New causes of mass mortality in modern bats
By reviewing the literature on causes of mass mortality in bats from 1790-2015, we found changes in the reasons that bats are dying in large numbers. While in the past, intentional persecution (killing, poisoning etc.) was a big factor, collisions with wind turbines and the disease white-nose syndrome in North America now lead the reported causes of mass death in bats. Interestingly, we found very little evidence for other viral or bacterial diseases causing large die-offs.
The absence of viral or bacterial infectious diseases associated with die offs is somewhat surprising given that bats often live in dense aggregations which should be ideal for disease spread. So, is disease-induced mortality really uncommon in bats? I think there are a few possibilities. First, perhaps many bat infections occur endemically at low prevalence rather than in epidemics that cause large scale die offs? If that were the case, bats could be dying from viral or bacterial infections, but since deaths occur over months or years rather than days, we fail to take notice. Rabies virus is a good example there – no die offs, but individuals certainly die of infection. On the other hand, could bats be somehow “special” in not actually showing disease at the individual-host level? This is commonly argued for other viruses like Nipah, Hendra and Ebola, where it is suggested that exceptional immune systems or ancient co-evolved relationships with viruses could protect bats at the individual level in a way that is somehow different from other host species.
Teasing apart these explanations requires a lot of work. We need long term monitoring to look at sublethal effects of infections on bats and longer term effects of infection on lifespan and we need tools to be able to actually measure the health of these bats beyond very crude measures of alive, dead or nearly dead in the field. We need experimental infection studies that replicate natural conditions of exposure and infection. All of these things are costly, but necessary if we are to fully understand bat mortality and the relationship between bats and their natural pathogens.
Check out the USGS Press release for this paper here