Spatiotemporal influence of permafrost thaw on anthrax diffusion

The recent 2016 outbreak of anthrax disease affecting reindeer herds in Siberia has been
associated to the presence of old infected carcasses released from thawing permafrost, underlying
the emerging character of such disease in the Arctic region due to climate change. Anthrax occurs
in nature as a global zoonotic and epizootic disease caused by the spore-forming bacterium
Bacillus anthracis. It principally affects herbivores and causes high animal mortality.
Transmission occurs mainly via environmental contamination through spores which can remain
viable in permafrost for many decades.
We propose and analyze a novel epidemiological model for anthrax transmission specifically
tailored for the Arctic region. It conceptualizes the transmission of disease between susceptible
and infected animals in the presence of environmental contamination, considering also herding
practices (e.g. seasonal grazing) and the seasonal environmental forcing caused by thawing
permafrost. We performed stability analyses and implemented Floquet theory for periodically
forced systems, and therefore applied our model to the 17-year-long records of permafrost
thawing depth available at the Lena River Delta (northern Siberia). Accordingly, in order to
spatialize potential anthrax incidence and consequently the possible hazardous areas in the Arctic,
we used the Maximum Entropy (Maxent) approach considering environmental variables and, in
particular, accounting for current and expected permafrost thawing rates.
Results show how temporal variability of grazing and thawing may influence and favor sustained
anthrax transmission. Also, particularly warm years are associated to increased risk of anthrax
incidence. Accordingly, we show that such risk could be mitigated with specific precautions
involving herding practices, for example by anticipating or postponing seasonal grazing. Finally, a
spatial map of the potential Arctic areas at risk is presented, providing a tool for local authorities in
view of eventual targeted prevention measures.