Vet Column 2-22-10 | TheFencePost.com

Vet Column 2-22-10

David L. Morris, DVM, Ph.D.
Fort Collins, Colo.

Many producers pick up the vaccine bottle, draw the reconstituted vaccine into the syringe for administration, and immunize their cattle against bovine respiratory disease. The primary concern often involves infectious bovine rhinotracheitis (IBR) and bovine virus diarrhea (BVD). For those that look at the bottle closely, they see BRSV. So what is bovine respiratory syncytial virus (BRSV)?

BRSV infects the upper and lower respiratory tract of cattle. Depending upon the previous exposure of cattle to the virus, the clinical signs may vary. In older calves and cattle with partial or waning immunity to BRSV, the primary clinical signs may be those of upper respiratory tract disease such as nasal or ocular discharge. These clinical signs are not as severe as with IBR (bovine herpes virus-1). Most of the problems with BRSV occur in the lower respiratory tract affecting the bronchi, bronchioles and alveoli.

Syncytial viruses can affect humans as well and it is referred to as human respiratory syncytial virus (HRSV). Although BRSV and HRSV are not the same, and BRSV does not infect humans nor HRSV cattle, there are often similarities in the clinical signs caused in the different species. The pathology caused by infection of the respective hosts is similar; infection can occur repeatedly throughout the lives of both hosts; and the development of the infection is similar.

One of the key issues regarding BRSV is the damage that can be done in the lower respiratory tract. The lower respiratory tract is considered to begin at the end of the trachea or windpipe in the chest. Here it branches into the two mainstem bronchi. The mainstem bronchi branch repeatedly to form additional bronchi, and then bronchioles. Terminal bronchioles end in the alveoli, where gas exchange occurs and oxygen is taken up and carbon dioxide is released.

BRSV is not the only major lower respiratory tract pathogen. Parainfluenza-3 (PI3) can disrupt normal respiratory tissue in the lower respiratory tract as well. What is important to know is that once the tissues are compromised by such viruses, bacteria such a Mannheimia hemolytica, Pasterurella multocida and Mycoplasma species can proliferate and cause further lung damage. In general, these complicating factors create the classical shipping fever which was the former name prior to bovine respiratory disease (BRD) complex.

When housing and management of calves in close confinement occurs, such as often the case with dairy calves, BRSV can surface. “Summer pneumonia” is a classical examples of a virus taking advantage of the “window of susceptibility” when antibodies from the dam have decayed to nonprotective levels and the calf has not developed active immunity through vaccination or exposure. Close confinement can also offer advantages, though, in that if BRSV is present, exposure can be occurring and opportunities for active immunity can exist outside of vaccination. For beef calves on pasture, exposure is reduced, but so is the opportunity to develop comparatively similar active immunity.

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Somewhat unique, BRSV only infects epithelial cells of the respiratory tract. As a result, circulating virus does not reach calves in utero. Calves contract the disease from other calves or from its dam after birth. To reduce the severity of the disease, good colostral transfer is important, beef or dairy. If calves are raised in close confinement, it may help to avoid housing animals with a wide range of age in the same group. Vaccine should be timed a couple of weeks prior to expected outbreaks if they have occurred previously. Generally vaccinating around 3-months-of-age provides for active immunity. If problems do occur, one should consider using a modified live vaccine and repeat at 2 to 4 weeks. Questions should be directed to your veterinarian.