In our search for emerging zoonotic viral diseases, perhaps one of the most difficult questions has been where to look.
It seems logical that, if you are concerned with animal viruses that are spilling over into human populations, it would probably be best to find sites where animals and people live in close proximity.
But that doesn’t really limit you in a country like Bangladesh: people live in rural thatched huts right below bat roosts and goats and cattle rest in the same room as their owners in Dhaka.
There are hundreds of thousands of places to look, but the resources exist to investigate only a few of them.
How do you focus in on the highest risk areas? The answer that we have settled on is to look where we already know that viral spill over occurs.
And Bangladesh has what could become a classic model of animal to human spill over: Nipah virus. From a medical standpoint, Nipah is terrifying. Fatality rates reported from outbreak investigations stand at around 80%. Even worse, the virus, once it infects an individual, can be spread person to person.
Studies link Nipah virus with Indian flying foxes (Pteropus giganteus), a large bat species that is common in Bangladesh, living in close association with humans and feeding on cultivated fruit.
Epidemiologic studies of Nipah outbreaks in Bangladesh have linked human disease with the consumption of raw date palm sap. Bats feed from the pots where sap is collected, contaminating the juice with feces, urine, and saliva. Drinking contaminated sap can lead to oubreaks of the deadly disaese.
But is Nipah the only virus that is spilling over in this close association between humans and bats?
We are guessing that there may be other viruses using the same pathways.
Following this hypothesis, we have narrowed this part of our search to selected villages throughout north western parts of the country where behaviors that expose villagers to Nipah virus are most common.
Of course this does restrict us to bats as the main animal culprit in the spill over event. But this too may be a good use of resources: bats harbor a lot of viruses. Besides Nipah, bats have been implicated in many human diseases including rabies, lyssaviruses, Hendra virus, SARS-coronavirus-like virus, Menangle virus, and Tioman virus. In addition, they harbor large numbers of viruses which have unknown, at least at this time, significance to human health.
Although this all makes sense in theory, it's hard to say if our assumptions are close to reality. The only way to figure that out is to get out on the ground and look. We just hope that we are searching in the right haystack.