The die-off began during calving, when females gather in dense aggregations. Whole herds went down within days, adults first and calves shortly after. By the time it stopped, more than half the world's saiga were dead.
Investigation led by an international team identified haemorrhagic septicaemia caused by Pasteurella multocida serotype B, an organism carried without harm in the tonsils of healthy saiga. Something had allowed it to invade the bloodstream in nearly every animal simultaneously. The strongest correlate found was weather: unusually high humidity combined with warm temperatures in the days before each local outbreak, matching conditions recorded at two earlier, smaller die-offs in the same region.
The population has since rebounded strongly, and the species was downlisted from Critically Endangered to Near Threatened in 2023. Why the bacterium turned invasive remains unresolved, and whether the trigger is purely climatic or involves some change in host physiology is the open question.