EPIDEMIC OF PLAGUE
In the southwestern part of the United States, plague [Latin plaga, pest] occurs primarily in wild rodents (ground squirrels and prairie dogs). However, massive human epidemics occurred during the Middle Ages, and the disease was known as the Black Death because one of its characteristics is blackish areas on the skin caused by subcutaneous hemorrhages. Infections now occur in humans only sporadically or in limited outbreaks. In the United States approximately 25 cases are reported annually, and the mortality rate is about 15%.
CAUSATIVE AGENTS
The disease is caused by the gram-negative bacterium Yersinia pestis. It is transmitted from rodent to human by the bite of an infected flea, direct contact with infected animals or their products, or inhalation of contaminated airborne droplets (figure 39.9). Once in the human body, the bacteria multiply in the blood and lymph. An important factor in the virulence of Y. pestis is its ability to survive and proliferate inside phagocytic cells rather than be killed by them. One of the ways this is accomplished is by the YOPS (yersinal plasmid-encoded outer membrane proteins) that are secreted by the bacterium and act as antiphagocytic proteins to counteract natural defense mechanisms and help the bacteria multiply and disseminate in the host (see figure 34.4).
SIGNS AND SYMPTOMS
Symptoms—besides the subcutaneous hemorrhages—include fever and the appearance of enlarged lymph nodes called buboes (hence the old name, bubonic plague). In 50 to 70% of the untreated cases, death follows in 3 to 5 days from toxic conditions caused by the large number of bacilli in the blood.
LABORATORY DIAGNOSIS
Laboratory diagnosis of plague is by direct microscopic examination, culture of the bacterium, serological tests, the PCR for detection of bacteria in infected fleas, and phage testing. Treatment is with streptomycin, chloramphenicol, or tetracycline, and recovery from the disease gives a good immunity. Pneumonic plague arises (1) from primary exposure to infectious respiratory droplets from a person or cat with respiratory plague or (2) secondary to hematogenous spread in a patient with bubonic or septicemic plague. Pneumonic plague can also arise from accidental inhalation of Y. pestis in the laboratory. The mortality rate for this kind of plague is almost 100% if it is not recognized within12-24hour Obviously great care must be taken to prevent the spread of airborne infections to personnel taking care of pneumonic plague patients.
PREVENTION AND CONTROL
Prevention and control involves ectoparasite and rodent control, isolation of human patients, prophylaxis or abortive therapy of exposed persons, and vaccination (USP Plague vaccine) of persons at high risk.