The temporal rhythm of rat epizootics and the role of rat fleas

IPRC started with basic experiments on the rat-and-rat-flea-borne theory of plague, using locally trapped black rats and laboratory rodents. These experiments were carried out in especially designed godowns constructed after the manner of Indian houses, which allowed experimental animals to roam quite freely, much as in a normal rat colony in a usual human environment. This permitted basic experiments, examining, for instance, the option that plague was spread by contact: 50 healthy guinea pigs were placed together with 10 guinea pigs inoculated with a virulent culture of Y. pestis from which all fleas had been removed. A week later, all inoculated animals had died of plague, and none of the healthy animals had died from plague. On 25 November 1905, this experiment was repeated with a normal presence of fleas: all inoculated animals were dead after a week and the epizootic flared up among the healthy. The first death among the healthy guinea pigs occurred eight days later, a period that would include the death of at least one inoculated animal, the transfer of at least one of its infective fleas to one of the healthy animals, infection, incubation and the time of illness. Later studies showed that the average time from infection to death of black rats is 3.66 days, which fits well, even perfectly, assuming the lapse of a few hours from the infective flea leaving the dead or dying guinea pig until it finds a new host among healthy animals. By 17 days later, all originally healthy animals were dead.

Significantly, 400 fleas were recovered from the last two guinea pigs, 326 from one which was moribund, and 74 from the other which was dead and had been deserted by many of its fleas on the hunt for a new host. This showed that, as the animals were dying or died, their fleas gathered together on the remaining live hosts, which meant that when these hosts died, hundreds of infective rat fleas would be released much at the same time. Numerous such observations were also made on wild rats dying or recently dead from plague: they carried on average about 80–100 fleas, which proved infective when introduced among healthy rodents.