Frequency of plague infections among rats in Hong Kong and Kowloon in 1902

The Hong Kong plague epidemic, which began in 1894, gave way to some of the most important discoveries in the field of epidemiology. Swiss-French scientist, Andre Yersin, successfully identified the plague-causing bacteria Yersinia pestis, and showed that this bacteria was also present in rodents during this time. Following this discovery, scientists then investigated the connection further, in order to establish the extent of the infection in rats.

Results

In these experiments, all of the rats tested carried the plague in their blood, and over 90 percent had it in their spleen. The four tests conducted had similar results in both regions, although the infections had typically spread further within the rats by June than it had in May. The significance of these infections was that each rat became more contagious as the disease spread to other parts of the body. For example, as all of these rats had contaminated blood, this meant that any fleas or other animals who fed on infected rats would likely contract the disease as a result; in contrast, only 15 to 34 percent of those studied had plague bacteria in their saliva, meaning that a bite from an infected rat would generally not transmit the plague.

Plague today

While human diseases such as smallpox and polio have been or are in the process of being eradicated through vaccination, the presence of plague in rat colonies makes it much more difficult to eradicate. While increased sanitation and control measures have made plague almost non-existent in most countries, it continues to be endemic to rural areas of sub-Saharan Africa, Madagascar, and Peru. Between 2010 and 2015, there were 3,248 cases and 584 deaths worldwide due to plague, mostly in the aforementioned areas. The reason for its continued existence is due difficulties in locating and exterminating the infected rat colonies which act as natural reservoirs for the disease. Nonetheless, while the number of cases and deaths has been very low for decades, these numbers continue to decrease and this trend is expected to continue into the future.