Bubonic plague is mainly a disease of rats and rat fleas (Xenopsylla cheopis). Infection in a human occurs when a person is bitten by an infected flea. The bacteria multiply inside the stomach of flea by forming a mucilaginous plug at the entry to gizzard that blocks its passage and causes the flea to starve. The ever-hungry flea then bites again and again to feed as it cannot satisfy its hunger.

In 1894, two bacteriologists, Alexandre Yersin of France and Shibasaburo Kitasato of Japan working in Hong Kong isolated the bacterium responsible for plague. Though both investigators reported their findings, a series of confusing and contradictory statements by Kitasato eventually led to the acceptance of Yersin as the primary discoverer of the organism. Yersin named it Pasteurella pestis in honor of the Pasteur Institute, Paris, where he worked.  In 1967 it was renamed as Yersinia pestis in honor of Alexandre Yersin.

In 1898, the French scientist, Paul-Louis Simond, who was working in China to battle the epidemic, established that rat-flea was vector of the disease.


 An infected flea bites man and contaminates the wound with regurgitated blood that contains the plague bacteria. Yersiniapestis can reproduce inside cells, so even if phagocytised, they can still survive inside macrophage. Once inside body, bacteria enter lymphatic system. Plague bacteria secrete several toxins, one of which is known to cause dangerous beta-adrenergic blockade. Y. pestis spreads through the lymphatics of the infected person until it reaches a lymph node, where it stimulates severe hemorrhagic inflammation causing the lymph nodes to swell, the condition is called the characteristic “bubo” that is associated with the disease.

Lymphatics ultimately drain into the bloodstream, so the plague bacteria enter blood and travel to any part of body. In septicemic plague, there is rupture of blood capillaries of the skin and other organs which creates black patches on the skin. There are red bite-like bumps on the skin. Untreated, septicemic plague is universally fatal, but early treatment with antibiotics reduces the mortality rate to between 4 and 15 percent. People who die from this form of plague often die on the same day symptoms first appear.

The pneumonic plague infects the lungs and infection spreads from person-to-person transmitted through respiratory droplets released by coughing. The incubation period for pneumonic plague is usually between two and four days, but sometimes can be as little as a few hours. The initial symptoms, of headache, weakness, and coughing with hemoptysis are indistinguishable from other respiratory illnesses. Without diagnosis and treatment, the infection can be fatal in one to six days and mortality in untreated cases is 50–90%.


Infected persons usually start with flu-like symptoms after an incubation period of 3-7 days. Patients typically experience the sudden onset of fever, chills, head and body-aches and weakness, vomiting and nausea. Clinical plague infection manifests itself in three forms depending on the route of infection: bubonic, septicaemic and pneumonic.

Bubonic plague is the most common form of plague resulting from the bite of an infective flea. Plague bacillus enters the skin from the site of the bite and travels through the lymphatic system to the nearest lymph node. The lymph node then becomes inflamed because the plague bacteria replicate there in large numbers. The swollen lymph node is called a “bubo” which is very painful and can become suppurated as an open sore in advanced stage of infection. Symptoms of bubonic plague generally appear within two to eight days after the flea-bite. The bubo is usually 1 to 10 centimeters in diameter, in groin, armpit or neck, swollen, painful and warm to touch. It can cause so much pain that you can’t move the affected part of body. More than one bubo can develop, but typically buboes affect only one area of your body.

Septicaemic plague occurs when infection spreads directly through the bloodstream without evidence of a “bubo”. Septicaemic plague may result from flea bites and from direct contact with infective materials through cracks in the skin.

Pneumonic plague is the most severe and least common form of plague that occurs due to a secondary spread from the advanced infection of an initial bubonic form. Primary pneumonic plague results from inhalation of infective droplets released into the air by coughing patients and can be transmitted from human to human without involvement of fleas. Untreated pneumonic plague has a very high fatality ratio.


People who live and work in areas with active plague infection should take these precautions:

  • Eliminate food and shelter for rodents around homes, work places, and certain recreation areas, such as picnic sites or camp-grounds where people congregate. Remove brush, rock piles, junk, and food sources, including pet food.
  • Health authorities should use appropriate and licensed insecticides to kill fleas during the plague outbreaks in wild animals.
  • Treat pets such as cats and dogs for flea control regularly.
  • Avoid sick or dead animals and report such animals to the health department. Hunters and trappers should wear rubber gloves when skinning animals.
  • Use insect repellents when outdoors in areas where there is a risk of flea exposure.

Preventive treatment with antibiotics is recommended for:

· People who are bitten by fleas or who are exposed to tissues or fluids from a plague-infected animal.

·  People living in a household with a bubonic plague patient, since they may also be exposed to infected fleas.

· People in close contact with a person or pet with suspected plague pneumonia.

People who travel to countries where plague occurs should take these additional precautions:

  • Avoid exposure to fleas from diseased rats. The risk of being bitten by infected fleas is especially high after large numbers of plague-infected rats have died. Therefore, avoid places that are infested with rats.
  • If travel to such areas is essential, apply insect repellents to legs and ankles. Also apply repellents and insecticides to clothes and outer bedding.
  • Take preventive antibiotics if the risk of exposure is high.


Vladimir Havkin, a doctor of Russian-Jewish origin who worked in India, was the first to invent and test a plague antibiotic. The traditional treatments are:

Streptomycin 30 mg/kg of body weight, twice daily for 7 days

Chloramphenicol 25–30 mg/kg of body weight, single dose, followed by 12.5–15 mg/kg of body weight, 4 times daily

Tetracycline 2 g single dose, followed by 500 mg, 4 times daily for 7–10 days. The drug is not suitable for children).

Gentamicin 2.5 mg/kg of body weight, twice daily for 7 days.

Doxycycline 100 mg (adults) or 2.2 mg/kg of body weight for children, orally twice daily have also been shown to be effective.


Plague vaccines at one time were widely used but have not proven to be successful in preventing plague effectively. Vaccines are not recommended for protection in outbreak situations. Vaccination is only recommended as a prophylactic measure for high-risk groups such as laboratory personnel who are constantly exposed to the risk of contamination.

A formalin-inactivated vaccine is available for adults of high risk group but severe inflammatory reactions frequently appear. Primary intramuscular injection is given, followed by boosters at 3-5 months and another booster at 5-6 months. Then 3 more booster shots are given at 6 months interval, followed by doses at 1-2 year interval.