Story highlightsPlague is caused by infection with the bacterium Yersinia pestisMore than 1,800 people were infected with the plague last year

(CNN)One of the worst pandemics in human history, the Black Death, along with a string of plague outbreaks that occurred during the 14th to 19th centuries, was spread by human fleas and body lice, a new study suggests.

It was previously suggested that rodents, whose fleas can also transmit the bacteria behind the plague, were the main culprits behind Europe’s second pandemic of the disease that saw a string of outbreaks occur in succession.How do we still have the plague, centuries after the Black Death? How do we still have the plague, centuries after the Black Death? How do we still have the plague, centuries after the Black Death? Plague is caused by infection with the bacterium Yersinia pestis and is typically spread through the bite of infected fleas, frequently carried by rats, causing bubonic plague. Symptoms include painful, swollen lymph nodes, called bubos, as well as fever, chills and coughing.Pneumonic plague is more virulent or damaging and is an advanced form characterized by a severe lung infection. The infection can be transmitted from person to person via airborne droplets — coughing or sneezing. Outbreaks still occur today, with an outbreak in Madagascar last year infecting more than 1,800 people.Animals that can carry plague bacteria found in the Western U.S.Animals that can carry plague bacteria found in the Western U.S. Photos: The deer mouse is <a href="https://www.aphis.usda.gov/wildlife_damage/nwrc/publications/12pubs/witmer127.pdf" target="_blank">a potential source</a> of diseases including the plague, salmonellosis and hantavirus.The deer mouse is <a href="https://www.aphis.usda.gov/wildlife_damage/nwrc/publications/12pubs/witmer127.pdf" target="_blank">a potential source</a> of diseases including the plague, salmonellosis and hantavirus. Photos: The deer mouse is a potential source of diseases including the plague, salmonellosis and hantavirus.Hide Caption 1 of 9Often, humans become infected after being bitten by fleas that have bitten infected animals like deer mice and voles.Often, humans become infected after being bitten by fleas that have bitten infected animals like deer mice and voles. Photos: Often, humans become infected after being bitten by fleas that have bitten infected animals like deer mice and voles.Hide Caption 2 of 9Another easy way humans become infected is after their pet, mainly a house cat, hunts an infected rodent and then bites the owner or is host to infected fleas.Another easy way humans become infected is after their pet, mainly a house cat, hunts an infected rodent and then bites the owner or is host to infected fleas. Photos: Another easy way humans become infected is after their pet, mainly a house cat, hunts an infected rodent and then bites the owner or is host to infected fleas.Hide Caption 3 of 9The first known outbreak of plague in the United States came after infected rodents from China that were on ships mingled with urban rats in port cities. Los Angeles had an outbreak of plague in 1924.The first known outbreak of plague in the United States came after infected rodents from China that were on ships mingled with urban rats in port cities. Los Angeles had an outbreak of plague in 1924. Photos: The first known outbreak of plague in the United States came after infected rodents from China that were on ships mingled with urban rats in port cities. Los Angeles had an outbreak of plague in 1924.Hide Caption 4 of 9The California ground squirrel is highly susceptible to becoming infected with the plague and transmitting it widely to other squirrels. The California ground squirrel is highly susceptible to becoming infected with the plague and transmitting it widely to other squirrels. Photos: The California ground squirrel is highly susceptible to becoming infected with the plague and transmitting it widely to other squirrels. Hide Caption 5 of 9Rock squirrels are also highly susceptible to plague bacteria and can spread it widely among its species.Rock squirrels are also highly susceptible to plague bacteria and can spread it widely among its species. Photos: Rock squirrels are also highly susceptible to plague bacteria and can spread it widely among its species.Hide Caption 6 of 9Prairie dogs are some of the animals most affected by large die-offs after becoming infected with the plague.Prairie dogs are some of the animals most affected by large die-offs after becoming infected with the plague. Photos: Prairie dogs are some of the animals most affected by large die-offs after becoming infected with the plague.Hide Caption 7 of 9Scientists are testing vaccines to protect the <a href="http://www.nwhc.usgs.gov/disease_information/sylvatic_plague/publications/protecting_black-footed_ferrets.pdf" target="_blank">black-footed ferret</a>, an endangered species, from getting the plague. The species has experienced devastating losses from the plague over the years.Scientists are testing vaccines to protect the <a href="http://www.nwhc.usgs.gov/disease_information/sylvatic_plague/publications/protecting_black-footed_ferrets.pdf" target="_blank">black-footed ferret</a>, an endangered species, from getting the plague. The species has experienced devastating losses from the plague over the years. Photos: Scientists are testing vaccines to protect the black-footed ferret, an endangered species, from getting the plague. The species has experienced devastating losses from the plague over the years.Hide Caption 8 of 9California voles <a href="http://icwdm.org/handbook/rodents/voles.asp" target="_blank">rarely come into contact</a> with humans, since they are easily frightened, yet they can carry the plague and tularemia.California voles <a href="http://icwdm.org/handbook/rodents/voles.asp" target="_blank">rarely come into contact</a> with humans, since they are easily frightened, yet they can carry the plague and tularemia. Photos: California voles rarely come into contact with humans, since they are easily frightened, yet they can carry the plague and tularemia.Hide Caption 9 of 910 plague animals08 plague animals02 plague animals01 plague animals03 plague animals04 plague animals RESTRICTED05 plague animals07 plague animals RESTRICTED09 plague animalsTo understand the historic outbreak, scientists from the University of Oslo modeled the three transmission routes for the disease — rats, airborne and human fleas and lice — using mortality data for nine outbreaks that spanned the time period of the second pandemic. Their analysis found that human ectoparasites, such as fleas, reflected the death trends most accurately.Read MoreThe model outcomes therefore suggest the spread of the plague back then was mainly attributable to human fleas and body lice, according to the study published Monday.”Our results support that human ectoparasites were primary vectors for plague during the Second Pandemic, including the Black Death,” the authors write.This challenges “the assumption that plague in Europe was predominantly spread by rats,” they added.Follow CNN Health on Facebook and Twitter

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“This study uses mathematical modeling to try and determine how exactly the plague spread in Europe during the pandemic known as the ‘Black Death’,” said Raina MacIntyre, professor of Infectious Diseases Epidemiology at the University of New South Wales in Australia, who was not involved in the study.”Most naturally occurring plague is bubonic (affecting the lymph nodes, rather than the lung). Studies like this can help better understand rapid transmission of plague, which then points to the most suitable control measures.”

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