ANTIGENIC DIVERSITY OF RESPIRATORY SYNCYTIAL VIRUSES AND ITS IMPLICATION FOR IMMUNOPROPHYLAXIS IN RUMINANTS

Bovine respiratory syncytial virus (BRSV) is a very important pathogen of cattle and perhaps other ruminants. It is a major contributor to t he incidence of respiratory tract disease in nursing beef and feedlot and dairy calves. The genome of respiratory syncytial viruses encodes 10 proteins translated from 10 unique mRNAs. The major glycoprotein (G ), fusion protein (F), 1A protein and the 22K protein are components o f the viral envelope. The nucleocapsid contains the nucleocapsid prote in (N), the phosphoprotein (P), and the large protein (L). The matrix protein (M) forms a structural layer between the envelope and the nucl eocapsid. Antibodies to all the structural proteins develop in convale scent calves. However, evidence suggests that immunity develops primar ily as a result of the antigenic stimulus by the major glycoprotein G and the fusion glycoprotein F. It is known also that activated cytotox ic T cells interact with N and F protein antigens and helper T cells i nteract with N, F, and IA protein antigens. With the exception of the major glycoprotein, the respective proteins of various respiratory syn cytial viruses share major antigenic domains. Based on antigenic diffe rences of the major glycoprotein, at least 3 subgroups of RSV are reco gnized; human A, human B, and bovine RSV. Indirect evidence suggests t hat a second subgroup of BRSV exists. However, we have identified only one BRSV subgroup based on our work with RNase mismatch cleavage anal ysis of the G protein gene from a limited number of strains. Furthermo re, our data indicated that a caprine RSV isolate is closely related t o the bovine strains, but an ovine isolate is not. The latter may cons titute yet another subgroup of RSV. These data affect decisions on opt imization of immunoprophylaxis since evidence suggests that protection against a homologous RSV subgroup virus is superior to that against a heterologous strain in immune subjects.