Replies

Joseph P Dudley, PhD
Senior Analyst, Biosecurity & Agriculture

Although SARS obviously didn't originate in rats, the current evidence seems to indicate that what we are witnessing and tracking at this time is a significant evolutionary event involving the process of the tentative early stages of the establishment ["adaptation"] of a newly-mutated viral pathogen (the SARS coronavirus) in a new stable reservoir host (rats, cats, humans, etc.).

If we are fortunate, the SARS virus, in its current form, may not be able to establish itself as a self-sustaining zoonotic pathogen within rat populations, and the disease may only occur in rats as a transient and dead-end secondary host (in which case, the closing of wildlife street markets -- for raccoons, dogs, palm civets, etc. -- could successfully prevent subsequent outbreaks). The current data do suggest that this may in fact be the case at present.

It cannot be ruled out that the hypothesized role of rats in the Amoy Gardens outbreak could have involved mechanical transmission rather than urine/fecal contamination (as might have occurred had a rat crawled/swam out of a SARS-contaminated toilet and then tried -- successfully or unsuccessfully -- to exit the toilet room by means of the ventilation fan opening or by means of a ventilation shaft).

It seems possible, however, that if the intensive rat-eradication efforts undertaken at the sites in Guangzhou or Hong Kong, where SARS outbreaks occurred, were, in fact, successful, or nearly so, we could still have a possibility of latent SARS infections circulating in such a very small number of individual rats that we would not expect the disease to have revealed its presence in urban rat populations at this early date; at least until the next time there is a recurrence of the reported scenario in Guangzhou this year, in which several rats infected with SARS were trapped in an apartment of an isolated SARS victim who has no known contacts with any of the suspected wildlife hosts for the SARS virus.

For all of these reasons, it is extremely important for any and all potential SARS reservoirs and transmission mechanisms to be thoroughly, and exhaustively, investigated. We must also recognize that what is true for the SARS virus today may not necessarily be true tomorrow, as demonstrated over the past year with the avian influenza A (H5N1) virus.

Demonstration of past recombination events in the SARS-CoV lineage indicates this virus has a clear potential for rapid unpredictable change that makes this pathogen an extremely challenging adversary for drug and vaccine development well as for public health management