Plague is a communicable disease, caused by bacterium Yersinia pestis. It is one of the oldest diseases known to man and still occurs in the tropics, subtropics and warmer areas of temperate countries. Past outbreaks have shown that plague may reoccur in areas that have long remained silent.
Plague is primarily a zoonotic disease (transmitted between vertebrate animals and humans)
of wild rodents and man is affected incidentally. It is transmitted from one rodent to another by fleas (ectoparasites) and from animals to humans either by the bite of infected fleas, or direct contact, inhalation and rarely, ingestion of infective materials when handling infected hosts.
Widespread pandemics (disease occurring over a wide geographic area and affecting an exceptionally high proportion of the population) of plague have occurred in past with high mortality. It was known as the "Black Death" during the fourteenth century, causing an estimated 50 million deaths. Nearly half of deaths were in Asia and Africa and the other half were in Europe.
Plague is endemic (a disease that occurs regularly in a particular area) in many countries in Africa, former Soviet Union, Americas and Asia. The distribution of plague coincides with the geographical distribution of the rodents it infects, which are found in all continents except Australia.
Plague epidemics (unusually high occurrence of a disease in a population or area) have occurred in Africa, Asia, and South America but since the 1990s, most human cases have occurred in Africa. In 2013 there were 783 cases reported worldwide, including 126 deaths. According to World Health Organization (WHO), 20 countries in Africa, America, and Asia are identified as natural foci for plague. The 3 most endemic countries are Madagascar, the Democratic Republic of Congo and Peru. In Madagascar, outbreaks have occurred nearly every year since 1980.
In India, outbreaks of plague occurred in Mamla village of district Beed, Maharashtra state (bubonic type) and district Surat in Gujarat state (pneumonic type) in September 1994. Subsequently, suspected cases of pneumonic plague were reported from Delhi, Varanasi, Karnataka and other states. A total of 876 cases and 54 deaths were reported. In 2002, outbreak of pneumonic plague occurred in Himachal Pradesh with 16 cases and 4 deaths. In 2004, outbreak of bubonic plague was reported from Dangud village, of district Uttar Kashi, Uttarakhand state with 8 cases & 3 deaths.
The National Centre for Disease Control (NCDC), Delhi has identified four sylvatic foci (affecting wild animals) in India; the tri-junction of south India (Karnataka, Andhra Pradesh and Tamil Nadu), Beed belt in Maharashtra, Rohru in Himachal Pradesh and Uttarakhand. NCDC is keeping routine surveillance in these areas.
Weak health systems and low standards of environmental hygiene are risk factors for outbreaks of human plague. As with many primarily zoonotic diseases, plague is a very severe disease in people, with case fatality rates of 50-60% if left untreated.
Dikid T et al, Emerging & re-emerging infections in India: An overview, Indian J Med Res 138, July 2013, pp 19-31 asccessed from http://icmr.nic.in/ijmr/2013/july/0704.pdf
Incubation period (time taken between catching an infection and appearance of symptoms) of the disease is 3-7 days. Disease starts with flu like symptoms such as sudden onset of fever, chills, head and body-aches and weakness, vomiting and nausea.
Depending on the route of infection, there are three forms of plague: bubonic, septicaemic and pneumonic.
Plague is caused by Yersinia pestis (non-motile, non-acid fast, non spore forming, gram-negative cocco-bacillus). Y. pestis is easily destroyed by sunlight, high temperatures, and disinfectants.
Vectors of plague- Fleas are the natural vectors of plague in the world. In India, the important fleas for plague transmission are Xenopsylla cheopis, X. astia and X. brasiliensis. Out of these X. cheopis is the principal vector.
Reservoir of infection – Wild rodents are the natural hosts of Y. pestis. At least 220 species of rodents which inhabit mountains, plains, deserts, cultivated fields and forests in both temperate and tropical climate are known to be infected with plague bacillus. Tatera indica among the wild rodents and Rattus norvegicus and R.rattus among the commensal rodents play an important role as reservoir of Y. pestis. In India Tatera indica acts as a main reservoir of infection, not the domestic rat (R.rattus).Generally the infection is maintained by the resistant species of the wild rodents. The susceptible rodents die due to the disease.
Mode of transmission-
Flea bites-Plague bacteria are mostly transmitted by bite of an infected flea. During plague epizootics (occurrence of disease at higher rates than expected, in animal population), many rodents die, and causing hungry fleas to seek other sources of blood. People and animals that visit places where rodents have recently died from plague are at risk of being infected from flea bites. Dogs and cats may also bring plague-infected fleas into the home. Flea bite exposure may result in primary bubonic or septicaemic plague.
Contact with contaminated fluid or tissue- Humans can become infected when handling tissue or body fluids of a plague-infected animal without using proper precautions such as hunters. This type of transmission results in bubonic plague or septicaemic plague.
Infectious droplets- Infected droplets from a case of pneumonic plaque can infect other person. Contact with infected cats can transmit the infection to humans by droplet infection.
When plague is suspected, clinical specimens should be collected immediately and specific antimicrobial treatment should be started before confirmation of laboratory diagnosis. Routine diagnostic specimens for smear and culture include the following: whole blood; aspirates from suspected buboes; pharyngeal swabs, sputum samples or tracheal washes (from those with suspected plague pharyngitis or pneumonia); and cerebrospinal fluid (from those with suspected meningitis).
Smear examination-During epidemics or epizootics, the examinations of smears (stained with Wayson's stain) that show the presence of characteristic bipolar stained plague bacilli are helpful in preliminary diagnosis.
Fluorescent antibody (FA) staining- the smears should be stained with the fluorescent-antibody test (FA).
Culture-Samples collected for culture can be transported to laboratory in Cary-Blair transport medium then cultured on other biological media. Presence of bacilli can be confirmed by Bacteriophage Lysis Test.
Serology- Presence of a single high titre of plague antibody in individual who has not been vaccinated earlier gives a presumptive diagnosis of plague. Whereas a fourfold rise in antibody in two serum samples collected at least 10 days apart gives the confirmation of plague diagnosis.
Other tests- Polymerase Chain Reaction for the detection of selected Yesinia pestis genes, ELISA based rapid serological tests, animal inoculation.
Laboratory diagnostic categories for human plague are as follows:
Post-mortem specimens, specimens from rodent tissues, flea specimens, and soil specimens are also used to determine the eco-epidemiology of the infection.
As the disease is fatal early diagnosis and treatment is essential for the good prognosis of the patient.
When human plague is suspected on clinical and epidemiological grounds, appropriate specimens/samples for laboratory confirmation of disease should be obtained immediately.
The patient should be started on specific antimicrobial therapy without waiting for a definitive answer from the laboratory.
Suspect plague patients with evidence of pneumonia should be placed in isolation, and should be managed under respiratory droplet precautions.
Prevention and control of human plague transmission-
Prevention and control of human plague transmission is a multidisciplinary approach. Preventive measures include regular surveillance for the zoonotic plague and informing local people if zoonotic plague is occurring in their surroundings.
In India, Plague laboratory at Zoonosis Division of NCDC, Delhi and Plague Surveillance Unit, at its Bengaluru Branch, Karnataka are involved with various activities in control measures and bacteriological and serological surveillance in rodents and rat fleas. Integrated Disease Surveillance Programme (IDSP), Government of India is managing surveillance activities in humans through its impending outbreak reporting and weekly surveillance reporting systems.
The information should be provided to local people regarding -
Local physicians and other health care workers must be familiar with the symptoms of plague and consider it in the differential diagnosis.
If plague is suspected, samples should be collected for lab confirmation. Treatment must be started without waiting for confirmation of diagnosis.
Active surveillance should be started for identification of other cases exist or have occurred recently in the same vicinity. Prompt reporting is important for cases of pneumonic plague because this form of the disease can be transmitted directly from person to person via infectious aerosols.
Notification: under the International Health Regulations, WHO Member States have to notify plague when it occurs in humans in their territories.
Control of flea: destruction of rat fleas by proper application of an effective insecticide is most effective method to break the chain of transmission of disease.
Control of rodents: Long term measures to control rodents should be based on improvement of sanitation in human dwellings and their vicinity.
Chemoprophylaxis: Post exposure prophylactic therapy with antibiotics may be given to following persons if the exposure was in the previous six days:
A plaque vaccine is available for human use, though it is not recommended for the public at large or for those with only casual potential exposures. The vaccine is therefore indicated for persons whose work routinely brings them into close contact with Y. pestis, such as laboratory technicians in plague reference and research laboratories and persons studying infected rodent colonies.