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-Len 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
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