A two-year study, conducted by School of Public Health under the financial grant received by Indian Council of Medical Research, has shown that intermixing of rats and rat- flea species between domestic and wild areas are present in plague prone areas of State of Himachal Pradesh. This is a matter of high concern as such intermingling can help in easy occurrence and spread of plague in the region.
Plague (also known as Black Death) has been occurring at regular intervals of 10-15 years in Himachal Pradesh. Plague still remains a significant global concern due to its persistence in many parts of the world. It is a zoonotic infection and can be transmitted from person to person through respiratory droplets. It undergoes sylvatic cycle, where in, it is maintained in wild rodent populations.
Occasionally it gets transmitted to domestic rats and then into human population. Infected rats die in large numbers, as they have less resistance to the disease. The fleas living on them then seek alternative hosts. Y. pestis taken up by the flea from an infected host in a blood meal undergoes a rapid multiplication in its stomach. The bacteria then form a viscous plug that blocks the stomach and prevents the flea from feeding normally. When the flea attempts to feed again, the blood comes up against the plug of bacteria and cannot enter the stomach. It is instead regurgitated by the flea back into the host. This process takes some bacteria with it, infecting the new host.
In Shimla district of Himachal Pradesh (H.P), a small outbreak of primary pneumonic plague took place during February 2002. Sixteen cases of plague were reported with a case-fatality rate of 25 percent. A previous outbreak in this region during 1983 was also suggestive of pneumonic plague in which 22 cases were reported out of which 17 persons died. There had been several un-official reports of plague in the same region over past five decades.
The study says that plague incidence is limited by temperature above 26.7°C or 29.4°C and humidity above 70 or 80 percent. This may be due to the fact that breeding of fleas normally occurs in temperature ranging from 18-27 °C and humidity at 60 percent.
The population of the rat fleas builds up quickly in these favorable conditions leading to plague out breaks. The flea infestation rate was more in colder months of December, which could be linked to the fact that snowfall could also drive rats from forests into human dwellings for the food, thus increasing the likelihood of human exposure to infected rats or their parasites.
All these circumstances lead to commingling (inter-mixing). This commingling can also allow Y. pestis to be maintained in both wild and domestic rodents. Very few studies globally reported that Yersinia.pestis was found on both wild and domestic rodents similar to our study. One of the factors contributing to increase commingling is urbanization.
It is reported that the natives of plague affected belts of Himachal Pradesh used to go for hunting in jungles during winter season. Simultaneously they also indulge in collection of ‘gucchhi’ a type of mushroom found in forest to earn their livelihood. This way, possibly they usually come in contact with the wild rodents or animals that were infected. It has been reported they often went in groups and stayed in caves for days together.
This activity will lead to influence the social behavior of vertebrates. The presence of hunters in the area can lead the animals to move out of their original home-rangers. This can break-up the social structure and can cause long-term effects on the demography of hunted as well as nearby populations, amplifying the impact of hunting.