BRSV-MoreThan Just Four Letters On A Vaccination Bottle

Bovine respiratory syncytial virus (BRSV) has been recognized as a pathogen in cattle since 1970. The presence of this virus in cattle herds is recognized world wide. In the United States antibody prevalence has been reported to 65% to 81% in the cattle population.

Aerosolized secretions absorbed through the respiratory tract is the source of transmission from infected cattle into susceptible cattle. While bovine cell lines can be persistently infected, there have been no chronic carrier cattle demonstrated. Latent infections may occur in calves infected at birth and may provide an explanation for sudden outbreaks in calves raised in the isolation of individual calf hutches.

Cattle most susceptible are beef calves six weeks to 13 months of age and dairy calves two weeks to nine months. In younger calves mortality rates are increased by secondary bacterial infections resulting in severe exudative, fibrinous, or suppurativebronchopneumonia. Infections with BRSV have been implicated as the initiating cause of shipping fever and other respiratory disease complexes.

Outbreaks of BRSV infection infrequently follow stressful weather changes. The morbidity of these infections usually ranges from 0% to 20%. By itself, BRSV can cause acute respiratory distress. Gross necropsy lesions are severe pulmonary edema and interstitial emphysema. There are often consolidated areas in the cranioventral lung lobes accompanied by severe emphysema and edema of the entire lung but primarily of the caudal lobes.

Diagnosing BRSV infections starts with recognizing the clinical signs. Infections tend to be acute with nasal and ocular discharge, pyrexia, cough, progressive dyspnea, and occasionally subcutaneous emphysema.

Laboratory diagnosis can be a challenge and diagnosis should not be made on any one test. Because the virus is so labile, virus isolation is rarely successful from the tissue of a post-mortem animal. The best sample for virus isolation is a tracheal wash or lung lavage.Nasopharyngeal swabs can also be used with good success. The IFA test is one of the most rapid, reliable, and sensitive tests currently available for the diagnosis of BRSV. The best samples for IFA are the same as for virus isolation. Other tests include immunohisto-chemistiy, virus neutralization tests, enzyme immunoassays for antibody detection,enzymeimmunoassays for antigen detection, nucleic acid detection tests and histopathology. Many of these tests are not used with very good success but many hold some promise for the future.

Treatment of BRSV infections consists of a three step regimen. The first step is supportive therapy. Supportive therapy consists of keeping the affected animal well hydrated, maintaining proper electrolyte balance and providing a clean, well ventilated environment. The second step is to treat affected animals with antimicrobials to combat any secondary pathogens that may invade the already compromised lungs. The third step is the use of anti-inflammatory   drugs.   Because   of  theimmunosuppression of corticosteriods these drugs are not recommended for use in the treatment of bovine respiratory disease. Non-steroidal anti-inflammatory drugs (NSAID) are the drugs of choice. Currently aspirin is the only NSAID that is approved for use in food animals.

Prevention is the key to dealing with BRSV infections. Good husbandry and well ventilated housing is mandatory for the prevention of all respiratory diseases. Vaccination is another tool available to prevent BRSV infections. Both modified live vaccines and inactivated vaccines are available. Both types have pros and cons. Modified live vaccines  tend  to  stimulate  stronger neutralizing antibodies responses. Both modified live   inactivated   vaccines   stimulate   non-neutralizing antibody and prime T cells in calves.

The presence of maternal antibodies in young calves can impede the immune response following vaccination with modified live vaccines. For this reason timing is crucial in the vaccination regiment. Beef calves should be vaccinated twice before weaning in order for them to be protected when they enter the feed lot. Vaccinating dairy calves should be done with inactivated vaccines or with modified live vaccines after the maternal antibodies have declined. Maternal antibodies have usually declined to undetectable levels by six months of age.

- by Fred Froderman, Class of 1997

- edited by Greg Stevenson, DVM,PhD

References:

Ames,TrevorR., DVM: The Epidemiology of BRSV Infection. Veterinary Medicine. 881-884;1993.

Baker, John C.,DVM: Treating BRSV. Veterinary Medicine.900-902; 1993. Belknap,Ellen.,DVM: Recognizing the Clinical Signs of BRSV Infection. Veterinary Medicine.886-887; 1993.

Bryson, David G.,DVM: Necropsy Findings Associated with BRSV Pneumonia. Veterinary Medicine.888-893; 1993.

Dubovi, Edward J.,DVM: Diagnosing BRSV Infection: A Laboratory Perspective. Veterinary Medicine.888-893; 1993.

Kelling,ClaytonL.,DVM: Controlling BRSV Infection in Calves. Veterinary Medicine. 903-906; 1993.