseases mutate is by jumping between species, though. Middle Eastern Respiratory Syndrome, Ebola, and the H1N1 swine flu all jumped from animal to animal before crossing into humans. MERS is a bat virus that established a hold in camel populations; humans in close contact with camels were most likely to get sick. Ebola, which originated in fruit bats, jumped into other species of great apes before coming to humans. And a variant of swine flu — which as the name suggests, is ordinarily found in pigs — jumped to humans, causing a pandemic in 2009. Now that CDV has switched hosts from dogs to lions, scientists are concerned about potential threats to other wildlife, researchers write in today's paper.
"Our study raises questions about which other carnivores in the Serengeti might be transmitting the virus between dogs and lions," says Tiziana Lembo, veterinary scientist at the University of Glasgow's Boyd Orr Centre for Population and Ecosystem Health. Lembo is one of the study's authors. "We need to consider how to broaden our strategies to control this disease."
Though domestic dogs are likely the original culprit in spreading CDV to other species — a major spike in dog infections was followed by an initial major spike in lion infections — over time, dogs played less of a part in the virus spread. Using decades of data collected from both the Serengeti lion and domestic dog population, researchers determined that the frequency of infection in lions has increased while the overall frequency in domestic dogs has decreased. Peak CDV infections in lions are no longer in sync with peak infections in the dog population, the study finds.
"We need to consider how to broaden our strategies to control this disease."
In response to the 1994 epidemic, vaccination programs for CDV were established for domestic dogs. While that's been effective in reducing infection in the dog population, the disease continues to spread in lions. Studies on CDV infection in Russia's endangered Siberian tiger population have revealed similar patterns in interspecies adaptation. The next question is to study what other animals are now carriers — particularly worrisome are roving animals, like hyenas, who can spread CDV more widely.
Trying to keep endangered species from dying off entirely is a tricky business. Today's study shows how encounters between our domesticated animals and wildlife can be unexpectedly dangerous for the wildlife. Knowing more about how viruses adapt to new hosts may help protect lions and other threatened animals. And given how often human infectious diseases start in animals, keeping track of how viruses jump species may help us monitor future threats to our own health.