Plague 

Plague is a zoonotic disease caused by the bacterium Yersinia pestis.^{[1, 2]} No disease has impacted civilization as deeply as the plague. As many as 200 million people have died from this disease. The first pandemic, known as the Justinian plague (AD 541-544), began in Egypt and spread throughout the Middle East and Mediterranean areas. Eventually, the entire known world was affected. By the 8th century, plague receded into scattered endemic areas.

The second pandemic began in 1347, when traders from central Asia introduced plague into ports of Sicily. This became the first epidemic, known as the Black Death, which killed over one third of the population of Europe. Later, following the Great Plague of London (1665), the disease subsided.

The third pandemic began in Hong Kong in 1894 and continues to the present. Alexandre Yersin discovered the plague bacillus, Y pestis, and effective antibiotics were introduced in the early 1940s; however, plague remains endemic in much of the world.^{[3, 4]}

Examples of findings are shown in the images below.

The classic mode of transmission to humans is a flea bite (see the image below).

Alternately, broken skin serves as a portal when tissue or blood of an infected animal is handled (skinning or evisceration of infected animals). Competency of the flea to serve as vector for transmission of plague to humans depends on its willingness to feed on a human host and its tendency to regurgitate intestinal contents during a blood meal. Fleas from sylvatic rodents feed on humans only reluctantly. However, the Oriental rat flea (Xenopsylla cheopis) is an effective vector because of its tendency to regurgitate and to feed on nonrodent hosts (see the image below).

When the flea takes a blood meal from an infected rodent, stomach enzymes cause a clot to form, blocking the flea's proventricularis. At its next attempt to feed, unable to swallow due to the blockage, the flea regurgitates plague bacilli into the bite wound.

The organisms invade the lymphatics and travel to regional lymph nodes, causing inflammation (see the image below).

Large, tender lymph nodes are termed buboes and give the bubonic form of plague its name. If the infection is not contained at this site, the organisms may be further spread via the bloodstream to organs such as lungs, spleen, liver, kidneys, and meninges. Bacteremia without the appearance of buboes is considered septicemic plague. Pneumonic plague occurs when pneumonia results from either hematogenous spread (secondary pneumonic plague) or inhalation (primary pneumonic plague) of organisms transmitted from animals or other humans.

Frequency

United States

Plague is considered endemic in all western and southwestern states.^[5] Native Americans who reside on reservations are at increased risk for acquisition of the disease. Most cases of plague are acquired in rural areas. Ground squirrels and prairie dogs serve as major enzootic foci (see the image below).^[6]

Dogs and cats are susceptible to plague. Domestic animals, cats in particular, have been responsible for human cases.

From 1990-2005, a total of 107 cases of plague were reported in the United States, a median of 7 cases per year. However, plague activity increased during 2006, when a total of 17 cases (2 fatal) of plague were reported. Of these, a cluster of 7 cases (likely due to peridomestic exposures) occurred in Bernalillo, Torrance, and Santa Fe Counties in New Mexico. A second cluster of 4 cases was noted in La Plata County, Connecticut. The remaining cases were noted sporadically in New Mexico (1 case in Lea County), California (2 cases in Los Angeles and Inyo Counties), Utah (1 case in Utah County), and Nevada (1 case in Lander county).^{[7, 8]}

In 2007, a total of 7 cases (2 fatal) were reported in the United States; 5 cases (1 fatal) were reported in New Mexico, and 2 cases (1 fatal) were reported in Arizona.^[9]

International

Plague reached a worldwide maximum of 5419 cases (274 fatal) in 1997, and the incidence has declined since that time.^[10] In 2003, 9 countries reported 2118 cases (182 fatal) to the World Health Organization (WHO).^[11] Algeria reported cases of human plague for the first time in 50 years. India and Indonesia also recently reported cases after a 30-year to 50-year quiescent period. Occurrence is thought to be underreported.^{[12, 13]} Currently, about 95% of the world’s human plague cases now occur in the African region, including Madagascar.^[14] World distribution of plague is shown below.

Mortality/Morbidity

Bubonic plague

Mortality is approximately 16%, which increases to 40-70% in untreated cases^[15] . Practitioners must maintain a high index of suspicion for plague, especially with patients exposed to animals or fleas in endemic areas. The most common complications are secondary septicemia, pneumonia, and meningitis. Polyarthritis, lung abscesses, and superinfection of lymph nodes also rarely occur.

Septicemic plague

Mortality ranges from 30-50% for patients with septicemic plague and increases to nearly 100% in untreated cases. This high mortality rate reflects the difficulty in diagnosis, given the disease's similarity to gram-negative bacterial sepsis. Diagnosis is often made postmortem.

Pneumonic plague

The fatality rate of untreated pneumonic plague approaches 100%. The last reported case of person-to-person transmission occurred during a plague epidemic in Los Angeles in 1924. Since then, cases of primary pneumonic plague have been acquired chiefly from infected cats.

Sex

Earlier reports demonstrated a male predominance in cases. A nearly equal sex distribution has been noted in more recent reviews.

Age

Plague can occur at any age. Approximately 45% of reported cases from 1947-2001 occur in individuals younger than 19 years. Both of the deaths reported in the United States in 1996 occurred in adolescents.