Plague is an acute and highly fatal disease caused by Yersinia pestis and transmitted by the bite of infected rat fleas. It is primarily a disease of rodents and small animals and man is affected incidentally.
No infectious disease other than plague has created greater havoc in the world. The first recorded pandemic occurred in the sixth century AD, and is believed to have killed more than 100 million people. The second pandemic occurred in the fourteenth century and destroyed approximately a quarter of the population of Europe and spread into the Middle and Far East. This pandemic was known as the ‘Black Death’ because of the severe cyanosis of an infected person at the terminal stage. With the development of rat control programmes and improvement in living conditions, the incidence of plague has greatly been reduced.
Plague is known to occur in India from ancient times. The last pandemic occurred in the year 1898 and took over 12.5 million lives. This episode of plague involved the states of Punjab, Uttar Pradesh, Bihar, Madhya Pradesh, Andhra Pradesh, West Bengal, Maharashtra, Tamil Nadu, Karnataka and Gujarat. The disease trend showed a continuous decline since then and it almost disappeared from the country in the early 1960s. The last case of human plague in India was recorded in 1966 and since then no proven cases of plague have been reported. However, plague reappeared in India in 1994 and claimed several lives. It was brought under control quickly.
Epidemiology: Plague is a natural disease of both domestic and wild rodents and exists in two forms – sylvatic (wild) plague and urban (domestic) plague. Sylvatic plague exists in the nature in wild rodents (squirrels, rabbits, pack rats, etc.) independent of human population. The infection is maintained in relatively resistant hosts called permanent reservoir hosts. These transmit the infection to resistant animal hosts resulting in epizootics. Some domestic rodents may also be involved and may cause outbreaks in man.
Domestic plague is intimately associated with man and rodents living with man. The infection is picked up from the permanent reservoir host by semi-domestic rodents which is turn transmit it do the commensal rodents and thence to man. Rats are the reservoirs and they usually die acutely. The disease is transmitted by the bites of fleas (e.g., Xenopsylla cheopis, the rat flea) which have previously sucked blood from an infected animal. The ingested bacilli proliferate in the intestinal tract of the flea and eventually block the lumen of the proventriculus. The hungry flea, upon biting another rodent, regurgitates into the wound a mixture of plague bacilli and aspirated blood. If its hosts dies the fleas promptly seeks a replacement. If no rodent is available it will accept a human host, an accidental intruder in the rat-flea-rat transmission cycle.
There are three main clinical types of plague occurring in man:bubonic, pneumonic and septicaemic plague. The incubation period is 2-7 days.
A. Bubonic plague: This is the most common type of the disease. Yersinia pestis enters into the human body through flea bites and initiates a progressive infection. The organisms become localized and causes inflammation of the regional lymph nodes usually in the groin. The enlarged lymph nodes are called buboes, from which the name of the disease is derived. The symptoms of bubonic plague include malaise, fever, and pain in the area of the infected regional lymph nodes. Severe tissue necrosis can occur in various areas of the body, and the skin appears blackened. It was this symptom that gave the name “Black Death” to the disease in the Middle ages. As the infection progresses, the symptoms become quite severe, and without treatment the mortality rate is 60 to 100 %. Death usually occurs within 3-5 days of the first symptoms. Bubonic plague does not spread from person to person.
B. Pneumonic plague: Primary pneumonic plague is rare. It generally follows as a complication of bubonic plague. Pneumonic plague is highly infectious and spread from man to man by droplet infection. Untreated patients rarely survives longer than 3 days. Pulmonary signs may be totally lacking until the final day of illness.
C. Septicaemic plague: Primary septicaemic plague is rare except for accidental laboratory infections. However, bubonic plague may develop into septicaemic plague.
Laboratory diagnosis: Rapid diagnosis is of paramount importance in view of the swift progress of the disease. The important laboratory method of diagnosis include the following:
1. Direct staining of smears prepared from sputum or of fluid aspirated from lymph
nodes for the presence of bipolar organisms.
2. Isolation of causative organism from sputum, throat swabs, autopsy material and aspirates from buboes.
3. Typing of the organisms with bacteriophages.
4. Bacterial agglutination test.
5. Immunofluorescence test.
6. Animal inoculation (usually lethal in mice or guinea-pig, with typical lesions).
7. Nucleic acid based diagnosis like PCR may be used for rapid and accurate diagnosis.
Control and prevention: Important methods of control and prevention are discussed here.
1. Control of rats: Mass destruction of rodents is an important plague-preventive measure.
2. Control on fleas: Destruction of fleas by spraying suitable insecticides.
3. Vaccination: Immunization with plague vaccine is a valuble preventive measure.
4. Health education: Health education to common people is an essential part of the plague control programme.