[Biology Class Notes] on Plague Pdf for Exam

What is Plague? 

Plague is one of the deadliest diseases which is caused by Yersinia Pestis. Yersinia Pestis is a form of gram-negative disease-causing bacterium from the rod-shaped coccobacillus family. Plague is caused by the bacterium which transmits from a rat, mouse, or any kind of rodent through the medium of flea. When the flea, having sucked blood from infected rodents, bite human beings, then the disease is transmitted. Plague is, therefore, a vector-borne disease; however, it is also known to spread through other mediums like air, water or one-to-one contact. It is not sexually transmittable but may spread through penetrative sex, as well.

Massive Impact Of Plague On Humanity

Plague is one the most feared pandemics due to a very high communicability rate (the rate at which a disease spreads). One type of plague disease is known to have been the cause of the Black Death in Europe during the mid-thirteenth century which resulted in an estimated number of 75 to 200 million (i.e. 7.5 to 20 crores) during the whole course of the pandemic. It is estimated that due to the Black Death, approximately half of Europe’s population died. The disease is prevalent more in South America and Africa. 

Types Of Plague

There are various types of plague that differ in terms of their physiological signs and symptoms. They are as follows.

1 – Bubonic Plague: It starts with a flea biting a human being and spreading the contamination into the tissue. The Yersinia Pestis bacteria can procreate even inside a cell even if engulfed by it. They later enter the lymphatic system; they then spread through the lymphatic tubes to the lymphatic node. This results in acute lymphadenitis, i.e., intense swellings of the lymphatic nodes. These swollen lymph nodes characterise the bubonic type of disease. These swollen lymph nodes also are haemorrhagic (causing bleeding) or necrotic (characterised by the death of cells). 

2 – Septicemic Plague: This is the secondary form of infection in which the over-swollen lymphatic nodes drain their fluids into the bloodstreams. This makes way for the bacteria to spread across the different parts of the body. The released endotoxins by the pathogen cause the blood throughout the body to coagulate (curdle). The semisolid blood is not effective in oxygenation and perfusion of the body anymore and causes necrosis of the tissues throughout several parts of the body. Consequently, there is obvious bleeding inside the skin, making it swell with extreme redness and blackness. There is also bleeding inside the organs. Patients are seen coughing and vomiting blood. This level of plague is usually fatal.

3 – Pneumonic Plague: In this type of plague, the infection enters into the patient’s lungs. This level of the infection makes the disease contagious. When the patient coughs, the droplets get airborne, and one can get infected by inhaling or ingesting the droplets. If untreated, the infection at this stage will definitely result in fatality. 

Methods Of Transmission In Details

At the onset of the disease, it is vector-borne; i.e., it is spread through infected rodents and their parasites, i.e., fleas. Once the infection jumps the species, i.e., from rodents to humans, it can spread to other hosts through other mediums. Upon passing of the infection from the phase of lymphatic infection to the pneumatic or septicemic stage, the bodily secretions can become transmittable in the following ways:

1. Direct Contact With The Bodily Fluids: The bodily secretions like mucus, faeces, urine etc., and bodily fluids like blood, saliva, tears etc. are highly contagious. One may come in contact with them by inhaling the contaminated airborne droplets expelled by the body via cough or sneeze. Even sexual contacts may be responsible for contact with the contaminated fluids.

2. Indirect Contact: Accidental contacts with the surfaces contaminated with the patient’s bodily fluids may result in infection. Holding items previously handled by the infected, cleaning and taking them to the toilet may be some types of indirect contacts. 

3. Water-Borne: Defecation or urination of the infected person into a source of water shared by others, too, for drinking or other household activities may too result in the infection.

Symptoms 

The symptoms of plague differ with the stage of infection. However, they are in general as follows: 

• Fever

• Seizures

• Nausea

• Diarrhoea

• Vomiting

• Physical weakness

• Bouts of or persisting excruciating headache

• Obstructed breathing

• Swelling in the joints

• Severe muscular pain

• Chest pain with severe coughs

• Reddening or blackening of the skin with swelling

Diagnosis