Plague squirrels spread ‘Black Death’ in Colorado and kill teenage boy in Mongolia

State's first cross-species transmission in five years comes as World Health Organisation 'carefully' watches outbreak in China

The "Black Death" has returned to Colorado after an infected squirrel passed the bubonic plague onto a human in the state for the first time in five years.

It comes as health authorities continue to monitor an outbreak of bubonic plague after a 15-year-old boy died in Mongolia from eating an infected marmot, a type of ground squirrel.

The World Health Organisation, meanwhile, is "carefully" watching China after local authorities ordered a herdsman near the border of Mongolia to quarantine in an effort to prevent the spread of the deadly infection.

The Black Death has made somewhat of a comeback in recent years after ravaging Europe in the middle ages. It has been classified as a re-emerging disease, with the WHO estimating between 1,000 to 2,000 infections annually.

Between one to 17 cases of human plague are reported in the US each year, with four people killed by the Black Death in the country in 2015, according to the Centres for Disease Control and Prevention.

The most recent case was spread from an infected squirrel that tested positive in Jefferson County last week, according to the Colorado Department of Public Health & Environment.

"Plague has been present in Colorado since at least the 1940s, and cases in wild rodents in Colorado are reported most years," said Dr Jennifer House, state public health veterinarian.

"While we see most plague activity during the summer, the disease can be found in rodents year-round and sometimes spills over into other wildlife species as well as domestic cats and dogs."

While no other cases were identified, health experts warned residents to take precautions to lower the risk of transmission during this time of year when the plague is more present in the state. There is no vaccine to prevent the spread of the plague, but if caught early it can be treated with antibiotics in both people and pets. Symptoms include fever, headache, chills, weakness, and tender, and painful lymph nodes.

The health department confirmed that the Colorado patient recovered, and that the version of the disease was septicemic plague, which is in the blood and cannot be spread to other people.

"The disease is most commonly spread by the bite of an infected flea but also may be transmitted by infected animal tissues, fluids, or respiratory droplets," the health department said in a statement.

A version of the plague was spread in western Mongolia after a teenager caught and ate an infected marmot, according to the country's Ministry of Health.

He died on Sunday.

Health imposed a week-long quarantine of 15 people that came into contact with the boy in the Turgug district of Gobi-Altai province.

The World Health Authority spokeswoman Margaret Harris said last week they were monitoring case numbers in China after

"It is being well managed," she said during a virtual press conference.

"At the moment, we are not considering it high-risk but we're watching it, monitoring it carefully."