November 3 , 2005
Bats ‘n Men Begin…
Batman Begins was all the rage last week. Number one in home DVD sales, I'm told. At least now we know the answer to “Why does he do the things that he does?”
But far more salient was the Perspective in Science October 28 by Andrew Dobson entitled “What Links Bats to Emerging Infectious Diseases.”
The story of SARS (severe acute respiratory syndrome) crossing to humans from another animal source, namely bats through civet cats and other mammals, is fascinating and illuminating.
It is interesting not only because of SARS but because of the recent paranoic panic concerning Avian Flu. How could you? How could the government allow a flu virus to mutate and move from birds to humans? Who's responsible for this? I want to know right now!!
Some aspects of recent human behavior that I find extremely entertaining include the following commonly held beliefs:
- Humans somehow exist outside of the environment and therefore control it.
- Humans are the exclusive despoiler of our world.
- If the environment changes it must be humans who did it. (and I thought physicians were egocentric)
- Environmental evolution including the loss of species is surprising and negative.
- It is shocking when a new mechanism of disease presents itself.
Instead of glorying in the wonderment of our ever changing world and in the wonderment of new ways of discovering knowledge about our challenges new and old, we recoil, as if new knowledge presents the specter that we can never know it all, damn it, if it keeps changing all the time!!!
Stasism vs. Dynamism. Most of us hate dynamism and crave stasism. Just hold the ball still and I'll hit it. I can't stand it when you keep moving all the time.
The Perspective is interesting in that in addition to pointing out this new information about bats and SARS the author proposes directional leads for “control” of the SARS virus in bats and the spread between bats and humans.
Here's the interesting news:
“Three species of horseshoe bats (Rhinolophus spp.) have now been officially recorded as the natural reservoir host of the coronavirus that causes severe acute respiratory syndrome (SARS). The emergence of this pathogen (SARS-CoV) in southern China in 2002-2003 almost brought the burgeoning economy of Southeast Asia to its knees. Bats are now known to be natural reservoir hosts to several other new emergent disease pathogens: Nipah and Hendra viruses and potentially Ebola and Marburg viruses. They are also reservoirs to "older" and more well-known pathogens, such as rabies virus, which frequently resurge into human populations or domestic livestock. Fieldwork on SARS illustrates not only the crucial role that conservation organizations play in frontline research on emergent diseases, but also the shortcomings in our understanding of the etiology of these diseases.”
Bats seldom bite humans, vampire stories aside. So how do bats transmit diseases to humans and other mammals? It's a fascinating unfolding story. Think “fruit” here…”many bats are frugivorous--that is, they meet their energy requirements by ingesting fruits. But instead of swallowing them, they chew them to extract the sugars and higher energy components, and then spit out the partially digested fruits, which drop to the ground. Other animal species can ingest these fruit remnants and may consequently become infected with virus particles in residual bat saliva.”
And if that weren't enough, in Bangladesh humans are complicit as this interesting set of observations about the Nipah virus illustrates: “In Bangladesh , the Nipah virus has been shown to be transmitted directly from bats to humans. There, during the fruiting season, young boys climb trees to pick fruit. They frequently add fruit that is partially chewed by bats to their collections, which they then sell to the local salesmen. The fruit is pulped to produce a drink that is sold in neighboring villages. The Nipah outbreaks there often follow the trails of these bicycle-borne salesmen.”
How do we control such diseases threats to humans? The Perspective suggest the following possiblities: “This means that control of the disease has to focus on either controlling its abundance in its reservoir, preventing its spillover between hosts, or rapidly reducing its spread once it has infected humans or domestic livestock. This creates a dilemma for both public health and conservation biology: Should we attempt to control potentially emergent pathogens by focusing on their reservoir hosts, or should we try and prevent the spillover events that allow the pathogen to spread in a new population? A third option is to develop a vaccine to protect hosts in the spillover population. Unfortunately, because spillover is likely to be a random event, effective protection requires that all individuals in the spillover population become protected. We have never achieved this level of coverage for well-known pathogens that have fairly safe and effective vaccines.”
Why don't bats get sick from these viruses? We don't know. Perhaps we can learn something from studying their immune systems. But apparently a whole new scientific discipline has been launched. “The scientists who revealed the bat reservoir of SARS-CoV operate within a new intellectual paradigm. They call their discipline "conservation medicine". It brings together the two areas of natural science that will be crucial to the future welfare of humans: health sciences (human, veterinary, and plant pathology) and the ecological sciences that monitor the health of populations, communities, and ecosystems.”
Conservation medicine. Live and learn. Perhaps it should be learn and live. While embracing our constantly changing environment.
