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:
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.
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.
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
Blood test
Bronchoscopy
Fluid test of the swollen lymphatic node
FAQs on Plague
1. What is plague in biology?
Plague is a serious infectious disease caused by the bacterium Yersinia pestis. It is a zoonotic disease, meaning it can be transmitted from animals to humans. It primarily affects rodents, but their fleas can transmit the bacteria to humans, leading to severe illness.
2. What pathogen causes plague and what is its primary mode of transmission?
The causative agent of plague is the gram-negative bacterium, Yersinia pestis. The most common mode of transmission to humans is through the bite of an infected flea that has previously fed on an infected rodent, such as a rat, squirrel, or prairie dog. This is known as the vector-borne transmission route.
3. What are the three main types of plague?
The three main clinical forms of plague are distinguished by the part of the body they affect:
- Bubonic plague: The most common form, characterised by the infection of the lymphatic system, causing swollen lymph nodes called buboes.
- Septicemic plague: Occurs when the bacteria enter the bloodstream directly and multiply there, causing blood poisoning.
- Pneumonic plague: The most serious form, which involves the infection of the lungs. It is the only form of plague that can be spread from person to person.
4. What are the key symptoms associated with each type of plague?
Symptoms vary depending on the type of plague:
- Bubonic plague symptoms include sudden onset of fever, headache, chills, weakness, and one or more swollen, tender, and painful lymph nodes (buboes).
- Septicemic plague symptoms include fever, chills, extreme weakness, abdominal pain, shock, and potentially bleeding into the skin and other organs, which can cause tissues to turn black.
- Pneumonic plague symptoms include fever, headache, weakness, and rapidly developing pneumonia with shortness of breath, chest pain, cough, and watery or bloody mucus.
5. How are animals like rodents and fleas involved in the plague transmission cycle?
Rodents, such as rats and squirrels, act as the primary reservoir hosts for the Yersinia pestis bacterium, meaning they can carry the bacteria, often without dying. Fleas act as the vector. A flea becomes infected by feeding on an infected rodent. The bacteria then multiply inside the flea. When this infected flea bites a human or another animal, it transmits the bacteria, thus continuing the cycle of infection.
6. How is plague diagnosed and treated in modern medicine?
Plague is diagnosed by taking samples of blood, sputum (from the lungs), or pus from a bubo and sending them to a laboratory for testing to identify the Yersinia pestis bacterium. The disease can be effectively treated with modern antibiotics. Prompt treatment is critical, as delaying it significantly reduces the chances of a full recovery, especially in cases of septicemic or pneumonic plague.
7. Why is pneumonic plague considered the most dangerous form of the disease?
Pneumonic plague is the most dangerous form for two main reasons. Firstly, it has an extremely high mortality rate if not treated with antibiotics within 24 hours of symptom onset. Secondly, and more importantly, it is the only form that allows for direct person-to-person transmission through inhaling infectious respiratory droplets from a coughing patient, enabling rapid and widespread outbreaks without the need for fleas or rodents.
8. What is the difference between the sylvatic cycle and the urban cycle of plague?
The difference lies in the host animals involved. The sylvatic (or wild) cycle is the natural circulation of plague bacteria between wild rodents and their fleas in a rural or natural setting. This cycle maintains the bacteria in nature. The urban cycle begins when the infection spills over into rodents that live in close proximity to humans, like city rats. Fleas from these rats can then transmit the infection to people, potentially causing an epidemic.
9. Can plague be eradicated like smallpox? Explain why or why not.
No, plague cannot be eradicated like smallpox. The primary reason is that Yersinia pestis has a vast and widespread animal reservoir in wild rodent populations across the globe. Unlike smallpox, which only infected humans, the plague can persist indefinitely in these wild animal populations. As long as this reservoir exists, there will always be a potential source for new human infections, making complete eradication practically impossible.
10. How does the Yersinia pestis bacterium manage to overcome the human immune system?
Yersinia pestis is highly effective at evading the immune system using a mechanism called the Type III Secretion System (T3SS). This system functions like a molecular syringe, allowing the bacterium to inject toxic proteins directly into key immune cells, such as macrophages. These proteins paralyze the immune cells, preventing them from engulfing the bacteria (a process called phagocytosis) and from sending chemical signals to alert other immune cells, allowing the infection to establish itself rapidly.
