Bubonic plague

Bubonic plague is one of three types of plague caused by the bacterium Yersinia pestis. One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and vomiting, as well as swollen and painful lymph nodes occurring in the area closest to where the bacteria entered the skin. Acral necrosis, the dark discoloration of skin, is another symptom. Occasionally, swollen lymph nodes, known as "buboes", may break open.

The three types of plague are the result of the route of infection: bubonic plague, septicemic plague, and pneumonic plague. Bubonic plague is mainly spread by infected fleas from small animals. It may also result from exposure to the body fluids from a dead plague-infected animal. Mammals such as rabbits, hares, and some cat species are susceptible to bubonic plague, and typically die upon contraction. In the bubonic form of plague, the bacteria enter through the skin through a flea bite and travel via the lymphatic vessels to a lymph node, causing it to swell. Diagnosis is made by finding the bacteria in the blood, sputum, or fluid from lymph nodes.

Prevention is through public health measures such as not handling dead animals in areas where plague is common. While vaccines against the plague have been developed, the World Health Organization recommends that only high-risk groups, such as certain laboratory personnel and health care workers, get inoculated. Several antibiotics are effective for treatment, including streptomycin, gentamicin, and doxycycline.

Without treatment, plague results in the death of 30% to 90% of those infected. Death, if it occurs, is typically within 10 days. With treatment, the risk of death is around 10%. Globally between 2010 and 2015 there were 3,248 documented cases, which resulted in 584 deaths. The countries with the greatest number of cases are the Democratic Republic of the Congo, Madagascar, and Peru.

The plague is considered the likely cause of the Black Death that swept through Asia, Europe, and Africa in the 14th century and killed an estimated 50 million people, including about 25% to 60% of the European population. Because the plague killed so many of the working population, wages rose due to the demand for labor. Some historians see this as a turning point in European economic development. The disease is also considered to have been responsible for the Plague of Justinian, originating in the Eastern Roman Empire in the 6th century CE, as well as the third epidemic, affecting China, Mongolia, and India, originating in the Yunnan Province in 1855. The term bubonic is derived from the Greek word βουβών, meaning 'groin'.

The bubonic plague is an infection of the lymphatic system, usually resulting from the bite of an infected flea, Xenopsylla cheopis (the Oriental rat flea), though other flea species are able to carry the bubonic plague, such as Pulex irritans (the human flea) and Ceratophyllus fasciatus. Xenopsylla cheopis was the most effective flea species for transmission.

The flea is parasitic on house and field rats and seeks out other hosts when its rodent host dies. Rats were an amplifying factor to bubonic plague due to their common association with humans (living around and within dwellings) as well as the nature of their blood, which allows the rat to withstand a major concentration of the plague.^{[clarification needed]} The bacteria form aggregates in the gut of infected fleas, and this results in the flea regurgitating ingested blood, which is now infected, into the bite site of a rodent or human host. Once established, the bacteria rapidly spread to the lymph nodes of the host and multiply. The fleas that transmit the disease only directly infect humans when the rat population in the area is wiped out from a mass infection. Furthermore, in areas with a large population of rats, the animals can harbor low levels (subclinical) of the plague infection without causing human outbreaks. With no new rat inputs being added to the population from other areas, the infection only spread to humans in very rare cases of overcrowding.

After being transmitted via the bite of an infected flea, the Y. pestis bacteria become localized in an inflamed lymph node, where they begin to colonize and reproduce. Infected lymph nodes develop hemorrhages, which result in the death of tissue. Y. pestis bacilli can resist phagocytosis and even reproduce inside phagocytes and kill them. As the disease progresses, the lymph nodes can hemorrhage and become swollen and necrotic. Bubonic plague can progress to lethal septicemic plague in some cases. The plague is also known to spread to the lungs and become the disease known as the pneumonic plague. Symptoms appear two to seven days after getting bitten and they include: