Deadly virus pushing tigers towards extinction

Adding to the existing pressures of habitat loss, poaching and depletion of prey species, a new threat to tiger populations in the wild has surfaced in the form of a deadly virus.
According to a new study from the Wildlife Conservation Society (WCS), canine distemper virus (CDV) has the potential to be a significant driver in pushing the tigers towards extinction.
While CDV has recently been shown to lead to the deaths of individual tigers, its long-term impacts on tiger populations had never before been studied, researchers said.
The authors evaluated these impacts on the Amur tiger population in Russia's Sikhote-Alin Biosphere Zapovednik (SABZ), where tiger numbers declined from 38 individuals to 9 in the years 2007 to 2012.

In 2009 and 2010, six adult tigers died or disappeared from the reserve, and CDV was confirmed in two dead tigers - leading scientists to believe that CDV likely played a role in the overall decline of the population.
Joint investigations of CDV have been an ongoing focus of scientists since its first appearance in tigers in 2003.

The finding shows that smaller populations of tigers were more vulnerable to extinction by CDV. Populations consisting of 25 individuals were 1.65 times more likely to decline in the next 50 years when CDV was present.
The results are profoundly disturbing for global wild tigers given that in most sites where wild tigers persist they are limited to populations of less than 25 adult breeding individuals.

The scientists used computer modelling to simulate the effects of CDV infection on isolated tiger populations of various sizes and through a series of transmission scenarios.
These included tiger-to-tiger transmission and transmission through predation on CDV-infected domestic dogs and/or infected wild carnivores.
High and low-risk scenarios for the model were created based on variation in the prevalence of CDV and the tigers' contact with sources of exposure.
Results showed that CDV infection increased the 50-year extinction probability of tigers in SABZ as much as 55.8 per cent compared to CDV-free populations of equivalent size.
"Although we knew that individual tigers had died from CDV in the wild, we wanted to understand the risk the virus presents to whole populations," said WCS veterinarian Martin Gilbert.

"Tigers are elusive, however, and studying the long-term impact of risk factors is very challenging. Our model, based on tiger ecology data collected over 20 years in SABZ, explored the different ways that tigers might be exposed to the virus and how these impact the extinction risk to tiger populations over the long term," said Gilbert.
The finding appears in the journal PloSONE.