On 20 April, Bob Martin asked about why alternative means can't be found to
detect rabies without killing an animal.  He noted that the rabies shedding
studies conducted so far suggest that rabies has been detected in the
salivary gland at least in one animal and wondered if a more sensitive test
(PCR) could be used to test salivary glands rather than brain tissue.
 
> How can it be that the University crowd (using PCR I think) can detect
>the rabies virus in the saliva, yet our fearless political leaders have
>to test the brain tissues of sacrificed animals.  <...snip...> My gut
>feeling is that the PCR process takes too long, or costs too much...
 
I don't know if I count as a higher authority, but I've done PCR and work
with people who use it.  PCR doesn't take all that long.  Detecting RNA adds
a few steps (rabies is an RNA virus), thus, more time and money, but the
results could be had within a day or two.  That would be plenty of time to
decide if a human bitten by a ferret needed rabies treatment.  The cost
wouldn't be totally unreasonable.  In North Carolina, the state contracts
with Roche Biomedical to do DNA fingerprinting for paternity suits which is
far cheaper than if an individual requests such a test.  A similar deal
could be worked out for PCR.
 
In this case, cost and speed aren't the issue.  Bob is right that PCR
(polymerase chain reaction) is exquisitely sensitive for detecting DNA or
RNA.  I have found papers studying this, but it turns out that PCR does not
work well.  The reason: rabies doesn't exist throughout the body.  It's only
in the nerves.  It travels from the site of a bite, up the nerves and into
the brain where it reproduces in the white matter.  After reaching the
brain, the virus can travel down the efferent nerves (those that transmit
impulses away from the brain) to other organs.  Normally it doesn't get far
before the animal dies, but the salivary glands aren't far and this is where
the virus can continue its "life" cycle by being transmitted through saliva
to another animal.
 
The shedding studies so far suggest that ferrets may be a dead- end host for
rabies and that infected ferrets may die of the virus before it ever gets to
the salivary gland.  If that is verified by other studies, it's good news,
because it means that there would be a very low risk of ever transmitting
the virus through a ferret bite.  (With viruses, never say there's a zero
risk.) However, if *anyone* can claim that a ferret has acted in any way
that suggests it is infected, the health authorities can call for it to be
tested for rabies.
 
Here's the unfortunate catch--If the virus never reaches the salivary glands
of most infected ferrets, then neither PCR nor any other test will detect it
there.  An examination of brain tissue is still necessary to detect the
virus.  Thus, PCR will not save any ferrets' lives for the moment.  The best
strategy is to encourage the additional shedding studies needed to convince
public health officials that following a bite, a quarantine is sufficient to
rule out rabies.
[Posted in FML issue 1547]