Yersinia pestis

Yersinia pestis

Flea infected with yersinia pestis

Preparações de Ricardo Jorge do bacilo da peste bubónica no Porto (1899) - fotografias de António Plácido da Costa (Museu de História da Medicina Maximiano Lemos, FMUP)

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Yersinia pestis is a Gram-negative bacterium that belongs to the family Enterobacteriaceae. It is a rod-shaped, nonmotile, and facultative anaerobic organism that is primarily known as the causative agent of plague, a disease that has caused several pandemics throughout human history. The most notable of these pandemics include the Justinian Plague, the Black Death, and the Third Pandemic, which collectively resulted in the deaths of hundreds of millions of people. Yersinia pestis is transmitted to humans through the bites of infected fleas, direct contact with contaminated fluids or tissues, and inhalation of respiratory droplets from a person or animal with pneumonic plague.

Biology and Genetics

Yersinia pestis is characterized by its ability to infect and multiply inside the lymph nodes, lungs, and blood of its hosts. The bacterium possesses a number of virulence factors that enable it to evade the immune system, including a type III secretion system (T3SS) that injects effector proteins into host cells. Its genome consists of a circular chromosome and several plasmids that carry genes critical for its pathogenicity and survival in both fleas and mammals.

Transmission and Disease

The primary vector for Yersinia pestis is the rat flea (Xenopsylla cheopis), although other flea species can also transmit the disease. Plague manifests in three primary forms: bubonic, septicemic, and pneumonic, with bubonic being the most common. Bubonic plague is characterized by the sudden onset of fever, chills, weakness, and swollen and painful lymph nodes (buboes). Septicemic plague occurs when the bacteria multiply in the bloodstream, and pneumonic plague, the most virulent form, involves severe lung infection.

Prevention and Treatment

Prevention of plague involves controlling the rat populations and reducing contact with fleas. Vaccines are available but are primarily recommended for those at high risk of exposure. Treatment typically involves the administration of antibiotics, such as streptomycin, tetracycline, or gentamicin, which are effective if administered early in the course of the disease.

Historical Impact

The impact of Yersinia pestis on human history is profound. The Black Death of the 14th century, caused by Yersinia pestis, killed an estimated 50 million people, or up to 60% of the European population at the time. The disease significantly affected the course of European history, leading to widespread social, economic, and cultural changes.

Research and Surveillance

Ongoing research on Yersinia pestis focuses on understanding its pathogenesis, developing effective vaccines, and improving diagnostic methods. Surveillance and monitoring of plague cases are crucial for preventing large outbreaks, especially in regions where the disease is endemic.