Host responses of Calves to a Group 5 Bovine Herpesvirus

Bovine respiratory disease (BRD) complex is of substantial economic importance to cattlemen in the United States. An estimated 40-80% of all cattle diseases involve the respiratory system. Although cattle may recover, cost of treatment and loss in production make respiratory disease an important factor to consider in the cattle industry. IN 1974, 407,000 yearling feedlot cattle in Colorado were surveyed for the causes of illness and death. The morbidity was 5.1% with a case mortality of 18.9%. Approximately 75% of the clinical diagnosis and 64% of the necropsy findings indicated respiratory tract diseases. Viruses are obligate intracellular parasites that must enter susceptible cells of the host in order to replicate and cause disease. Specific defense mechanisms such as cell-medicated immunity (CMI) and/or hormonal immunity (HI) become active in preventing viral multiplication. Three bovine herpesviruses, Bovid herpesvirus 1 (BHV-1), Bovid herpesvirus 3 (BHV-3), and Bovid herpesvirus 5 (BHV-5), have been associated with respiratory disease. The bovine immune response to BHV-1 has been studies using the serum neutralization (SN) test to evaluate levels of humoral antibody and the lymphocyte immunostimulation (LI) test to monitor CMI. The ability of BHV-1 to become latent in host cells and to be reactivated in experimentally infected calves following treatment with corticosteroids has also been reported. While the HI response of cattle to BHV-3 has been monitored using the SN test, no data is available on the CMI response or the reactivation of this herpesvirus in naturally or experimentally infected cattle following corticosteroid treatment. A search of the literature did not reveal studies monitoring the HI or CMI responses of naturally or experimentally infected cattle to BHV-5 or the recrudescence of this virus following corticosteroid treatment. Since such studies and in understanding the specific immune responses of calves to herpesvirus (HV) infections, experiments were carried out using the indirect immunofluorescent (IIF) test to monitor the HI response, and the LI assay to monitor the CMI response of calves to BHV-5. The effect of a corticosteroid on the immune response to and recrudescence of this virus was also studied