Number of dead rats and human plague cases in Hong Kong 1903

It was not until the Third Plague Pandemic where humans began to understand the relationship between bubonic plague, rats and their fleas. Today, we know that the most common way that plague spreads to humans is through the bites of infected fleas. During the Black Death, there were a variety of explanations for the plague's origin, such as punishment from God, bad air or poisoned wells; as the impact of the plague on humans and livestock were the most noticeable features of epidemics, the connection to rats was not established until much later. When the Third Pandemic spread through China, an increase in the number of dead rats was observed in Canton (Guangzhou), and again in Hong Kong during the outbreak of 1894.

Investigations

This prompted European scientists in Hong Kong to investigate this further as the plague progressed through 1902, and the experiment was repeated in 1903. The 1902 study showed that plague was at epidemic proportions among rats, and the number of dead rats observed correlated with the number of human plague cases. When the experiment was repeated in 1903, plague in rats was at a much lower level, however the correlation was much more noticeable. The conclusions drawn at the time were that the outbreaks peaked when the population of young rats was at its highest, but then declined as the weather cooled and rats were exterminated in larger numbers.