Polymerase chain reaction (PCR) is nowadays the technique of choice for the diagnosis of infections by Feline herpesvirus (FHV); this technique is based on a molecular biology method that, compared to the traditional methods, has greater sensitivity and specificity. PCR is commonly used to detect FHV-DNA in conjunctival, corneal, and oropharangeal samples and in corneal sequestra and blood.
The advantages of PCR analysis are:
- the possibility of identifying the target genome even when there is only a small amount of virus in the initial samples;
- easier storage and transport of the samples to the laboratory;
- faster aetiological diagnosis.
However, since even small amounts of virus can be identified by PCR analysis, it is not possible to determine a priori whether the presence of the virus is really associated with the clinical disease being investigated. Positive results must, therefore, be interpreted with care. If a cat shows the characteristic clinical signs of FHV infection, a positive test strongly supports the diagnosis. However, given that almost all sick cats can be latent carriers and intermittent shedders of the virus, a positive result may be incidental, related to stress-induced reactivation of the virus.
PCR can also detect the virus in conjunctival scrapings and/or tonsils in non-productive infections; its diagnostic value is, therefore, low and should be assessed in the light of the general condition of the animal and also the type of population tested (cats from catteries are more frequently positive than are domestic cats).
Finally, PCR can also be positive due to the FHV-DNA present in live attenuated vaccines, although it is unclear which vaccine strains can be detected in recently vaccinated animals. It is, therefore, important to collect information on the most recent vaccinations that the animal has undergone in order to be able to interpret the results of the test correctly.
Real-time quantitative PCR, which can be used to measure the concentration of the virus, can provide some additional useful information; a high level of viral elimination in nasal or conjunctival secretions is suggestive of active replication and, therefore, the involvement of FHV in the clinical signs. A low number of DNA copies in the corneal scrapings often indicates a latent infection.