This is a virus that affects sheep herds causing chronic interstitial pneumonia, mastitis and neurological disorders. The study confirmed that horizontal transmission (direct contact between infected animals) of the virus is the most likely path of infection, rather than vertical transmission (from infected suckling milk or colostrum). Since there is no current effective treatment against this virus, the study demonstrated the importance of control methods aimed at reducing direct contact between infected animals.
Visna/maedi virus (VMV) causes a slow and fatal disease in sheep. It is mainly associated with clinical symptoms of chronic interstitial pneumonia, but it is also linked to other clinical conditions such as arthritis, encephalitis and mastitis. The disease is widespread throughout the world and the infection has considerable historical importance. It gave rise to the discovery of slow viral infections and pointed to the difficulty in controlling them in international trade. Moreover, while it does not affect humans, it was first described within the group in which AIDS was subsequently included.
The studies carried out by Dr. Iratxe Leginagoikoa, PhD, have shown that VMV horizontal transmission is more important than vertical transmission. Thereby the need to detect infected animals rapidly, rearing the flocks in good conditions of space and ventilation and, reduce contact with infected animals in order to avoid its propagation within the flock.
Intensive stabling increases infection
During the first part of the study transmission of VMV through direct contact was monitored amongst 190 one-year-old lambs of the Latxa breed. Animals were divided into two groups: one subjected to horizontal infection at high pressure and the other kept in a well-aired enclosure without contact with infected animals. In the first group infection increased by 57 %, while no infection was detected in the second group.
This first study identified a close association between flock handling and stabling conditions and transmission of the infection by direct contact. To test this hypothesis, Neiker-Tecnalia scientists designed a three-year research programme with 38 ovine flocks under three different husbandry systems in Spain: the Latxa dairy sheep under semi-intensive farming (the Basque Country), the Assaf dairy sheep produced in Castile-León under intensive farming and the extensive farming of crossed Manchego sheep in Castile-La Mancha. The study determined a seroprevalence (percentage of animals having antibodies against visna maedi virus) of 25 % of the semi-intensive flocks of the Basque Country, 77 % amongst the intensive flocks of Castile-Leon and 5 % amongst the extensively grazed flocks of Castile-La Mancha.
These results showed that whereas visna/maedi virus is not very prevalent amongst grazing flocks in extensive farming it is potentially highly prevalent amongst flocks in intensive sytem, where animals spend longer periods indoors thus increasing chances of virus transmission. Therefore, it is concluded that vertical transmission, which was once thought to be the only relevant transmission route, is, indeed, incapable by itself of maintaining VMV in the population. From this conclusion it can be gathered that eliminating the virus from flocks in extensive production systems could be simple and economically achieved by culling infected animals and feeding replacement lambs with virus-free colostrum and milk.
The possible link between new yearly acquired infections with variables indicative of horizontal infection pressure and maternal inheritance was also investigated. Results showed that lambs born to seropositive mothers became infected with greater frequency than the offspring of seronegative mothers.
ELISA and PCR
The results of this work also indicated that the analysis of blood using commercial ELISA methods for the detection of antibodies enables the reliable detection of VMV infection in adult sheep. However, these methods were not so effective in young animals or when viral load was low. In these cases, PCR, more expensive but able to detect the presence of the virus directly, would enable the detection of those infected animals which had not yet developed sufficient antibodies against the virus.
To verify this hypothesis, serum, blood clots and white blood cells were collected from 25 seronegative animals and 25 seropositive animals, confirmed as such with the ELISA technique. PCR detected the presence of the virus in up to 25 % of the negative ELISA samples and in all the positive ones, demonstrating the better performance of PCR. Especially noteworthy was the ability to detect VMV in blood clots, this representing the first report on the use of PCR protocols for VMV detection based on blood clot DNA. In addition to the higher sensitivity of the test, this novel protocol avoids the need for purifying leucocytes before DNA extraction, and opens a new door for the diagnosis of the infection through blood clot samples.