Genetic variation in paratuberculosis in dairy populations

Paratuberculosis, also known as Johne’s disease, is caused by oral uptake of Mycobacterium avium subspecies paratuberculosis (MAP) from the environment. MAP causes granulomatous lesions in the distal part of the ileum in domestic and wild ruminants.Ileal lesions limit sufficient nutrient uptake leading to weight loss and additionally, in animals used for dairy production, infection with paratuberculosis leads to decreased milk production and fertility. In the Dutch Holstein-Friesian dairy cattle population, prevalence of paratuberculosis based on a positive ELISA test in milk was 46.7% on the herd level and 2.4% on the animal level in 2008. Prevalence of paratuberculosis in dairy goat herds is unknown but suspected to be higher than in cattle herds based on clinical and routine pathological observations.The aim of this thesis is to contribute to control of paratuberculosis by investigating genetic variation in the pathogen and studying genetic variation in host susceptibility. Genetic variation of the pathogen was investigated within and between herds in the Netherlands. For host susceptibility to paratuberculosis, genetic variation and heritability were estimated for dairy cattle and dairy goats. For dairy cattle, results from the quantitative genetic analysis were used to perform a genome-wide association study to find chromosomal regions associated with susceptibility to disease. Finally, a genetic-epidemiological model was used to study the effect of genetic selection for increased resistance to paratuberculosis on the prevalence of infection in the dairy cattle population