Friday, July 14, 2006

Happy hour: Sacred cows and sympathetic squirrels

The hour is not really that happy, since I’m still very sad after the tragic terror attacks in Mumbai. I’m even more saddened by the rather ineffective leadership we have, and the absolute lack of even the concept of bipartisan efforts, or an understanding of national interest or national security by India’s pathetic leaders.

But life must go on, and as Mumbai’s citizens have fearlessly shown, it will. And there’s no better way to take your mind of tragedy than by exploring Science’s fascinating problems, revelations and possibilities.

And that leads me to today’s happy hour topic. I came across an absolutely fascinating essay in PLoS Medicine. How much did you think human disease risk is influenced by biological diversity? Did you ever think that the cow serenely obstructing traffic in a Delhi main road could serve a greater cause for humanity? Or the poor squirrel that was yesterday’s garden destroyer and today’s road kill could actually be helping humanity?

If this were a trivia quiz, and I asked you “what is common between cows and squirrels when it comes to infectious diseases in humans?” what would you say?


Ah, but science tells us the answer.

Both animals receive bites from insect vectors that would otherwise have bitten us, and infected us with pathogens, thus breaking a chain of pathogen transmission.

Still confused?

Think malaria. If the damn mosquito carrying the dreaded plasmodium, bit a holy cow instead of sleeping me, I’m not going to get malaria. Squirrels are bitten by ticks (that are perfectly capable of biting humans) that carry infectious spirochetes which cause lyme disease. And cows don’t get malaria easily, just as squirrels don’t get lyme disease easily. So, in effect the rate of transmission of both diseases to humans decrease.

And geeky scientists even have a name for such a process.

Zooprophylaxis (as defined by Allan Saul,
Malaria Journal 2003, 2:32) is “the diversion of disease carrying insects from humans to animals, may reduce transmission of diseases such as malaria”

What will they think of next?
And this is not just for malaria or lyme disease, but our friendly neighborhood animals seem to be lending a hand with various other diseases. Tick borne encephalitis (TBE) circulates among yellow-necked mice. If an infectious tick bites a human, the nasty disease is transmitted. So, you’d think if there are a lot of mice, then there might be more infection of humans. But here’s a quote from that paper:

”When the density of mice is high, then the probability of two ticks feeding on the same host at the same time is very small, so transmission declines to levels where the pathogen cannot persist. In contrast, when the mice are at very low density, not enough infectious ticks are produced for the disease to persist.”.

Similar stuff happens with house sparrows and west Nile virus. And a host of other examples.

And ecologists niftily term this a “dilution effect”, where increased host diversity results in an increase of the proportion of the bites that are “wasted”.


Cows rule. A little problem though. As that paper states, the “by-products of cows” provide breeding material for our dear bloodsuckers, the mosquitoes. Not good that.

Lovely read, and fascinating paper. You can read all about it here.

Postscript: My only “beef” with the authors is this statement “…they have been considered sacred since the Aryans invaded in the 2nd century, B.C…..”. Which history books have they been reading? Most historians are questioning that old theory.


Anonymous said...

Well, I knew biodiversity was good to deal with environemntal stress. And I have come across zooprophyllaxis during my studies. But it's fascinating to put them togther and see the benefit to human health !

Sarat said...

Really fascinating article! If I understood it right, when at some point in the future, we find out the 'pathogen hosting capabilities' of all the animals commonly found in human habitats, we could potentially customize our habitat by getting rid of the 'bad' species. Ofcourse, the customizations might differ in different geographical we may ultimately end up with a Goldilocks kind of situation...not too much, not too less, just the right amount of biodiversity!
Obviously, this has disaster written all over it. But you never know what George Bush might do with this data.

Sunil said...

Ash....i thought this article was really interesting though like you I knew about diversity and environmental stress. Fascinating.

Sarat.....we'll never find out everything about all animals. Heck, we still don't know all the species of insects, reptiles, birds, fishes or even mammals. Thats how little we know. So, it's often better that we don't try to "manage" biodiversity, though a lot of people naively assume we can. But we're now understanding how biodiversity can not just be important for environmental stress, but also for human health, or even economic prosperity. That'll take some years to creep down the mainstream.

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