Bubonic plague is a zoonosis. The plague bacilli are transmitted from animal to animal and from animal to man by fleas, notably Xenopsylla cheopis„ one of the ectoparasites of rats. It has been shown experimentally that fleas, lice or ticks are essential for the spread of infection among rodents. Transmission from rats to man is dependent on fleas. In cool humid weather fleas multiply and plague spreads readily among susceptible rats.
Hot, dry weather, on the other hand tends to limit spread of the infection. Its ability to spread rapidly and exponentially in a population of rats depends on the animals having a high flea index, or mean number of fleas per rat (e.g. index of 2 or more), since an infected rat is unlikely to pass on the infection to a larger number of other rats than the number of its fleas. In the sick animal, an intense septicaemia develops and when a flea feeds on the blood of such an animal, plague bacilli are sucked into the midgut where they multiply to such an extent that they block the proventriculus.
On the death of the animal the flea seeks an alternative host which may be another rodent or man. Because the 'blocked' flea is unable to suck readily, some of the infected blood of the previous host is regurgitated and injected into the bite wound of the new victim. 'Unblocked' infected fleas may also pass on infection, but less readily than 'blocked' ones.
When the weather changes to hot, dry conditions, the fleas tend to die. This may happen at the time when the epizootic among the rats has reached a stage where the number of susceptible animals has greatly decreased through death or developing immunity, Those two factors contribute to the termination of the rat epizootic and thus, also of the human epidemic. The renewal of an epizootic and epidemic in the following or later years depends on the growth of a fresh population of young susceptible rats and their heavy infestation with fleas. The infection may be introduced into this fresh population from an old, surviving 'carrier’ rat with intermittent bacteriaemia or from infested wild rodents.
EPIDEMIOLOGY OF PNEUMONIC PLAGUE
In some patients contracting bubonic plague from a rat or sylvatic rodent (e.g. tarbagan in Eastern Siberia) the bacteriaemia may lead to infection of the lungs and the development of severe bronchopneumonia. The sputum from these pneumonic patients contains plague bacilli and under certain circumstances the infection may spread epidemically, from man to man, by the respiratory route. Epidemics of pneumonic plague have occurred particularly in colder regions such as North China where conditions have been favorable to airborne transmission of infection within dwelling houses. Whether the bacilli are transmitted in respiratory droplet-nuclei or sputum-dust is unknown.
Plague prevention consists essentially in measures to control both wild and domestic rodent population in towns and villages. The construction of dwelling houses to a standard that facilitates the exclusion of rats from them is of primary importance. Rat-proofing of buildings especially warehouses in dock areas, the fumigation of ships or the building of ships in which rats can no longer establish themselves have succeeded in preventing the spread of plague from one country to another. Periodic surveys in areas where wild plague is endemic are advocated to determine the prevalence of rodents and fleas and the degree of infection among wild and domestic rodent hosts in order that, where necessary, control measures can be put into effect. When an epidemic is in progress, control is most quickly achieved by the eradication of fleas from the rats by the liberal application of insecticide to rat runs.