Spread of Plague by Respiratory Droplets or Ectoparasites.

To the Editor: Drancourt and Raoult () have emphasized the risk of overestimation of pneumonic plague contagion by respiratory droplets and hypothesize that only transmission of Yersinia pestis by ectoparasites, such as lice and fleas, by close contact with infected humans can sustain outbreaks and epidemics. The outbreak of pneumonic plague in Madagascar in 2017 () reminds us that plague remains a potential serious threat in locations that are relatively inaccessible or have limited capacity for a robust public health response. Records describe substantial outbreaks of pneumonic plague () but portray a more dangerous disease than that described by Drancort and Raoult. High rates of transmission are possible () when pneumonic plague is spreading through social networks, in a way similar to that observed in West Africa during the recent epidemic of Ebola virus disease (). The Ebola virus is not thought to be easily transmitted but is clearly capable of generating a sustained epidemic.

The role of ectoparasites in the transmission of Y. pestis should not be dismissed. However, until a substantial epidemic has been documented with this proven etiology, this explanation of plagues, both historical and modern, must remain in the realm of conjecture.