Declines in large wildlife lead to increases in disease risk

University of California – Santa Barbara
In the Middle Ages, fleas carried by rats were responsible for spreading the Black Plague. Today in East Africa, they remain important vectors of plague and many other diseases, including Bartonellosis, a potentially dangerous human pathogen. The researchers concluded that the “spike in disease risk results from explosions in the number of rodents that benefit from the removal of the larger animals.”

In the Middle Ages, fleas carried by rats were responsible for spreading the Black Plague. Today in East Africa, they remain important vectors of plague and many other diseases, including Bartonellosis, a potentially dangerous human pathogen.

Research by Hillary Young, assistant professor in UC Santa Barbara’s Department of Ecology, Evolution and Marine Biology, directly links large wildlife decline to an increased risk of human disease via changes in rodent populations. The findings appear today in theProceedings of the National Academy of SciencesEarly Online Edition.

With an East African savanna ecosystem as their research site, Young and her colleagues examined the relationship between the loss of large wildlife — defaunation — and the risk of human disease. In this case, they analyzed Bartonellosis, a group of bacterial pathogens which can cause endocarditis, spleen and liver damage and memory loss.

“We were able to demonstrate that declines in large wildlife can cause an increase in the risk for diseases that are spread between animals and humans,” said Young. “This spike in disease risk results from explosions in the number of rodents that benefit from the removal of the larger animals.”

The researchers discovered this effect by using powerful electric fences to experimentally exclude large species like elephants, giraffe and zebra from study plots in Kenya. Inside these plots, rodents doubled in number. More rodents meant more fleas, and genetic screens of these fleas revealed that they carried significantly numbers of disease-causing pathogens.

The study was concentrated in an area where rodent-borne disease is common and sometimes fatal. According to Young, these rodent outbreaks and associated increases in disease risk may be exacerbating health problems in parts of Africa where diminishing wildlife populations are rife.

“This same effect, however, can occur almost anywhere there are large wildlife declines,” Young said. “This phenomena that we call rodentation — the proliferation of rodents triggered by large wildlife loss — has been observed in sites around the world.”

Downturns in wildlife numbers can cause rodent increases in a variety of ways, including by providing more access to food and better shelter. “The result is that we expect that the loss of large animals may lead to a general increase in human risk of rodent borne disease in a wide range of landscapes,” Young said.

“In this study, we show the causal relationship between disturbance and disease is alarmingly straightforward,” she added. “We knock out the large members of ecosystems, and the small species, which generally interact more closely with humans, dramatically increase in number, ultimately brewing up more disease among their ranks.

The study provides ecosystem managers with yet another reason to protect large and at-risk wildlife species. “Elephants are an irreplaceable part of our global biodiversity portfolio,” Young said, “but they also appear to be circuitously protecting us from disease.”

Story Source:

The above story is based on materials provided by University of California – Santa Barbara. The original article was written by Julie Cohen. Note: Materials may be edited for content and length.

Journal Reference:

  1. H. S. Young, R. Dirzo, K. M. Helgen, D. J. McCauley, S. A. Billeter, M. Y. Kosoy, L. M. Osikowicz, D. J. Salkeld, T. P. Young, K. Dittmar. Declines in large wildlife increase landscape-level prevalence of rodent-borne disease in AfricaProceedings of the National Academy of Sciences, 2014; DOI:10.1073/pnas.1404958111
University of California – Santa Barbara. “Declines in large wildlife lead to increases in disease risk.” ScienceDaily. ScienceDaily, 29 April 2014

Canine distemper virus: An emerging disease in rare Amur tigers

Rare Amur tigers in Russia are succumbing to infection with canine distemper virus (CDV), a pathogen most commonly found in domestic dogs, according to the authors of a study published in mBio®, the online open-access journal of the American Society for Microbiology.

Pressure from poaching, decimation of their prey base, and habitat fragmentation have diminished the population of Amur tigers (also called Siberian tigers) to fewer than 500. In the study, a team of scientists from the US and Russia show that CDV infected and caused fatal neurological disease in members of this critically endangered species. They estimate that the virus has killed at least 1% of Amur tigers since 2009.

“Losing 1% of an endangered population is pretty significant,” says corresponding author Denise McAloose, Head Pathologist at the Wildlife Conservation Society in The Bronx, New York. “And these losses represent only the deaths we know about. I imagine that there were others that we just never saw,” says McAloose.

Since 2001, several rare Amur Tigers have exhibited a set of strange behaviors. Normally a reclusive species, tigers have been seen entering villages and wandering onto roads in the Russian Far East, stumbling, emaciated, and unafraid of humans. (One example can be found on YouTube: In each of the documented cases, the tiger eventually died or was destroyed after its condition worsened. Early findings showed that at least one of the tigers was infected with a member of the morbillivirus family of viruses, but conclusive answers had evaded scientists and wildlife managers until now.

Using tissue samples from five wild Amur tigers that died or were destroyed due to neurological disease in 2001, 2004, or 2010, McAloose and her colleagues proved that infection with CDV, a type of morbillivirus, is to blame for the deaths of two of the tigers and caused a serious infection in a third. Under the microscope, the brains of the two tigers that died of CDV infection were riddled with lesions, indicating they suffered from severe viral encephalitis, consistent with their clumsy, abnormal behavior. Molecular analyses to identify CDV-specific proteins and immunolabelling with CDV-specific antibodies confirmed that CDV was present in these tissues. A gene for a CDV-specific gene was detected in the third tiger.

The problem isn’t limited to one location, says McAloose. The three tigers that tested positive for CDV were distributed across the Russian Far East.

“That tells us this is a disease that is distributed all across Amur tiger range,” McAloose says. “And it also appears to be a relatively new threat to tigers since blood samples from wild tigers prior to 2000 tested negative for antibodies to the virus”.

But how do tigers contract a CDV infection? Relatively few domestic dogs in the Russian Far East are vaccinated against CDV, McAloose says, and tigers do kill and eat dogs, so they represent one possible source. But domestic dogs aren’t the only suspects.

“In the Russian Far East, domestic dogs are one of the biggest concerns, but other species, like raccoon dogs or foxes, can also harbor the disease,” says McAloose.

McAloose and her colleagues are now working on collecting samples from dogs and small wild carnivores in the Russian Far East to get a more complete picture of the various strains of CDV in circulation in the hopes of linking tiger infections to a source, knowledge that would hopefully aid in preventing more infections among tigers.

“The situation is quite serious”, says McAloose, and when asked if CDV could spell the demise of Amur tigers, she says, “It’s possible.”

“It’s the first infectious disease that we know is a significant risk to Amur tiger survival,” says McAloose.