The Biology of the Goat
Testing for the CAE virus with PCR
PCR or the Polymerase Chain Reaction test is different from ELISA and AGID in that instead of searching for antibodies to the virus, this test looks for the viral DNA. Small ruminant lentiviruses, like all retroviruses, are unique in that they insert their genome into the DNA of the cell it infects. Called the provirus, integrated viral DNA is very small compared with the long strands of cellular DNA. The PCR technique is able to amplify just the viral DNA through a series of chain reactions into many copies which can be further analyzed.DNA polymerase is an enzyme found in cells that catalyzes the synthesis of a new strand of DNA from a single strand DNA template. In the cell this is how DNA is replicated when the cell decides to divide. The double strand DNA opens up and new DNA is created from the separated strands with the help of DNA polymerase. Scientists have found a way to use a DNA polymerase that is stable at high temperatures to make multiple copies of a small segment of DNA. This process is quite simple and can be done in one test tube.
The test requires that a portion of the DNA sequence of the virus is known so that an appropriate primer can be made. A primer is very short section of nucleic acids that helps start DNA replication. DNA polymerase can only add on to an existing strand of DNA so the primer gives it something to work with. Primers must exactly match the end points of the genetic material to be copied. The illustration shows how multiple copies of the DNA segment of CAE is synthesized. Once a large quantity is obtained it can be further analyzed.
When the PCR test was first developed it was initially thought that it would be a highly sensitive way to look for the presence of small ruminant lentiviruses. Unfortunately, due to the low level of infection of CAEV and MVV and more importantly to the high degree of genetic variablity of lentiviruses PCR tends to be less sensitive than ELISA. It also proves to be difficult to construct all the possible primers that would work with all the variants of small ruminant lentiviruses. The PCR test can detect early infection before seroconversion takes place so has some benefit when combined with ELISA.