Study shows how KHV persists

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Viral propagation and transcription are turned off when Koi Herpes Virus cultures are kept outside the permissive temperature, says new research.

Koi Herpes Virus (KHV), which is now formally known as Cyprinid herpesvirus 3 (CyHV-3), affects fish only when the water temperature is between 18-28°C, and now scientists know why.

Virologists from the the Hebrew University Hadassah Medical School have just reported that viral propagation and transcription (the process through which genetic sequences are copied by polymerases) stops when the cell cultures are kept at temperatures outside the 18-28°C range.

The paper, which is due to be published in the May issue of the Journal of Virology, says that when cell cultures were returned to the permissive temperature range, transcription of viral genes was reactivated.

The sequence for reactivation of viral genes differs to that seen when naiive cells (those previously unexposed to the virus) are first infected.

The authors said: "Our results show that CyHV-3 persists in cultured cells maintained at the nonpermissive temperature and suggest that viruses could persist for long periods in the fish body, enabling a new burst of infection upon a shift to a permissive temperature.

"The disease afflicts fish in the transient seasons, when the water temperature is 18 to 28 degrees C, conditions which permit virus propagation in cultured cells. Here we report that infectious virus is preserved in cultured cells maintained for 30 days at 30 degrees C.

"CyHV-3-infected vacuolated cells with deformed morphology converted to normal, and plaques disappeared following shifting up of the temperature and reappeared after transfer to the permissive temperature."

Since the virus typically only shows its symptoms when the water is in the permissive temperature, the now recommended method for quarantining carp is to keep them at this elevated temperature range for a 2-3 week period in order to attempt to reactive any virus present.

For more information see the paper: Dishon A, Davidovich M, Ilouze M, Kotler M (2007) - Persistence of cyprinid herpesvirus 3 in infected cultured carp cells. J Virol. 2007 May;81(9):4828-36.