The global epidemiological trends of many viral diseases clearly indicate resurgence of some hitherto quiescent yet virulent pathogens and emergence of new strains that pose a challenge to medical science. The Nipah outbreak with many sero-positive cases and 13 fatalities so far in two districts of Kerala — Kozhikode and Mallapuram — has once again brought this zoonotic virus into focus.
The virus typically survives in a non-human ‘reservoir host’ such as the ‘Flying Fox’, a bat species of family pteropodidae; that is immune to its pathogenic potential, but can jump into humans through contact with infected animal or its bite, body secretions and excretions. Often the aerosol transmission spread the virus from roost sites, where large congregations of these bats engage in a grooming frenzy using their copious saliva. The virus causes acute respiratory syndrome and fatal encephalitis and the prognosis is poor as humans have low immunity.
The Kerala outbreak is perplexing because the bat samples from the epicentre referred to National Institute of Virology in Pune have tested negative, absolving these flying mammals of being viral reservoir in Kozhikode at least. None the less, mixed species roosts are common in some species of bats including the one implicated in Nipah virus transmission.
Of the 52 species of bats recorded for Western Ghats, six pteropodid species occur in the state with overlapping foraging grounds. In Goa, three species — Rousettus leschenaulti, Cynopetrus sphinx and Pteropus giganteus — forage in common areas, though at different times. When several species of bats roost or forage together, there are raised chances of inter-specific transfer of disease causing agents between bats.
Repeated outbreaks of Nipah virus or NiV have been noted in select districts of neighbouring Bangladesh almost every year, with occasional outbreaks in India. The bad news is that as of now, neither is there a vaccine nor a cure, predisposing medical and nursing staff to a risk of infection, if the mandatory quarantine protocols are compromised.
Now let’s reflect on the situation in Goa. The state health authorities while issuing necessary guidelines to all concerned to be vigilant, have assured its citizenry that there is “nothing to worry and that Goans are safe”. But this notion of safety warrants a technical discussion.
The state has bats, pigs and a vulnerable population. While much of the focus here is on consolidating and scaling up medical response to any exigency likely to be set off by NiV ‘spillover’ into the state, the fact remains that reservoiring and transmission of the virus involves species of bats. Therefore any serious discussion on NiV control cannot ignore the subject of epizootiology, or the study of occurrence and distribution of the pathogen in the triad of host, agent and environment.
Interestingly, the state committee constituted for the purpose of NiV prevention and management in Goa, comprises men of medicine and bureaucracy. While these two domains are important in implementing the prevention and control of NiV in Goa, there is clearly a dearth of expertise on zoological dimensions of the transmission.
Goa has some remarkably inviolate bat roosting sites in its caves, forests, abandoned dwellings and ancient structures. A proactive surveillance by sample-screening bats for NiV can be a pragmatic step. Bats have the power of flight and can cover long distances. Given the proximity of Kerala and Karnataka to Goa, some ‘virus reservoir bats’ can easily ‘spillover’ into Goa, just as infected travellers too can import the virus.
Unfortunately, we have not learnt lessons from the Kyasanur forest disease (KFD), another zoonotic transmission that continues to intimidate people in Goa’s hinterlands.
In conclusion, we need to have a multidisciplinary disease-ecology approach incorporating spatial relationship, population biology, behaviour of the pathogen, vector and the host within the context of a given environment to effectively mitigate the presently endemic infection from scaling up to epidemic levels. Understanding bat diversity, their ecology and behaviour has implications in deciphering transmission of zoonotic diseases including NiV, ebola, hendra and rabies.
The writer is a senior zoologist and has researched ecology and endocrinology of some Indian bat species.