Bubonic plague’s association with moderately warm temperatures and seasons

Bubonic plague has a distinctive seasonal pattern. According to observations and studies performed in India and other developing countries with a more or less tropical climate, when humidity is reasonably favourable, favourable temperatures for plague epidemics are generally around 20 °C, while temperatures much above or below this range, generally above about 30 °C and below 10 °C, will weaken or suppress them. In the words of G. Lamb, Director of the IPRC: ‘While plague can exist and spread under a great range of climatic conditions’, ‘the most striking feature of plague epidemics is their seasonal prevalence.’ Lamb considers the seasonality of bubonic plague in terms of the concept of defining feature as unique and that, therefore, this feature alone can serve for identification of the disease. He also emphasizes that it can spread efficiently under ‘a great range of climatic variation’.

This defining feature is mainly due to two central structural aspects of the epidemiology of bubonic plague springing from its basis in (black) rats and their fleas:

• 1. The natural mortality rate of fleas is high. Levels of temperature and humidity affect the reproduction of fleas, the rates of egg-laying and development of fertilized eggs, and the rates of survival and development of larvae and cocoons on the ground. This means that in adverse temperatures and conditions of humidity, the population of fleas will not be replenished by reproductive processes and their number will fall. This will affect the intensity with which a rat-flea population can transmit and disseminate plague and therefore the dynamics of the epidemic process.

• 2. Plague septicaemia in rats falls strongly at temperatures below 10 °C. Consequently, in chilly and cold circumstances rat fleas that draw blood from plague-diseased rats will become much less infected and will, therefore, develop blockage far more slowly and at much lower rates. Protraction of this process also implies higher death rates of fleas from other causes, which will contribute to a corresponding reduction of infected fleas and the further weakening of the disseminative powers of the flea population.