Why use PCR to detect infectious diseases?
Molecular testing using PCR (Polymerase Chain Reaction) has revolutionized the way veterinarians, researchers, zoos and animal care facilities maintain animal health and obtain disease data.
The sensitivity, specificity and speed with which pathogens are identified by PCR enable diagnostic, research and monitoring possibilities unavailable using other techniques. Molecular techniques represent a highly effective tool for the diagnosis of disease in dogs, cats and other animals.
In PCR testing, nucleic acid -- DNA or RNA -- is isolated from the specimen, then amplified (after an extra reverse transcription step in the case of RNA) using highly specific primers complementary to known target pathogen sequences. References and controls are run simultaneously; internal controls monitor extraction and PCR efficiency. Amplified sequences are visualized and recorded using state-of-the-art protocols and instrumentation.
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, 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 actually 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