Etiology
Yersinia species are gram-negative coccobacilli bacteria in the Enterobacteriaceae family.
Plague is caused by infection with the gram-negative bacteria Yersinia pestis. A variety of small mammals, including rats, squirrels, cats, and rabbits, maintain a natural reservoir. Infection is transmitted to humans and between animals by flea bites. Primary pneumonic plague as a result of inhaling plague-containing droplets from an infected person or animal is uncommon.[7]
Yersinia enterocolitica may be found worldwide in both wild and domesticated animals and a range of environmental sources, including water. Infection occurs by ingestion of contaminated food or water. The organism can grow at 39°F (4°C). Refrigerated meats are a potential source.[19] Secondary cases may occur in affected households as a result of fecal-oral transmission.
Yersinia pseudotuberculosis is the least common yersinia infection. The animal reservoir includes a variety of birds and mammals.[3][Figure caption and citation for the preceding image starts]: A female Xenopsylla cheopis flea, also known as the oriental rat fleaFrom the image collection of the CDC; used with permission [Citation ends].
Pathophysiology
Yersinia pestis is thought to have evolved from Yersinia pseudotuberculosis 10,000 to 40,000 years ago when it acquired genes that enabled it to infect fleas. Unlike Y pseudotuberculosis, Y pestis is able to infect fleas in large numbers and can aggregate to block the flea mid-gut. Blocked fleas regurgitate esophageal contents and make repeated attempts to take a blood meal leading to thousands of bacilli being inoculated into skin. Bacilli multiply within macrophages and are transported to regional lymph nodes. Dissemination around the body follows, and foci of extracellular bacilli form in tissues.
All Yersinia species produce siderophores that enable the bacilli to sequester iron. Y pestis expresses specific genes that enable it to survive in the flea and to disseminate and evade the immune system of its mammalian hosts. A high-density bacteremia is necessary for Y pestis to be efficiently absorbed and transmitted by fleas, and this may explain the increased pathogenicity of Y pestis above the enteropathogenic Yersinia species.[20]
Y pestis expresses a capsule-like antigen fraction 1 (F1) and is a major diagnostic marker for the detection of the bacteria.[21] All three Yersinia species possess a plasmid-encoded type 3 secretion system, which aids their resistance to host defences.[22]
The incubation period is 1 to 7 days.[23]
Classification
Clinical classification
Bubonic plague
Accounts for approximately 80% to 95% of all cases worldwide and has a case fatality rate of 10% to 20%.[1]
Characterized by the sudden onset of systemic symptoms and painful swelling of lymph nodes possibly with necrotic areas (buboes) draining the area of the flea bite.
Septicemic plague
Accounts for approximately 10% to 20% of all cases worldwide.[1]
Plague may present with systemic features such as fever and hypotension, but without clinically detectable lymph node involvement.
Pneumonic plague
Rare, but has the highest case fatality rate (close to 100% if left untreated).[1]
Primary pneumonic plague results from person-to-person transmission, although this is rare. The incubation period is short (from a few hours to 2-3 days). Onset of symptoms is rapid and may include pleuritic chest pain, hemoptysis, and dyspnea.
Secondary pneumonic plague results from systemic infection.
Meningeal plague
Typically occurs as a complication of delayed or inadequate treatment of another clinical form of primary plague and is characterized by fever, nuchal rigidity, and confusion.
Pharyngeal plague
Uncommon clinical manifestation of plague that follows inhalation or ingestion of plague bacilli. The anterior cervical nodes are usually inflamed and pharyngeal inflammation is common.
Yersiniosis
Infection caused by the enteropathogenic Yersinia species: Yersinia enterocolitica and Yersiniapseudotuberculosis. Symptoms are mostly confined to the gastrointestinal tract. Reactive arthritis is a common complication.
Far East scarlet-like fever (scarlatinoid fever)
Some strains of Y pseudotuberculosis can cause Far East scarlet-like fever with clinical features of erythematous skin rash (usually of the face, elbows and knees), skin desquamation, exanthema, hyperemic tongue, and toxic shock syndrome. This condition is beyond the scope of this topic.
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