Viruses in a vector

[Adrian.Philbey at SMTPGWY.AGRIC.NSW.GOV.AU]

          Ursula Keuper-Bennett (howzit at io.org) some time ago asked
          about detection of a suspected oncogenic virus of turtles in
          a putative piscine vector:

          >I believe that a disease is possibly being transmitted by a
          >vector.  The vector is a fish. (Actually several species of
          >fish have been observed performing "cleaning" behaviour)

          >I figure tissue, material and the viruses gets carried
          >around by the fish
                             
          >My question is this.

          >If I captured one or several of these suspected mechanical
          >vectors, would a lab be able to detect the viruses after
          >(say) 24 hours or more has passed. (Could be in the
          >dentition for example)

          If you knew what virus you were looking for, it should be
          feasible to prepare samples from the fish that would allow
          you to detect the virus if viral material was present in
          sufficient quantity. For example, viral sequences might be
          detectable by polymerase chain reaction (PCR) or viral
          particles might be detectable by electron microscopy in fish
          stored at 4oC for short periods in suitable transport
          medium, frozen at -20oC or -80oC or preserved in formalin,
          depending on the particular virus and its properties of
          environmental resilience. My opinion is that it is unlikely
          that a virus would be detected in a fish acting as a
          mechanical vector, because only a low proportion of fish
          would be carrying the virus at any sampling point and the
          quantity of virus carried by the vector would probably be
          very low. Even if you did detect virus in the fish, you
          could not be certain that the fish was acting as a
          mechanical vector, because it might be an inconsequential
          side-effect of the close contact between the fish and the
          turtles while feeding/cleaning around the turtles' eyes.

          If I am right in assuming that so far you have not
          identified the suspected virus causing the eye tumours in
          the turtles, I think it would be more productive to search
          for evidence of viral infection by examining tumour material
          directly. Electron microscopy would be a starting point.
          Attempts to isolate the virus in cell culture could also be
          done. Molecular techniques that could be used to identify
          sequences of viruses in neoplasms include PCR using primers
          specific for known oncogenic viruses or degenerate or
          consensus primers designed to amplify conserved sequences
          from known oncogenic virus groups, as well as
          representational difference analysis, differential display,
          arbitrarily primed PCR and subtractive hybridisation. A lot
          of effort is required to identify oncogenic viruses.

          Are the tumours transmissible? To determine this, it would
          be necessary to maintain a sufficient number of turtles
          under controlled conditions. The number of tumours that
          develop in unaffected turtles exposed to affected turtles
          would be compared to the number of tumours that develop in
          turtles in isolation. The presence or absence of the
          putative piscine vector could also be included in the
          experiment to incriminate the fish you suspect is acting as
          a vector. An experiment of this scale with turtles would be
          difficult to set up.

          >I know from my readings that viruses can't survive on their
          >own for any length of time.  Hmmmm. .. I guess I have
          >another question.

          >Can dead viruses be detected in such a scenario?

          PCR can be sometimes be used to detect nucleic acids in
          othrwise degraded material. The specific sequences of the
          virus must be known, however.


          Adrian W Philbey
          Veterinary Research Officer
          Elizabeth Macarthur Agricultural Institute
          Private Mail Bag 8
          Camden  NSW  2570
          Australia
          
          Telephone: 61-46-293332
          Facsimile: 61-46-293429
          email: philbea at agric.nsw.gov.au