In May 2015, more than 200,000 saiga antelope (Saiga tatarica) suddenly died in Kazakhstan, reducing the global population of the critically endangered species by two-thirds.

Research indicates the saigas were likely killed by hemorrhagic septicemia caused by a type of bacteria called Pasteurella multocida. But P. multocida generally exists harmlessly in healthy saigas and other animals, so the question remained: Why did so many saigas become infected so suddenly and severely by a normally benign type of bacteria?

A new analysis may have solved part of this mystery, linking the spread of P. multocida to unusually high humidity levels and temperatures.

The results indicate that saigas may be particularly sensitive to climate change, which stands to increase both temperature and humidity in Kazakhstan.

In May 2015, more than 200,000 saiga antelope (Saiga tatarica) suddenly died. Entire herds perished, their bodies littering the steppe grasslands of central Kazakhstan as if dropped from the sky. Biologists were left stunned, baffled and very concerned. Already critically endangered, the mass die-off reduced the species by two-thirds globally. Only around 31,000 remained in Kazakhstan and 100,000 worldwide.

Research indicated the saigas were likely killed by hemorrhagic septicemia caused by a type of bacteria called Pasteurella multocida. But P. multocida generally exists harmlessly in healthy saigas and other animals, so the question remained: Why did so many saigas become infected so suddenly and severely by a normally benign type of bacteria?

In 2016, disaster struck the saiga again. This time in Mongolia and from a known entity: a virus from domestic goats and sheep. Between December 2016 and February 2017, an estimated 2,500 saigas died from infection, representing the loss of a quarter of the country’s entire population.

Once the 2016 virus was confirmed, conservation organizations and local communities raced to stop its spread and reduce the risk of another outbreak by immunizing livestock. But with the ultimate cause of the 2015 die-off still largely a mystery, efforts to stop future events were stymied.

However, a new analysis may have solved part of this mystery, linking the spread of P. multocida to unusually high humidity levels and temperatures in the region. The analysis was conducted by an international team of researchers and published today in Science Advances.

The researchers used statistical modeling to explore how environmental conditions lined up with three different saiga die-offs in 2015, 1988 and 1981. They discovered the three events happened during temperature and humidity spikes, suggesting a possible connection.

They write that temperature and humidity anomalies have been associated with P. multocida-caused hemorrhagic septicemia in other species. However, they caution that the scale of the 2015 saiga die-off is far beyond that documented for other animals, so “additional factors must also be at play.”

Saigas are naturally prone to large die-off events, and scientists hypothesize this may be due to the harsh climate of their cold, semi-arid steppe habitat. Saigas also give birth to huge babies – the largest calves of any ungulate compared to the size of the mother – which could strain populations during calving seasons.

Adding to natural pressures are unnatural ones. Poaching has significantly depleted herds since the 1990s, and conservationists worry hunting coupled with die-offs could drive saiga numbers down to levels from which they can’t return.

Questions still linger, such as why higher temperatures and humidity levels triggered a bacterial invasion. The researchers recommend more comprehensive study of how bacteria spread in response to environmental changes.

What their results do indicate is that saigas may be particularly sensitive to climate change, which stands to increase both temperature and humidity in Kazakhstan. If this happens, the saigas’ resident colonies of P. multocida could act like time bombs, residing harmlessly in the animals until, one day, the temperature goes up and sets them off, ushering in another massive die-off.

The researchers say conservationists and scientists should be ready to act quickly in the case of such an event.

“The scale and nature of this event also point to the need for ongoing scientific and veterinary monitoring of wildlife populations and the need to be prepared for rapid and rigorous responses to disease outbreaks when they occur,” they write in their study.

Citation:

Kock, R. S., et al. (2018) Saigas on the brink: Multidisciplinary analysis of the factors influencing mass mortality events. Science Advances 4(1) DOI: 10.1126/sciadv.aao2314

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