Though there still may be some lingering doubt about the cause of the Black Death and subsequent outbreaks of plague, the pathogen behind the outbreaks that have taken place in the last 150 years or so is much less ambiguous.
While Koch and Pasteur ushered in the golden age of microbiology, an outbreak of plague began in China and spread from there. In 1894, while plague was raging in Hong Kong, the Pasteur Institute sent Alexandre Yersin, a physician who had trained with both Pasteur and Koch, to investigate. Yersin was able to access material from a corpse, and inoculated material he withdrew from a deceased soldier’s bubo into guinea pigs. His examination of the material revealed bacteria that were gram-negative: that is, they ended up pink when the Gram stain was carried out.
The guinea pigs Yersin inoculated died, and upon necropsy, he saw the same gram negative bacteria he’d found in the deceased soldier. He also was intrigued by the large numbers of dead rats he saw in the city (and even in the hospital). Upon examination, he saw the rats also were infected with these bacteria–concluding that the bacterium could infect both humans and other animal species.
However, Yersin didn’t finish figuring out the transmission of the bacterium. Another researcher, Paul-Louis Simond, experimented using infected rats and fleas, and noted that even if a sick rat was placed in a jar with healthy rats, the healthy rats didn’t typically become sick in the absence of fleas–suggesting fleas could act as an intermediate between infected and healthy animals, or between sick animals and the human population.
Remnants of this outbreak of plague, beginning in the mid-1860s, are still with us, as the bacterium rapidly spread across the globe by train and boat. And though most human cases are of the bubonic type, a handful of outbreaks of pneumonic plague have been documented in the last century. For example, in 1910-11, and again a decade later, two large outbreaks of pneumonic plague hit in Manchuria and China, erupting and spreading rapidly in the crowded conditions. Approximately 60,000 deaths were estimated from the first outbreak alone.
More recently, a 1994 outbreak of pneumonic plague in India resulted in over 600 confirmed cases and over 50 deaths. However, even worse than the death toll was the panic plague brought to the country, which had been plague-free since 1966. Physicians fled the area, and the outbreak severely affected the economy. I’ve seen estimates of the cost to tourism and trade range from $600 million up to $4 billion, so even with a relatively small outbreak, it has the potential to be a serious matter for the health of both the population and the economy.
Plague is considered a re-emerging disease. This means that it’s not only still with us, but it’s increasing in many geographic areas (particularly in Africa, including 2005 and 2006 outbreaks of pneumonic plague in the Democratic Republic of Congo [DRC]). And the U.S. isn’t exempt. This review, for example, details outbreaks in 1919 and 1924 in Los Angeles. And while human plague is infrequent enough to make news when it pops up in the United States (we average ~10-20 cases per year, mostly in the southwest where plague is endemic in animals), it can still be fatal if not treated quickly. Worldwide, between 1000 and 5000 cases of plague are reported every year, with 100-200 deaths (and these likely represent only a fraction of the actual cases each year). Though the epicenter of plague cases used to be Asia, in the past 20 years most cases have occurred in Africa, especially the DRC, Uganda, and Madagascar, which has experienced a recent outbreak of plague.
There are concerns that we may see more plague in the future as well. A new review in PLoS Medicine suggests that global warming could seed additional plague pandemics:
Recent analysis of data from Kazakhstan shows that warmer springs and wetter summers increase the prevalence of plague in its main host, the great gerbil. Such environmental conditions also seem to have prevailed during the emergence of the Second and Third Pandemics–conditions that might become more common in the future.
This could happen in the U.S. also, since the same warm, wet conditions that can lead to an increase in other diseases associated with increased rodent populations (such as hantavirus) may also increase plague here as well. Another pandemic on the scale of the Black Death is unlikely, given that we have improved sanitation, less crowding, and effective antibiotics, but recent outbreaks demonstrate that even a small number of cases can have a huge impact.
Finally, there is the potential to be infected with plague via malfeasance: a terrorist attack. This has been attempted in the past, via the release of Yersinia-infected fleas over Manchuria and China in 1941, or perhaps more famously, the catapulting of corpses who’d died of plague over the walls of the city during the siege of Kaffa by the Tartars in 1347. Today, Yersinia remains a top concern as far as potential bioterrorism pathogens go for a number of reasons. First, unlike anthrax, Y. pestis can be transmitted person-to-person, so any initial outbreak seeded by a terrorist release would spread beyond just the initial exposed group. Additionally, the infectious dose via inhalation is estimated to range from ~100 up to 20,000 organisms, so if it’s spread via aerosol by someone who knows what they’re doing, it has the potential to cause a lot of damage.
Y. pestis has been a scourge of humanity for centuries. It’s shaped our society in a myriad of different ways, and potentially even played a role in shaping our genomes. It may not be a killer of the magnitude of HIV, TB, or malaria, but the swiftness of a plague death and the high mortality rate of those infected means that it remains a threat–and potentially a greater one than it’s been in years due to the threat of bioterrorism and expansion of the range of its natural hosts due to climate change and other human factors (such as the crumbling of public health infrastructure and increased contact with rodents due to human encroachment into wildlife areas). The heyday of plague may be hundreds of years in the past, but with infectious diseases, it pays to be ready for the unexpected.
Previous posts in the series
References and further reading
Gubler D. et al. 2001. Climate variability and change in the United States: potential impacts on vector- and rodent-borne diseases. Environ Health Perspect. 109 Suppl 2:223-33. Link.
Stenseth, N.C., Atshabar, B.B., Begon, M., Belmain, S.R., Bertherat, E., Carniel, E., Gage, K.L., Leirs, H., Rahalison, L. (2008). Plague: Past, Present, and Future. PLoS Medicine, 5(1), e3. DOI: 10.1371/journal.pmed.0050003