SARS bats and palm civet cats

When SARS first occurred in China, epidemiologists sought the animal reservoir from which humans acquired the infection. It appeared that the reservoir might be palm civet cats which are caught for meat in parts of China and with which humans come in close contact in meat markets. However, the SARS virus is not found to be widespread in the Chinese civet cat population and so the latter is not likely to be the main animal reservoir.  In addition, civet cats have little immunity to the virus suggesting that they are not the reservoir host because they also die from the viral infection. These viruses are, however, widespread in horseshoe bats in southeast and east Asia (up to 40% of individuals in some bat colonies are infected) and once again bats turn out to be the reservoir for a human viral infection. In bats, the virus has little morbidity as would be expected of a reservoir host which has adapted to it.

Using genomic sequences of the SARS coronavirus found in bats, civet cats and humans, scientists were able to put together a phylogenetic tree and a likely sequence of events. The SARS coronaviruses that infect human and civet cats are very closely related (they are both in the same clade) and are more distantly related to the bat SARS virus, although it is 92% similar to the human virus. It is likely that some civet cats were infected by horseshoe bats and then the virus spread to humans in meat markets. In this case, civet cats act as an “amplifier” host because, unlike in bats which control the virus and thus have a very low viral load, civet cats produce a high viremia and can readily pass the virus to another animal.

So why do bats so often harbor virus that skip to the human population? Bats form large colonies in which close contact between animals is inevitable and the virus may pass from bat to bat in saliva during mutual grooming. In the bats, natural selection results in evolutionary changes, thus giving rise to a virus that can infect other mammals. The bats and civets may come in contact in the wild or when caught by humans and caged in markets. The resulting high viremia leads to spread to other animals including humans. Thus, as with other human infections (e.g. Ebola and Nipah), SARS entered the human population because of behavior changes, that is capturing animals, including bats, and bringing them into close contact.

SEE:

http://evolution.berkeley.edu/evolibrary/news/060101_batsars

http://news.bbc.co.uk/2/hi/science/nature/4291386.stm

http://link.springer.com/content/pdf/bfm%3A978-3-642-03683-5%2F1.pdf