Bovine Respiratory Disease
Bovine respiratory disease (BRD), also known as shipping fever or pneumonia, has been estimated to cost the U.S. cattle industry over $500 million each year. Backgrounding and finishing operations bear the majority of the costs, which stem from treatment expenses, reduced performance and death loss. This disease complex is the single biggest killer in the United States among newly weaned cattle with several factors playing a role.
BRD is typically caused by a combination of viruses, bacteria and stress (Figure 1). Research has shown that stress plays a major role in morbidity rates associated with BRD. Stressors for cattle, including respiratory irritation from dust, overcrowding, dehorning, castration, commingling, poor nutrition, transport, weaning and handling, can contribute to the onset of illness.
The immune status of the individual animal determines the severity of illness. Viruses play a key role in initiating BRD. Usually, a viral infection is the primary or initial challenge to the respiratory tract, especially the upper respiratory tract. The most common viral agents that play a role in respiratory disease are infectious bovine rhinotracheitis (IBR), bovine viral diarrhea (BVD), parainfluenza type 3 (PI3) and bovine respiratory syncytial virus (BRSV). The latter, BRSV, is of particular concern because viral particles directly invade the lungs. These viruses enter the respiratory tract through the nose or mouth and quickly impair the protective barriers of the trachea and lungs. With the protective barriers gone, opportunistic bacteria are allowed to multiply and infect the respiratory tract, which causes inflammation and tissue damage. Most animals that have a healthy immune system can successfully fight off a viral infection, thus avoiding severe disease. However, cattle with immune suppression due to stress factors are not able to combat viral infection efficiently.
Mannheimia haemolytica (formerly known as Pasteurella haemolytica), Pasteurella multocida and Haemophilus somnus are the bacterial agents most commonly identified in advanced cases of BRD. Another opportunistic bacteria, Mycoplasma bovis, has been isolated in cattle with chronic pneumonia. All of these bacteria flourish after an initial viral infection when stress factors have weakened the animal’s immune system. Each of these bacterial agents has unique mechanisms that cause illness in cattle and must be treated accordingly.
For more information on clinical signs and prevention of BRD click on the link for our fact sheet