The bovine viral diarrhea indicators in the cattle on the big dairy farms in Siberia

The Russian livestock industry, notably the dairy cattle industry, is currently facing serious changes. The number of large dairy farms, counting from up 1,500 dairy cows with an average annual milk production of about 12,000 kg, is increasing. Extensive movement of animals from multiple sources bears the risk of introduction of infectious diseases including bovine viral diarrhea (BVD), an economically significant disease caused by BVD virus (BVDV). BVDV is a member of the Pestivirus genus of the family Flaviviridae, and presented the types 1 and 2 and cytopathic or noncytopathic biotypes. Infection of not immune to the virus heifers and cows results in an unsuccessful fertilization, repeated coming in heat, and, at early stages of pregnancy, in infertility, fetal mortality, abortion and stillbirth, arising due to the dysfunction of the ovaries, uterus inflammation and direct impact on the embryo. The pathogen also causes fetal immunotolerance infection, leading to the birth of persistently infected (PI) calves becoming permanent endogenous source of the pathogen in the herd. The aim of our work was to identify the main gender and age groups of cattle at the highest risk of virus infection to be a kind of indicators for type 1 BVDV circulation on dairy complexes. This research was carried out in 2006-2014 on Holstein-Friesian cows from six large dairy farms in Siberia (Tyumen region), where the specific prevention of the disease has not been conducted. Model age and gender groups of 100-400 cows were tested for the presence of PI animals. In this, the imported animals at quarantine and during the next 5 years were epizootically surveyed with regard to health parameters, morbidity, mortality, and gynaecologic pathology including repeated coming in heat, barrenness, abortion and stillbirth. Additionally, we used serological tests and PCR. It was found that up to 8.8 % per 100 imported heifers were PI BVDV carriers at risk to produce PI calves. The first generation heifers respond to 71.4 % seroconversion to the virus within 1-3 months after artificial insemination, and dry heifers are the most susceptible to infection. These animals may be an indicator of virus circulation in the dairy herd and the risk of producing PI calves. A total of 10.6 % heifers, 8.8 % dry heifers and 5.5 % calves born from them were the PI carriers, which is higher than in cows of lactations 2 and 3. The portion of PI animals among these cows was 1.6 %, while in calves born from them it reached 3.7 %. For heifers, the virus genome was mostly detected in organs of aborted fetuses on months 4 to 6 (40.0 %) and 6 to 9 (24.0 %) of gestation, and in stillbirth calves (29.2 %). For cows, these parameters were 17.5 %, 10.3 % and 20.5 %, respectively. The frequency of viral RNA detection in the vaginal and uterine discharge amounted to 29.0 % and 13.5 %, respectively. Thus, when importing animals for big dairy farms, mass reproductive problems occur for a certain period. Its duration depends on a number of factors. These are animal welfare, the time required for the herd formation, the number of incoming animals, the number of sources from which they have been delivered, the frequency of input of new heifers, the separation of pregnant cows from young animals, lack of vaccination, etc. Testing more susceptible animal groups in the herd may be helpful to establish the role of BVDV in the reproduction pathology.