Immunisation research challenges the maedi visna virus
The maedi visna virus, MVV infects the respiratory system, udder and blood forming tissues in sheep and goats. This in turn causes respiratory problems and loss of body condition. It is passed on in utero during pregnancy and in colostrum to the young suckling animal. Horizontal transmission occurs through the respiratory route, particularly in enclosed barns. The incubation period is a lengthy three to four years. Eradication and control of the disease therefore requires an effective vaccination strategy. As part of this, a reliable test for the detection of the infection within herds must be available. In response to this demand, members of MVAC developed an assay based on real-time polymerase chain reaction (PCR) to determine the presence and levels of infection of the virus. To accomplish this, the team foused on two genes, gag and pol. Gag codes for the viral matrix, proteins on the inner surface of the envelope, the capsid and nucleoproteins. Pol is crucial in that it codes for reverse transcriptase which converts the viral RNA into DNA that is then incorporated into the host nuclear material. Primers, starting points for transcription and dual-labelled probes were designed for gag and pol for strain EV1 of the MVV virus. When the sensitivity of both gene detection protocols was evaluated, it was found that the gag assay had superior sensitivity. The test can be used to detect pro-virus DNA, that is, DNA that has been integrated into the host. It can also determine the presence or absence of mRNA (messenger RNA) in blood and tissues. The protocols developed could form the basis of field tests although the test was specifically designed for strain EV1. Further work and field trials would therefore be required for the test to be used for routine diagnosis.
