Veterinary Clinical Medicine-I (General and Systemic)

Bovine respiratory diseases

BOVINE RESPIRATORY DISEASE

• Bovine respiratory disease is not a single entity nor is it attributable to a single cause. Three different categories of problems occur in affected cattle.
• Bovine respiratory disease complex (BRDC) is the most common and most devastating form. This is commonly called “shipping fever” or enzootic calf pneumonia. Acute interstitial pneumonia also occurs and is most commonly associated with toxins. The third form of disease in metastatic pneumonia that occurs as a result of septicemic conditions such as discussed in the GI section.

• Bovine respiratory disease complex refers to bacterial bronchopneumonias that may are may not be complicated by previous or concurrent viral or mycoplasma infections. Numerous bacteria can be isolated from the lungs of affected cattle.
• In cattle the most important bacterial pathogen is Mannheimia hemolytica, with Pasturella multocida and Hemophilus somnus playing lesser roles. These organisms play the same role in pathogenesis in younger calves and Mycoplasma spp. are of additional concern.

Viral pathogens are implicated in the BRDC even though the final pulmonary pathology is caused by the previously mentioned bacteria.

• The primary viruses involved include herpesvirus 1 (IBR), parainfluenza type 3 (PI3), respiratory syncytial virus (BRSV), and bovine viral diarrhea virus (BVD). Other viruses may cause lesser disease at times but these four are the most problematic for producers and veterinarians.
•  These viruses infect the upper respiratory tract resulting in rhinitis, tracheitis and bronchitis. Their ability to cause direct pulmonary disease is limited except for BRSV. However, all of these pathogens predispose the lung to bacterial infection and bronchopneumonia by compromising the respiratory tract defense mechanisms.
• The persistence of pathogens in the environment obviously contributes to exposure. Survival times of the pathogens depend on many factors including the amount of organic material present, moisture, sunlight and exposure to disinfectants.
• Survival times for most viral pathogens are on the order of minutes to several hours, while bacterial pathogens may be longer. Airborne transmission of a typical virus can occur over distances of about 4 meters, although some viruses are capable of survival over several miles. The complexity of the problem is increased by the knowledge that survival varies between different strains of the same pathogen.
• Ventilation improvement is an important means of reducing pathogen concentration. As pathogen load increases in the air, ventilation provides less protection against infection.
• Increased animal activity increases dust exposure and ventilation rate, effort and tidal volume, which in increases the amount of pathogen shed by infected animals and the amount of pathogen inhaled by susceptible animals. The increased dust exposure also will adversely affect the mucociliary clearance and respiratory defense mechanisms.