Bovine respiratory syncytial virus (BRSV) was identified as a cause of respiratory disease in the 1970s. In the four decades since, BRSV became a problem in cattle herds worldwide. In the UK it is the most important primary viral cause of respiratory disease in young calves affecting approximately 1.9 million calves at a cost of 54 million (C$99 million). Respiratory disease in young calves, apart from being a significant barrier to production, is a major animal welfare issue.
Studies in Canada and the U.S. have shown that between 65 and 80 per cent of cattle carry antibodies to BRSV, indicating widespread exposure. It is probably safe to assume 100 per cent of cattle get infected. While some BRSV infections produce severe disease, most cases are either mild or inapparent. Scientists believe that multiple reinfections occur during the life of cattle. In short, the outcome of BRSV infection is very unpredictable.
Despite its persistent and sweeping presence, there are still many things unknown about BRSV. The erratic nature of the virus on its own or as an inciting cause of respiratory disease in cattle in combination with other pathogens creates many questions about its prevention and control.
Determining whether BRSV is present and then deciding whether the virus is a primary cause of disease or simply an unwelcome visitor in a mix of other viral and bacterial pathogens is a critical first step.
Singly or in combination the following signs may signal the presence of BRSV:
Outbreaks of respiratory disease in younger calves marked by fever, nasal discharge and persistent cough affecting up to 25 per cent of a group.
The sudden onset of respiratory disease following stressful events like transport, inclimate weather and weaning
Respiratory disease unresponsive to treatment with antibiotics.
Mild respiratory infections that progress into more-serious conditions involving other bacteria and viruses.
Signs resembling mild to severe allergic reactions. These signs may be highlighted by the sudden onset of open mouth breathing and coughing. Severely affected calves often appear anxious and reluctant to move. In severe cases, animals may have pockets of air under the skin over the neck and shoulders as a result of extensive lung lesions.
Producers need to consult their veterinarian about BRSV. Managing BRSV infections can be very challenging, beginning with making an accurate diagnosis. Then come the tasks of properly assessing risk, treating sick animals and ultimately designing vaccination programs that make sense. As with other viruses, antibiotics have no affect on the BRSV infection yet antibiotic treatment may be indicated in attempts to control secondary bacterial infections.
At necropsy, animals dying from BRSV infections exhibit lungs that contain fluid, feel very firm and display areas of trapped air. BRSV causes a type of pneumonia called interstitial pneumonia because air, fluid and inflammatory material collects in areas outside and between air pas-
sages of the lung. As a result, lungs fail to collapse as they normally due when the chest is opened at necropsy. The virus does not survive well and is hard to isolate. Its presence in tissue can readily be demonstrated using a technique called immunohistochemistry (IHC).
The immune system holds important clues
Veterinary research has provided a much clearer picture of how the immune system responds differently to individual viruses. Understanding the role of individual cell types and chemical mediators called cytokines partially explains why BRSV does what it does.
For years scientists were stymied why certain vaccines rather than protecting against BRSV actually increased the likelihood of clinical signs in vaccinated animals following exposure to BRSV. A similar phenomenon was recognized in human medicine following respiratory syncytial virus infection, a common cause of respiratory disease in infants. It seems the immune response to this family of viruses and the vaccines created to prevent them is a complicated interplay between the type of immune helper cells initially stimulated and the cytokines they excrete, antibodies already present in the system and the vaccines themselves. Adjuvants (immune enhancers), proteins and chemicals integral to vaccine manufacturing have all been incriminated in wreaking havoc with the immune system; actually causing disease rather than preventing it.
Maternal antibodies in calves acquired through colostrum from BRSV vaccinated dams only provide partial protection by reducing the severity of disease. The presence of circulating maternal antibodies (in the view of some authors) interfere with development of optimal immunity to injectable BRSV vaccines until maternal antibodies disappear around three to four months of age. On the other hand, mucosal antibodies — the kind produced on surfaces lining the respiratory tract following intranasal vaccination — are highly protective even in the face of circulating maternal antibodies.
There is some evidence that susceptibility to BRSV has a genetic component, making some breeds more susceptible than others. BRSV is structurally related to human HRSV, which is the single most important cause of bronchitis and pneumonia in infants. The high degree of similarity between HRSV and BRSV suggests that comparative studies of these viruses will yield important insights that should benefit both man and cattle.
A host of factors will influence decisions whether to include BRSV vaccine in vaccination protocols. A few include:
Safety of newer vaccines. Today’s vaccines do not contribute to disease when vaccinated animals are exposed to BRSV infection following vaccination.
Overall susceptibility and risk of the herd acquiring BRSV infection. A closed herd with basic biosecurity practices in place is at much lower risk than operations with frequent movement of livestock on and off the farm, or feedlots buying replacements in mixed groups through auction markets.
Breeding herds on community pastures or shared grazing leases are probably at risk.
Preconditioning programs for weaned calves and yearlings moving into background or finishing lots may require BRSV vaccine in health protocols.
Herds with known BRSV problems. Vaccination under these conditions slows down the spread of virus and the number of acute infections. Presently all BRSV vaccines are non-replicating and can be administered to both pregnant and non-pregnant cattle, regardless of whether they are modified live or killed virus vaccines.
Proper BRSV vaccination requires two doses initially, followed by an annual booster of one dose. The initial two doses should be administered at least 21 days apart and started when the animal is over 4 months of age.
BRSV vaccine can be purchased alone or in several combinations with other vaccines such as IBR, BVD, PI3, and Lepto-5.
Calves should be vaccinated prior to weaning.
— Ron Clarke
Ron Clarke is a veterinarian who writes from Stony Plain, Alta.