Why use PCR to detect infectious diseases?
Due to their sensitivity, specificity and rapid turnaround, molecular techniques provide a highly effective tool for the diagnosis and monitoring of disease in dogs, cats, horses and other animals.
In PCR testing, nucleic acid -- DNA or RNA – is extracted from the specimen. It is then amplified using specific primers for known target pathogen sequences. Amplified sequences are identified with real time detection allowing a semi-quantitative indication of the organism load in the original sample. PCR testing performed at Vetnostics utilises multiplex tandem technology which has increased sensitivity compared to some other PCR methods.
In contrast to serological testing where antibodies to an organism are identified, molecular assays detect the genetic material of the pathogen itself -- a direct indication of presence of the pathogen in the sample. Early infections and infections with very high organism load (including some wet form FIP cases) may have negative serology but positive PCR detection. This is particularly important in infections with rapid progression such as Leptospirosis, where PCR detection may be possible from 3 days post infection, but infected animals may take 2 weeks to seroconvert.
How is molecular, PCR-based testing complementary to other kinds of testing?
- Cross-reactivity reduces the specificity of many serological tests - a positive result can be caused by organisms other than the target. Molecular assays, on the other hand, are highly specific because they detect a genetic sequence unique to the target pathogen. Even closely related pathogen strains can often be distinguished.
- The influence of some sources of false positives is reduced in molecular testing. This is because, while other testing methods detect antibodies to pathogens which may not still be present, molecular assays detect the genetic material of the pathogen itself -- a direct indication of presence of the pathogen in the sample.
- Latent or early infections can often be detected by PCR before disease symptoms are apparent. Detection is not dependent on elevated antibody levels.
- Animals with high titres of some viruses can exhibit very low antibody levels as their immune systems are challenged by an infection. This can result in false negative diagnoses if antibody testing alone is used.
- PCR's exquisite sensitivity allows detection of a target pathogen in a sample even if present in very low titres, and even from very small sample volumes.
