As the Nipah virus outbreak in Kerala claims one more life, making the death toll 11 in the southern state of India, we look at the carriers of the deadly virus. Fruits bats, also called flying foxes, are native to the sub-continent and have shown to be the carriers of the virus that currently has no preventative vaccine or cure. We look at why the bats in question haven't died or gotten sick yet.
Bats are known to be carriers of over 60 viruses that are harmful and in most cases lethal to humans and many other mammals. Being mammals themselves, it is intriguing that the carriers of Ebola, Sars, Nipah and a dozen other deadly viruses aren't affected. Here, we aren’t talking about just fruit bats but other types as well.
Are bats reservoirs; what is a reservoir?
When a virus host can carry and transmit pathogens of such deadly impact without getting any symptoms, the host is called a reservoir. Since 1932 research has been going on to find out how bats have proven to be the perfect reservoirs and transmitters of zoonotic diseases (transmitted from animals to humans).
It started in 1932
The first documented and widely researched incidence of zoonotic disease transmission from bats to humans is of rabies in the early 1930s. Vampire bats were found to be natural reservoirs for the fatal disease, and the bats survived for five to eight months without being affected, and without showing any symptoms themselves.
In 1996, a Hendra virus outbreak in Australia was transmitted through native fruit bats. 2000 saw the very first Nipah outbreak, the virus was named after the village in Malaysia where it happened. The scary Sars (severe acute respiratory syndrome) outbreak in 2003 in China and some other countries has also been traced back to transmission through cave-dwelling Chinese horseshoe bats.
Ebola, Marburg, Reovirus, Mers (Middle East Respiratory Syndrome) and now this outbreak of Nipah have all been transmitted to humans through various sub-species of bats.
Why are they immune or less susceptible?
There are various theories and research hypothesis about why bats are such great reservoirs for pathogens that can kill other mammals.
They have constant immunity
First published in 2012, a report by Dr Michelle Baker, a bat immunologist at the Commonwealth Scientific and Industrial Research Organisation in Australia claims that studying the immune responses of bats to these viruses is the way forward to finding a cure or treatment for human infections.
According to the results of the research, Baker maintains that the immune system of bats is not as strong or diverse as of humans, but it is constantly ‘switched on’. In humans our immune responses are triggered when there is an ongoing infection in the body, while in bats the immune system is always sending out a fight response which maybe what is keeping the virus in check.
They fly over long distances
Another theory is that since bats fly over long distances, their metabolic rate is quite high, as are their body temperatures, twice as high of birds. A research team led by Dr Thomas O’Shea of USGS Fort Collins Science Centre in the United States published a report in 2005 about this phenomenon.
“If the elevated metabolic rates and body temperatures accompanying flight facilitate activation of the immune system of bats on a daily cycle, then flight could be the ultimate explanatory variable for the evolution of viral infections without overt signs of illness in bats”, the report says. Basically the high body temperatures and metabolic rates ensure that immune responses to infection take longer to assimilate in bats, when compared to other mammals like mice or humans.
The high fever-like temperature also has favourable immune responses in mammals, which is constant during the time bats fly. Thus, their body is constantly immune, which in turn may also make viruses stronger as they reside in this perfect flying reservoir.