Yersinia - Plague
Plague (Latin pestis: plague) is an infectious disease caused by the bacterium Yersinia pestis. An infection initially leads to symptoms similar to those of a severe flu, then lymph nodes swell to form thick bumps (= bubonic plague). If diagnosed early, the chances of cure are very high with antibiotics. In advanced stages, untreated bubonic plague can lead to pneumonic plague. In the case of pneumonic plague, which is often fatal, human-to-human transmission can then also take place. This acute pneumonia can be transmitted aerogenically by droplets and spread rapidly. It quickly leads to death if left untreated.
Wild rodents and their fleas are the natural reservoir of Yersinia pestis. The pathogen is most commonly transmitted to humans by infected fleas from rats or, in the western United States, from ground squirrels and prairie dogs. The wild and domestic animal population of the European Union is free of plague bacteria.
Yersinia, although growing at the same rate at 37 °C as at 25 °C to 28 °C, are not overgrown by the accompanying flora at the lower incubation temperatures.
The first documented plague pandemic began in Egypt in 542 AD and subsequently spread to Europe via Turkey. The second wave began in Africa and Asia Minor in the 14th century and, after spreading to Europe, was responsible for the death of a quarter of the European population. Europe was also mainly affected in the third pandemic between the 15th and 18th centuries. The fourth and so far last pandemic started around 1860 in Yunnan in China and spread primarily southwards, reaching the southern coast of China and Hong Kong. The plague subsequently spread by sea to other regions of Asia, including India, to Brazil and North America, notably California.
Among animals, plague is currently still widespread in many countries in Africa (e.g. Uganda, Madagascar, Democratic Republic of Congo), the Americas and Asia. There are no areas of distribution in Australia. Smaller outbreaks of bubonic plague are not uncommon in Madagascar. However, the 2017 epidemic was significantly more severe, and the majority of illnesses were also cases of the easily transmitted pneumonic plague. In the United States (particularly southwestern states such as New Mexico, Arizona, California, and Colorado), approximately 5 to 15 human cases of bubonic plague are diagnosed each year. Although 20% of these cases result in secondary pneumonic plague, no human-to-human transmission (= primary pneumonic plague) has been documented in the USA since 1924. To date, however, five cases of primary pneumonic plague have been documented in the U.S. that were attributable to cat-to-human transmission. Since affected animals also become ill and die, the pathogen can sometimes be detected in wild and domestic animals before humans are affected. This approach has proven successful in the USA, where entire national parks have repeatedly been temporarily closed for safety reasons due to cases of plague in ground squirrels or prairie dogs.
In bubonic plague and plague sepsis, direct human-to-human transmission is extremely rare: Transmission of the plague bacteria occurs primarily through the bite of infected fleas. In this case, human-to-human transmission would be possible through bodily fluids (e.g., bubonic fluid in bubonic plague). Human infection can also occur through contact with infected animals, e.g. when a hunter comes into contact with blood or other bodily fluids of the animal while skinning infected animals. The situation becomes far more dangerous when an untreated bubonic plague progresses to pneumonic plague: In this case, aerogenic transmission (= transmission by infectious droplets) is possible through the person suffering from pneumonic plague or through the diseased animal.
In terms of clinical symptoms, the following forms can be distinguished depending on the stage/severity/progression:
Bubonic plague (bubonic plague): the disease is most commonly diagnosed at the bubonic plague stage. Typically, the incubation period after an infectious flea bite is 2-5 days, but it can vary from a few hours to twelve days. Initial symptoms such as chills and a rapid rise in temperature to often over 40°C occur suddenly. The lymph nodes in the area of the flea bite site swell (1-10 cm) and form the typical bumps. There are skin changes in the region of the affected lymph nodes, such as pustules or even small ulcers. If left untreated, bubonic plague leads to death in more than 50% of cases.
Plague sepsis: If plague bacteria get into the blood, there is bleeding under the skin, which can also affect larger areas. Probably because of the increasingly dark discoloration of these areas, the disease was given the name "the black death". In extreme cases, the course of plague sepsis is lightning-like, in which a large number of plague bacteria enter the blood, but the lymph nodes are not enlarged.
Pneumonic plague: Secondary pneumonic plague can develop as a result of bubonic plague. In this case, the pathogens trigger pneumonia, which leads to coughing and expectoration of highly infectious mucus, and subsequently to the destruction of lung tissue. If the plague infection occurs through direct contact with infectious (cough) droplets (also from domestic animals such as cats with pneumonic plague), the result is primary pneumonic plague. This has a very short incubation period of 2-3 days maximum, and mortality is high. It starts with high fever, chills and severe headache. After about 24 hours, the first coughing attacks occur. Very soon, bloody sputum (light red/foamy, "raspberry syrup/jelly") joins in. Pneumonic plague is almost always fatal if left untreated. In individual cases, patients fall ill and die within one day of infection.
Special forms of plague: Pharyngeal plague occurs when plague bacteria are ingested through the mouth. Sore throat dominates the clinical picture. Plague meningitis usually develops when bubonic plague is inadequately treated. In endemic areas, immunity can lead to so-called "pestis minor" ("reduced plague"), in which infected persons only become slightly ill.
The plague can be treated well with antibiotics. However, intravenous therapy should be started as soon as possible (within 24 hours of the appearance of the first symptoms) and given for 10 to 14 days or until 48 hours after fever has broken. Persons ill with plague should be hospitalized. Patients with pneumonic plague must be isolated there. There is no protective vaccination: several poorly effective vaccines against plague have been tried in the past.
In principle, the cultivation of plague bacteria does not pose any particular problem. However, the cultivation of Y. pestis requires the infrastructure of a BSL-3 laboratory safety level. In addition, plague pathogens cannot be reliably identified by commercially available identification systems, so there is a risk of misdiagnosis. In cases of suspicion, the result is available within a few hours by means of molecular biological examination.