Y Pestis Research Paper

Y pestis is the causative agent of the bubonic plague. An infectious disease which has claimed the lives of millions of people across the globe. The purpose of this essay is to give a brief overview of the history of the plague and epidemiology, to discuss both the microbial characteristics and virulence of Y.pestis, and the way in which it is transmitted. This essay will also discuss and compare the different forms of the plague and the ways in which it is currently diagnosed and treated.
Three major plague pandemics have occurred throughout history. The first originated in Africa in the 6th century and claimed the lives of one hundred million people within 60 years. The second pandemic, known as ‘The Black Death’ began in the14th century and killed up to one quarter of Europe’s population. The final pandemic in 1860 began in China and was transmitted via infected rats on steamships to port cities over the following 20 years.
Y pestis is a gram negative, facultative anaerobic rod shaped bacterium that has the ability to enter and live inside of host cells (Tucker, 2010) It is non motile and typically grows at 35-37C.
Y pestis survives as a result of disease transmission within animal reservoirs, particularly rodents. Transmission occurs as a result of the differing resistance levels
of Y pestis in the domestic and wild rat population. The transmission to humans mainly occurs in the domestic rat population which are less resistant to the causative agent of the plague. When these rat populations overlap, the domestic rat becomes infected by the bite of an infected rat flea causing the domestic rat fleas to become infected thus spreading the agent of the plague amongst the domestic rat population. As the domestic rats continue to die as a result of the plague, the infected rat fleas will find another blood source in either domestic animals or humans, instituting a new epidemic amongst humans (Wolk, 1992).
Y pestis invades directly into the host via a break in the skin where a number of macrophages are present. Here it infects the macrophages and survives at the early stage of infection by the secretion of yersinal outer membrane proteins which allows the organism to then penetrate the cell surfaces of the host in order to counteract any initial defence mechanisms. Intracellular growth within the macrophages allows y pestis to avoid phagocytosis and remain undetected by the hosts immune system. 4-5 days’ post-infection, Y pestis numbers have increased rapidly forcing it to escape into the extracellular compartment with phagocytosis resistance (Li, Yang, 2008). The macrophages continue to circulate within the blood, leaving behind a toxic trail of bacilli proving fatal to the host. In the pneumonic plague Y pestis targets both alveolar macrophages and neutrophils, this causes considerable tissue destruction within the lungs and suggests why it is such a lethal form of the plague (Pechous et al, 2013). Its ability to survive and proliferate inside host cells is an important factor in the virulence of y pestis (Sherwood et al, UN date).

There are 3 forms of the plague; bubonic, septicemic and pneumonic.
The bubonic plague has an incubation time of 2-6 days. It is diagnosed by an enlargement of the lymph nodes; mainly in the neck, groin, or armpits. This is accompanied by extreme exhaustion, fever, confusion and headaches. Whilst Y pestis is present in the nodes, it avoids the immune system response and replication ensues resulting in extreme swelling of the lymph nodes. As the plague progresses the bacteria move into the bloodstream and can result in death by endotoxic shock due to the large numbers of bacilli in the blood. According to the World Health Organisation, death occurs in 50-70% of untreated cases
(2016)
Septicemic plague is a direct invasion of the bloodstream, avoiding the lymph nodes. The blood-infection causes small blood clots to occur, which in turn cuts off the circulation to areas of the body. This leads to uncontrolled bleeding, fever, nausea, vomiting blood, and diarrhea. Due to a lack of oxygen to the tissues gangrene can become visible. According to the World Health Organisation (2016) the general flu like symptoms this form of the plague presents made diagnosis somewhat difficult resulting in a death rate of 100% if untreated.
The pneumonic plague can develop in two ways, either if the bacteria move from the blood into the lungs or via human transmission. It is at the lungs Y pestis can replicate more frequently with a short incubation time of 1-3 days. Symptoms include severe and overwhelming pneumonia, fever and tightness of the chest accompanied by an overwhelming cough. This forces the host to cough droplets containing the bacterium into the air thus infecting more humans when these droplets are inhaled. Pneumonic plague, although rarer, is the most serious form of the disease and is 100% lethal if untreated within 12-24 hours (WHO, 2016).
According to the CDC (2015) Diagnosis involves laboratory tests on either blood cultures, sputum or lymph node fluid to determine if the Y pestis F1 antigen is present. It is then treated with specific antibiotic therapy such as Streptomycin, Gentamicin or fluoroquinolones to specifically target the bacteria. A report by the CDC (2016) confirms that antibiotic therapy has significantly reduced the mortality rate of the plague from 66% to 11%.
According to the WHO (2016) the plague is presently endemic, mainly occurring in rural unsanitary areas such as southern Africa, Central Asia and Southern America. 1,000 – 2,000 cases are reported worldwide with the highest number in Africa. Whilst the plague is considerably more controlled in the present day, Y pestis has already shown to develop resistance to antibiotic therapy, possibly creating further pandemics in the future (Sample, 2007).