Background and Aim: Bacterial and viral infections affect the welfare of animals and lead to large economic losses in dairy cattle breeding due to decreased productive indicators and increased culling rates. In modern dairy farming, farmers are looking for effective solutions to prevent and minimize infectious disease risks. To this end, the most relevant study field is the search for gene sites that impact production and health. This study aimed to determine the nature of the distribution of the relative frequencies of alleles and genotypes of polymorphic prolactin (PRL) and nitric oxide synthase (NOS2) in Holstein cows and identify the relationship of these genes with resistance to mastitis and bovine leukemia.

Materials and Methods: For this study, we chose cows because infectious diseases affect the amount of lactation and milk quality. Holstein cattle with mastitis and bovine leukemia were selected. Animal genotypes were determined by restriction fragment length polymorphism (RFLP) of polymerase chain reaction (PCR) products. The results were analyzed using a nonparametric statistical method using Microsoft Excel 2010 and Statistica 6.0.

Results: In healthy animals, 94 genotypes were identified for both genes under study. For bPRL, bPRL-RsaI^AA (72) was the most common genotype and bPRL-RsaI^BB (4) the least; for NOS2, bNOS2-HinfI^AB (47) was the most common genotype and bNOS2-HinfI^AA the least (21). In animals with leukemia, 34 genotypes were identified. For PRL, bPRL-RsaI^AA (25) was the most common genotype and bPRL-RsaI^BB (2) the least; for NOS2, bNOS2-HinfI^BB (17) was the most common genotype and bNOS2-HinfI^AA (3) the least. In animals with mastitis, 67 genotypes were identified. For PRL, bPRL-RsaI^AA (43) was the most common genotype and bPRL-RsaI^BB (6) the least; for NOS2, bNOS2-HinfI^BB (31) was the most common genotype and bNOS2-HinfI^AA (7) the least. The distribution of genotypes of polymorphic bPRL and bNOS2 generally coincides, and bPRL-RsaI^BB is the most common genotype. In groups of sick animals, the number of bNOS2-HinfI^AA homozygotes was lower than that of the control group. In particular, the proportion of animals with the bNOS2-HinfI^AA genotype with bovine leukemia was 8.7% and with mastitis was 10.3% compared with 22.4% in healthy animals. These data support the possible association of the bNOS2-HinfI^AA genotype with resistance to infection. The frequency of the bPRL-RsaI^B allele was higher in groups of sick animals. This allele is associated with increased milk productivity, suggesting that highly productive animals are less resistant to the incidence of viral bovine leukemia and mastitis of bacterial etiology.

Conclusion: DNA amplification of Holstein cattle for the polymorphic regions of PRL and NOS2 using the PCR-RFLP method revealed a possible connection between the distribution of relative allele frequencies of bPRL and bNOS2 and resistance to viral and bacterial infections. Thus, in groups of sick animals, the frequency of bPRL-RsaI^BB, associated with increased milk production compared with the theoretically calculated equilibrium value was higher and the number of homozygotes bNOS2-HinfI^AA was lower than in the control group. In conclusion, animals with increased milk production were more prone to diseases, such as mastitis and bovine leukemia.

Keywords: bovine leukemia, cattle, Holstein breed, mastitis, polymerase chain reaction-restriction fragment length polymorphism, polymorphism.