Plague

Plague is caused by bacteriaYersinia pestis.In the Middle Ages it was known as black death. That is because, it often leads to gangrene and blackening of various parts of the body. Capillary fragility causes subcutaneous bleeding, which also leads to black spots.

Morphology and physiology
Yersinia pestis is a pleomorphic, Gram-negative, polar-staining, facultatively aerobic, immobile bacteria. The optimum temperature for this organism is 28°C. It is an facultative intracellular parasite.

Epidemiology, transmission and symptoms
Three documented pandemic plagues have killed hundreds of millions of people. Even today, sporadic infections occur. The infection is endemic among rodents in remote agricultural areas. Yersinia pestis is transmitted between rats and human fleas (rat fleas or plague fleas, Xenopsylla cheopis). When a human is bitten by a flea-transmitting yersinia, these organisms get into the wound. Most are phagocytosed and neutralized neutrophils. However, some histiocytes are carried away, which are not able to kill them, allowing them to form a capsule and multiply. Encapsulated organisms (after release from histiocytes) are resistant to phagocytosis and neutrophil destruction. The result is an infection that spreads to lymph nodes (they become warm, swollen, stiff and bloody). Inflamed lymph nodes acquire a characteristic black color, which gives the name of the disease: black (bubonic) plague. Within hours, the parasite will spread to spleen, liver and lungs, resulting in pneumonia. Bacteria in the circulation causes diffuse intravascular coagulation, which is the cause trombosis and purpuric lesions all over the body. Untreated infection has a high (up to 90%) mortality rate. Developed plague is transmissible by droplet infection with cough. Droplet infection leads to pulmonary form plague, which is very progressive and has up to 100% mortality rate.

Pathogenesis
In pathogenesis of Y. pestis many factors play a direct or indirect role:
 * Low calcium response (lcr): It is a gene encoded by a plasmid that allows the bacteria to grow in a calcium-poor environment (ie, inside a cell). It also coordinates the creation of several other factors virulence, such as V, W and so-calledyops (Yersinia outer proteins).
 * V and W proteins: These plasmid-encoded proteins are associated with rapid proliferation and septicemia.
 * Yops: Group of 11 proteins encoded by plasmids. They are essential for rodent pathogenesis and are responsible for cytotoxicity, inhibition of phagocyte migration and aggregation of platelets.
 * Coating (F-1) antigen: It is a complex of proteins and polysaccharides that is expressed in an absorbent organism (not fleas) and has an anti-phagocytic effect.
 * Coagulase and akctivator of plasminogen (PA): coagulase is responsible for the formation of microthrombi. Plasminogen activator promotes hematogenous spread of infection.

Diagnosis and treatment
We clinically diagnose plague in areas of its endemic occurrence. The causative agent of the disease is shown in Gram staining or can be cultured on blood agar. It is treated with tetracycline, chloramphenicol or aminoglycosides.

Related articles

 * Yersinia pestis
 * Yersinia enterocolitica
 * Yersinia pseudotuberculosis

Literature

 * GILLESPIE, SH a KB BAMFORD. Medical Microbiology and Infection at a Glance. 1. vydání. London : Blackwell Science, 2000. ISBN 978-1405111737.
 * BERAN, GW a KB BAMFORD. Handbook of Zoonoses, Section A: Bacterial, Rickettsial, Chlamydial and Mycotic. 2. vydání. Florida : CRC Press, 1994. ISBN 978-0849332050.
 * University of South Carolina. Microbiology and immunology online [online]. ©2007. Poslední revize 2009, [cit. 22. 11. 2009].