BOVINE RESPIRATORY DISEASE UPDATE 0749-0720/97 $0.00 + .20 INFECTIOUS BOVINE RHINOTRACHEITIS, P ARAINFLUENZA-3, AND RESPIRATORY CORONAVIRUS Sanjay Kapil, DVM, PhD, and Randall J. Basaraba, DVM, PhD Bovine respiratory disease (BRD) complex is caused by a variety of viral, bacterial, and fungal pathogens. Economic losses from BRD in the United States have been estimated to be $640 million annually, and46 this is a more serious disease in stocker and weaned cattle than in yearling cattle. Viruses commonly involved in BRD are infectious bovine rhinotracheitis virus (IBRV), bovine viral diarrhea virus (BVDV), bovine respiratory syncytial virus (BRSV), and parainfluenza-3 (PI-3) virus. Bovine respiratory coronavirus (BRCV) is an emerging pathogen causing upper and lower respiratory tract disease in feedlot cattle, and clinical disease occurs when the animals are stressed, such as during shipment.48 INFECTIOUS BOVINE RHINOTRACHEITIS Role of IBR in BRD As a result of IBRV infection, necrosis of the epithelium of the respiratory tract compromises ciliary clearance of bacteria of the upper This work was supported in part by grants to SK and RJB from the Kansas Agricul- tural Experiment Station. This is publication number 97-268-B from the Kansas Agricultural Experiment Station. From the Department of Diagnostic Medicine and Pathobiology, College of Veterinary Medicine, Kansas State University, Manhattan, Kansas VETERINARY CLINICS OF NORTH AMERICA: FOOD ANIMAL PRACTICE VOLUME 13 NUMBER 3 NOVEMBER 1997 455 456 KAPlL however, they can be differentiated by restriction enzyme analysis of the viral DNAs. Three genotypes of IBRV (I, 2, 3) have been identified. Subtype 1 causes infectious bovine rhinotracheitis, subtype 2 causes infectious pustular vulvovaginitis, and subtype 3 causes neurologic signs. The genome of the IBRV is about 135 to 140 kilobase pairs,34 and the virus makes 54 transcripts (mRNAs).55 Definite gene assignments are available only for a few IBR genes. The IBRV has 70 proteins, of which 33 are structural proteins.38 Ten glycoproteins have been identified in IBRV,53 and have homology with herpes simplex virus 1 glycoproteins. Epidemiology The primary portal of entry for respiratory IBRV infection is the nasal cavity. Aerosols laden with the virus are inhaled, and can initiate respiratory infection. Thus, most infections occur when an infected ani- mal is introduced into a herd or when animals are confined in crowded pens in feedlot situations. The main source of genital disease is venereal transmission, and in bulls the infection can be subclinical. 54 Most bulls RHINOTRACHEms, PARAINFLUENZA-3, AND RESPIRATORY CORONA VIRUS 457 for artificial insemination are required to be seronegative for IBRV. In carrier animals the virus could persist in the trigeminal ganglia in a latent stage. If an animal is sufficiently stressed, as during shipping, the virus may become reactivated, with the host becoming viremic again, and virus may be shed into the environment. Infections by IBR start as foci with the herd of infection and spread within 2 to 4 weeks through- out the herd; the outbreak ends in 4 to 6 weeks when all animals have been exposed to the virus. Most infections occur in animals over 6 months of age, after maternal antibodies have waned. Both humoral and cell-mediated immune responses provide protection against the disease. Wild ruminants, such as deer, have been reported to be reservoirs of the virus. Other ruminants, such as goats, are also susceptible and can transmit the virus to domesticated cattle. Thus, cattle from endemic areas should be considered as carriers and capable of transmitting to previously unexposed animals. The severity of clinical disease is related to the strain of the virus, the immunologic status of the animal at the time of the infection, environmental stressors, and the age of the animal. The presence of even a low level of passive antibody in neonates is sufficient to reduce virus replication and the severity of the IBR infection. Even modified live IBR vaccines are not innocuous, and can cause death and generalized disease and death in young calves.8 Face flies can retain BHV -1 for a short time, but they are not involved in either mechanical or biological transmission.20 Latency Latent infection in cattle can be produced by IBRV, even by attenu- ated vaccine virus. The virus may travel from the sites of initial local infections by axonal migration to the sites of latency, such as trigeminal ganglia and sacral ganglia.1 A variety of factors, such as host immune system, have been implicated in regulating the establishment of IBRV latency. During the latent stage, limited transcription occurs from a small region of the IBRV genome.42 The latency-associated transcripts have been mapped to 0.740 to 0.748 map units, and about 1.16 kb fragment of genomic DNA has been reported to be involved in latency- associated transcription27; however, the precise feedback mech