Forms antiphagocytic capsule at 37°C
Role in attachment to host cells
Protects against lysis by activated complement
Traps hemin and other iron-containing substances; stores iron compounds intracellularly
The macrophages die and release the bacteria, which now are encapsulated and express Yops, pilus adhesin, complement resistance, and heme storage. After several days, an acute inflammatory reaction develops in the nodes, producing enlargement and marked tenderness. The lymph nodes become necrotic, allowing large numbers of virulent Y. pestis to spill into the bloodstream. This stage of the disease is called septicemic plague, and endo-toxin release results in shock and disseminated intravascular coagulation (DIC). Infection of the lung from the bloodstream occurs in 10% to 20% of the cases, resulting in pneumonic plague. Organisms transmitted to another person from a case of pneumonic plague are already fully virulent and, therefore, especially dangerous. The dark hemorrhages into the skin from DIC and the dusky color of skin and mucous membranes probably inspired the name black death for the plague.
The mortality rate for persons with untreated bubonic plague is between 50% and 80%. Untreated pneumonic plague progresses rapidly and is nearly always fatal within a few days.
28.3 Bacterial Diseases of the Lymph Nodes and Spleen 725
Plague is endemic in rodent populations of all continents except Australia. In the United States, the disease is mostly confined to wild rodents in about 15 states in the western half of the country. Prior to 1974, only a few cases of plague were reported each year. Over the last few decades, however, the number of cases has generally been higher, averaging about 15 reported cases per year, as towns and cities expand into the countryside. Prairie dogs, rock squirrels, and their fleas constitute the main reservoir, but rats, rabbits, dogs, and cats are potential hosts. Hundreds of species of fleas can transmit plague, and the fleas can remain infectious for a year or more in abandoned rodent burrows. Epidemics in humans, initiated by infected rodent fleas, can spread from person to person by household fleas, as well as by aerosols produced by coughing patients with pneumonic plague. Yersinia pestis can remain viable for weeks in dried sputum and in flea feces and for months in the soil of rodent burrows. ■ endemic disease, p. 486 ■ reservoir, p. 486
Plague epidemics can be prevented by rat control measures such as proper garbage disposal, constructing rat-proof buildings, installing guards on the ropes that moor oceangoing ships to keep rats from entering, and rat extermination programs. The latter must be combined with the use of insecticides to prevent the escape of infected fleas from dead rats. A killed vaccine that gives short-term partial protection against plague is available to control epidemics and for those who are at high risk in laboratories or endemic areas. The antibiotic tetracycline can be given as a preventive for someone exposed to plague. The antibiotic is useful in controlling epidemics because of its immediate effect. Treatment with gentamicin or tetracycline is effective, especially if given early in the disease.
The main features of plague are presented in table 28.5.
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