Yersinia pestis causes bubonic plague, a disease of great historical importance.  In medieval times, bubonic plague (called the "Black Death") killed nearly a fourth of Europe's total population.  The bacteria is found in wild animals and is transmitted to humans by flea bites.  Epidemics have been caused by poor sanitation conditions in cities where the rat population skyrocketed.  Fortunately, in modern societies, cases of plague are rare.  Symptoms of bubonic plague include fever and the characteristic "bubos" which are swollen, painful lymph nodes found in the neck, under the arms or in the groin area.  Untreated, bubonic plague causes death in over 50% of cases.  Antibiotics such as gentamicin are effective for treating plague when started early in the illness.

Diseases

Bubonic plague - fever, nausea, vomiting and painful, swollen lymph nodes.  Death occurs in over 50% of untreated cases.

Habitat and Transmission

• Yersinia pestis is carried in animals such as rodents, pigs, birds and other wild animals. 
• The bacteria is spread to humans by fleas that first bite an infected animal and then an unlucky human.

Weapons and Defenses

• Survives well in animals and fleas
• Once blood from an infected animal is eaten by a flea, Yersinia pestis causes the blood meal to clot in the gastrointestinal tract of the flea.  This allows the bacteria to reproduce and when the flea bites a human, it will vomit the blood to enable transmission of the bacteria to the unlucky human.
• Once in humans, the bacteria resist destruction by white blood cells and set up camp in lymph nodes.
• Once thriving in a lymph node, Yersinia pestis can spread to the entire body through the bloodstream, causing an overwhelming infection (sepsis).

Weaknesses

• Susceptible to antibiotics such as gentamicin
• Vaccines have been used in the past for treating military members traveling overseas

Photo - Dark stained bipolar ends of Yersinia pestis can clearly be seen in this Wright's stain of blood from a plague victim. CDC. 1993. Used with permission.

Photo 2 - Xenopsylla cheopis, oriental rat flea, with a proventricular plague mass. CDC. Used with permission.