Story highlights Thirteen people were hospitalized in western Siberia with suspected anthrax infections

Experts suspect a reindeer carcass that thawed during a heat wave

(CNN) Thirteen people have been hospitalized amid an outbreak of anthrax in western Siberia, the governor's office of the Yamalo-Nenets autonomous region reported Thursday. Experts with the Russian Ministry of Agriculture believe the cause of infection is the thawing of the frozen carcass of a reindeer that died 75 years ago.

Anthrax is a potentially deadly bacterial disease caused by Bacillus anthracis, according to the World Health Organization

The hospitalizations came after up to 1,200 reindeer died over the past month, which officials originally blamed on a heat wave in the region. Unusually high temperatures of up to 35 degrees Celsius (95 degrees Fahrenheit) recorded over the past month might have weakened the reindeer.

Now, laboratory tests have confirmed that the reindeer died of anthrax, Gov. Dmitry Kobylkin's office reported.

However, Moscow laboratories have not confirmed that the patients' illnesses were caused by anthrax. But in the meantime, the patients are being treated as though they have the illness, including treatment with antibiotics.

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