Pneumonic plague is a difficult phenomenon to model. We really don’t have much data from the modern medical era. Hinckley et al. (2012) argue that most of the data studied to date have been biased by taking it from well-established epidemics. To better study all transmission conditions, they gathered all of the cases of primary pneumonic plague in the United States between 1900 and 2009. Their data set, divided into pre- and post-antibiotic era, is really what interests me the most.
The demographics of the two eras tell very different stories. There were 60 cases before 1943, 92% of which occurred in California, 86% in three epidemics (San Francisco 1904, Oakland 1919, and Los Angles 1924). A full 83% of these cases came from documented human contact. After 1943, there have only been 14 cases from south-western states. Of these, 64% came from animal contact and another 21% from laboratory exposure. None of the post-1943 set obtained the infection from human contact. Obviously, the use of antibiotics is not the only difference between these two sets. The sheer number of cases and sources of infection are very different.
In both data sets, the overwhelming number of cases failed to transmit the infection. In the antibiotic era, there have been no documented cases of human-to-human transmission of pneumonic plague in the US. On the other hand, in the pre-antibiotic era, where most cases come from primarily bubonic epidemics, 8% of the cases transmitted to two or more people.
When it comes to pneumonic plague, antibiotics are not the panacea that is usually implied. In the antibiotic era, the mortality rate was “only 36%”, down from 92% in the pre-antibiotic era. So in other words, pneumonic plague today has approximately the same mortality rate as smallpox. Comforting.
Their models suggest that in the vast majority of instances pneumonic plague quickly reaches extinction due to poor transmission with traditional control measures. The important caveat is that super spreading events early in the outbreak can lead to an explosive increase in case numbers. In one case from 1924 Los Angles, one man transmitted pneumonic plague to 26 others. With the primary concern in the United States being bioterrorism, response planning for a large plague outbreak will continue with the goal of identifying and containing super spreaders as soon as possible, while knowing that most cases will fail to transmit to anyone.
Hinckley AF, Biggerstaff BJ, Griffith KS, & Mead PS (2012). Transmission dynamics of primary pneumonic plague in the USA. Epidemiology and infection, 140 (3), 554-60 PMID: 21733272