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On the rat track

Plague epidemics are widely prevalent in Africa, Asia and South America. In the US, there are about 10-15 cases per year.

The endemic zones of plague in India include the tri-junction of Maharashtra, Karnataka and Tamil Nadu, foothills of the Himalayas and the watersheds of Vindhyas. But the disease extends to areas such as West Bengal in the east and Kerala in the south.

Ishwar Prakash, principal investigator, desert regional station, Zoological Survey of India, Jaipur, however, says: "The ecology of rats has completely changed during the past few decades. We just do not know how the pathogen might be moving around."

Structures such as the Indira Gandhi Canal in Rajasthan have brought different types of rats together and this is one of the hotspots for the occurrence of plague. Other high-risk zones that have emerged include Mumbai and Kolkata. Experts feel that anything that might disturb rat ecology has the potential to cause plague.

Deforestation could be one such change. The rodents end up moving to nearby habitations, taking with them the pathogen which can then cause an epidemic. Studies on rat ecology reveal that regions which are dry and rocky attract the animal. Lack of sanitation and easy availability of food can also draw the rodents to residential areas.

Natural calamities such as flood, drought and cyclone favour the growth of rodent populations. The Technical Advisory Committee, constituted by the Union government in the aftermath of the Surat plague in 1994, had studied the epidemiology and ecology of the disease at Beed in Maharashtra and Surat in Gujarat.

Beed is located near Latur, which was rocked by a major earthquake in 1993. After the disaster, houses were abandoned giving the rats (including wild rodents) a chance to grow and multiply on the food that was left behind. This was accompanied by a sudden surge in the population of pathogen-bearing fleas, which led to ratfall. Human beings were then targeted by the fleas. In the case of Surat, heavy rain and flood led to the death of a large number of animals, compelling the fleas to look for an alternative host. In Andhra Pradesh, cyclones caused a four-fold increase in rodent population.

"The effect of the environment on the disease can be understood only after detailed studies," says Kalyan Banerjee, ex-director of National Institute of Virology, Pune. Areas in Himachal Pradesh received high rainfall and there were also instances of floods. But a linkage between these and the occurrence of plague has not been established as yet.

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