The Immunology of Bats

A few years ago I suffered a needle-stick injury at work. The needle was attached to a syringe containing Nipah virus. I spent the next two weeks alone in a quarantine unit, watching DVDs and checking my temperature regularly. My food was left at the door and I was not allowed within ten metres of another person. My enforced holiday ended without incident which is fortunate as there’s no effective treatment for Nipah virus encephalitis.

Nipah virus is a relatively new discovery. It was discovered during an outbreak in Malaysian pig farms in 1999. A mysterious illness began killing the pigs and soon spread to the farm workers. By the time the epidemic was over, 257 people had been infected, 105 were dead and a million pigs had been slaughtered to control the spread. Further outbreaks have occurred sporadically since then, mostly in Bangladesh, with fatality rates of up to 90%.

A related pathogen called Hendra virus is present in Australia. Since its discovery in 1994, numerous cases in horses and humans have occurred along the east coast of Australia. So far, infection of humans has only occurred after exposure to infected horses.

Hendra and Nipah virus both come from the same source: bats. Fruit bats in Asia and Australia act as reservoirs for the virus. Spillover into other species occurs when food or water is contaminated with bat urine or saliva. A number of the outbreaks in Bangladesh have been linked to uncooked fruit or date palm juice. Date palm juice is the sap of the date palm and is collected overnight in pots. Fuit bats feeding in the trees at night can contaminate the juice which is drunk fresh the following morning before it begins to ferment.

It’s not known how bats are able to harbour and circulate these viruses without falling victim themselves. And the henipaviruses are not unique. Bats are reservoirs for a large number of pathogenic viruses including SARS coronavirus, Ebola virus, Marburg virus and rabies virus as well as a host of lesser known viruses such as Mokola virus and Duvenhage virus that emerge from time to time, claim a small number of victims and disappear again.

The mobility of bats combined with their habit of roosting in very large, closely packed groups may have contributed to their resistance to viruses. A long evolutionary history in such virus-friendly conditions may have selected for an immune system that is uniquely capable of harbouring pathogens without succumbing to them.

As human populations encroach further into bat habitats, virus spillover events are likely to become more frequent and demands for culling of bats louder. By protecting themselves from the effects of viruses, bats may have inadvertently  made themselves the target of an even bigger threat.

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