Question

Explain in a paragraph format the infection cycle of Yersinia pestis. State why some of the reasons that the morbidity and mortality of the bubonic plague was so high.

Answer 1

Plague is caused by the bacteria Yersinia pestis, which is an infectious disease spread by rats. The symptoms of the disease include fever with chills, severe abdominal pain, diarrhea and vomiting, bleeding from the mouth, rectum or nose, gangrene and lethargy.

Plague mainly transmitted by rodents, especially, the rats. It can form biofilm inside the digestive system of flea. After entering into the host organism and phagocytosis, the growth conditions inside the phagocytes activate virulence genes. The shape of Y. pestis resembles a safety pin in some stained preparations.

The infection cycle of the plague:

The organism causing plague is harboured by the endemic reservoir hosts (mice), which does not develop the disease. The organism spread from the reservoir hosts to the amplifying hosts (chipmunk) through the flea, and the amplifying hosts develop the disease. The flea again carry the organism towards the human hosts, or the organism can reach humans through the aerosol. After infecting humans, the organism can cause bubonic plague or pneumonic plague.

A simple illustration of the infection cycle of Yersinia pestis is given below:

Flea Amplifying hosts (Chipmunk, develop infection) Bubonic plague Pneumonic plague Aerosol - Flea (carry organism to the humans) Endemic reservoir hosts (mouse, does not develop infection) Infected amplifying host Flea k Life cycle of Yersinia pestis

The two types of plague include,

• Bubonic plague
• Pneumonic plague

Bubonic plague occurs if the individuals are infected through a skin cut or bitten by an infected flea. The symptoms appear once the organism moves from the site of infection to the lymph nodes. The enlarged lymph nodes are called “buboes.” If the organism enters into the systemic circulation, it is called “septicemic plague.”

In the speticemic plague, the organism enters the blood circulation, thus, infect the lungs and cause “pneumonic plague.” Pneumonic plague is most contagious because the organism can spread through the aerosol droplets (produced during cough). So, it is considered as the most dangerous form of plague, the patient may die within 24-48 hours if left untreated.