Population Distribution of TMEM154 Genotypes in Sheep Breeds and Their Association with Maedi-Visna Virus Infection in Endemically Infected Flocks in the Netherlands

The maedi-visna virus (MVV) remains endemic in many agricultural countries, including the Netherlands, causing chronic disease, animal welfare concerns, and economic losses. In addition to conventional control programs based on serological surveillance and flock management, genetic selection for reduced susceptibility using variants of the transmembrane protein 154 (TMEM154) gene may contribute to sustainable disease control. This study evaluated the distribution of TMEM154 genotypes and their association with MVV infection and clinical outcomes. A Kompetitive Allele-Specific PCR assay was used to genotype 187 sheep from eleven breeds and 150 ewes from five MV-endemic flocks. The serological status was determined by ELISA, while viral detection and characterisation were performed using q(RT)-PCR and sequencing. The breed TMEM154 genetics were 33% EE-, 39% KE- and 28% favourably KK-genotyped. Among 150 ewes, 12% were homozygous KK, with the flock prevalence ranging from 0% to 40%. The overall seroprevalence rose in consecutive years. The q(RT)-PCR results differed between a single reactor up to 20 out of 30 per flock. The seroprevalence was significantly lower in KK sheep than in EE/KE sheep in both 2025 (11% vs. 69%; p < 0.001) and 2026 (8% vs. 78%; p < 0.001). Furthermore, respiratory and other clinical signs were less frequent and less severe in KK animals. All detected viruses belonged to MVV subtype A1. These findings support selective breeding for favourable TMEM154 alleles as a complementary strategy for MV control in populations where subtype A1 predominates.