Bovine lymphocyte antigen class II alleles as risk factors for high somatic cell counts in milk of lactating dairy cows.

Lactating Holstein cows (n = 1100) from 93 dairy herds in Iowa, Wisconsin, Minnesota, and Illinois were genotyped at the bovine lymphocyte antigen DRB3.2 locus by a genotyping system that used polymerase chain reaction and restriction fragment length polymorphisms. Milk samples were obtained after routine processing at a Dairy Herd Improvement Association facility and returned to the National Animal Disease Center for DNA extraction. Somatic cells were used to classify cows that had acutely elevated SCC (one test of SCC > 500,000; group 1), or chronically elevated SCC (three consecutive tests of SCC > 300,000 or two consecutive tests of > 500,000; group 2), or that were eligible as controls. For each cow in groups 1 and 2, controls were selected that were matched for breed, lactation, herd, and days in lactation (+/-60 d). A conditional model for stepwise logistic regression was used to determine the relative odds for the 10 alleles with a frequency > 3%. No significant associations were observed when the 292 cows in group 2 were compared with their 292 controls. Allele *16 was associated with an increased risk of disease for cows classified with an acute SCC (258 cases and 258 controls). This study has identified DRB3.2*16 as a potential risk factor for acute intramammary infection and established the use of Dairy Herd Improvement Association milk samples as a source of DNA that is useful for genetic epidemiologic studies.