The siblings – aged 27 and 16 – are believed to have caught the potentially deadly disease after hunting and then consuming the animal in the country, authorities have said.
Two suspected cases in the western Khovd province came back as the plague in laboratory tests, according to Mongolia’s National Centre for Zoonotic Diseases.
The ministry of health said earlier this week a 27-year-old patient was being treated with “marmot plague and secondary lung disease” according to a preliminary diagnosis.
He was in a “very severe condition” and had “multiple organ failure”, the department said.
His younger brother was also receiving treatment linked to the plague, according to the ministry of health.
Authorities are working to put in place a quarantine for the relevant areas in Khovd province, and have identified hundreds of people who came into first- or second-hand contact with the infected people.
Marmots have been linked to other cases of the plague in the past.
Several years ago, a 10-year-old boy caught the disease while out hunting with his grandfather in the Siberian mountains.
It was thought he became infected after skinning a marmot.