Serological ELISA results are conditioned by individual immune response in ovine maedi visna

The marked differences in sensitivity existing among maedi visna ELISA tests is a striking issue in the control of the disease, which so far have mainly been related to the different circulating viral strain or to the particular test used. The aim of this work is to discern whether or not the ELISA results could also be associated to different histological lesion patterns and therefore conditioned by individual immune response. Fifty infected animals and eight negative controls were used and histological, inmunohistochemical, PCR and serological studies were performed. Histological patterns were classified based on previous described immunophenotypical criteria: a lymphocytic pattern, characterized by a clear predominance of T cells, especially CD8+ T cells (n = 19), and a histiocytic pattern, with a high quantity of macrophages mixed with B cells (n = 23). A third mixed pattern characterized by a mixed inflammatory infiltration was observed (n = 8), predominantly in animals with minimal lesions with no clinical signs being observed (75%). An association between these lesion patterns and the ELISA optic density values exists (p < 0.001). Sheep with a histiocytic pattern (n = 21) showed higher titers of antibodies compared to sheep with lymphocytic pattern (n = 18), where values were much lower or even negative. Animals with histiocytic pattern are easily recognizable using the ELISA test, while sheep showing lymphocytic lesion pattern could go unnoticed in the flock as serological false negative animals, being a likely remaining source of infection. Animals with mixed pattern showed mixed values, and despite showing only minimal lesions, they are also carriers of the virus and can be easily underdiagnosed.