Plague
Plague (L. plaga = pest) causes more human deaths than any other infectious diseases except malaria and tuberculosis. It is caused by Yersinia pestis, an aerobic gram-negative rod-shaped bacterium. Plague is a natural disease of domestic and wild rodents; rats being the primary disease reservoir, Fleas are the intermediate hosts responsible to spread the disease from rats to mammals (fig. 16.4). Rat flea (Xenopsylla cheopis) ingests the pathogen by sucking blood from an infected rat. Bacteria multiply in flea’s intestine and can be transmitted to a healthy animal (including man) in the next bite. Once in man’s body, the pathogens reach the lymph nodes where they result in swollen areas called buboes hence the disease called bubonic plague.

Symptoms are usually absent until the last day or two when large amounts of bloody sputum are produced. Untreated victims rarely survive more than two days. Septisemic plague is caused due to the rapid spread of Y. pestis throughout the human body via the bloodstream without the development of bulboes and usually in death before the diagnosis could be made.

Prevention and control involves the control of fleas and rodents, isolation of human victims, treatment with tetracyclin for prophylaxis, and vaccination (USP Plague Vaccine) of persons at high risk.