Bovine Respiratory Disease

Respiratory disease is the most significant health problem of the beef industry.    Bovine Respiratory Disease (BRD) cost the US cattle industry an estimated loss of $624 million in 1991. This economic loss is due to costs of treatment, production loss, and death.

Cattle seem to be very susceptible to respiratory disease. One reason is that the bovine respiratory tract is small relative to body size.   Small nostrils limit airflow, increasing breathing effort. A narrow throat passage can easily become dry and irritated, allowing viruses and bacteria to invade.

There   are   many   organisms contributing to the incidence of Bovine Respiratory Disease (BRD).   The viral entities include Infectious Bovine Rhino-tracheitis(IBR), Bovine Viral Diarrhea (BVD), Bovine Respiratory Syncytial Virus (BRSV), and Parainfluenza Virus (Ply). The bacteria   involved   in   BRD   include Pasteurellahemolytica,Pasteurellamulto-cida,Haemophilussomnus,Mycoplasmasp. and Actinomycespyogenes.

A complex series of events occurs which is typically associated with BRD. Calves are stressed by weaning, shipping, processing, adverse weather, and over­crowding.   This stress compromises the defense mechanisms of the immune system. Viruses invade the nose and lungs because of weakened immune barriers.   Viruses damage the epithelium of the upper airways and compromise the effectiveness of the mucociliary  apparatus,   which   sweeps particles (bacteria, dust, mold, and pollen) up and away from the lungs. This lack of ciliary clearance allows overgrowth of normal respiratory inhabitants as well as bacterial pathogens.   Secondary bacterial invaders move in, proliferate, and may potentially cause death if the disease is not detected and treated properly.

Prevention of BRD is much more successful and economically feasible than treatment.   An ideal processing protocol should include vaccination of these calves two weeks before shipping to allow development of an adequate immune response, and to minimize pre-shipping stress.   Calves should receive a booster vaccination   once   they   reach   their destination. Vaccination will merely prime an immune response in a healthy immune system.    Animals that are immuno-compromised, have severely hampered this response.

Identifying the causative agents of BRD can often be difficult and frustrating. Necropsy usually reveals lesions charac­teristic of secondary bacterial infection, i.e. Pasteurellahemolytica. Lesions character­istic of a primary viral agent are often absent due to the lesions of the bacterial agent at the time of death. Viral isolation attempts are usually negative. Antibiotic treatment of these animals prior to death hampers both the ability to isolate bacteria and may affect sensitivity testing.

In the acute phase, viral pneumonias usually cause a bronchointerstitial pattern. Mycoplasmas cause bronchiolitis, necrosis, and an interstitial reaction. Mycoplasmas, unlike viral pneumonias, will additionally progress toward a chronic stage characteri­zed by peribronchiolarlymphoidhyper-plasia. Viral or mycoplasmal lesions can change from a pure bronchointerstitial to a suppurative   bronchopneumonia    when complicated by a secondary bacterial infection.

The   classic   gross   lesions   of pasteurellosis   are   fibrinous   broncho-pneumonia with a fibrinouspleuritis and pleural effusion. Lesions are cranioventraland often are ventral to the level of the tracheal bifurcation.   Yellow, gelatinous edema and fibrin distend the interlobular septa. Histologically, "marbling" of lobules results from coagulative necrosis, interstitial edema, and congestion.

Definitively, virology, serology, and bacteriology may be utilized for diagnosis of the various agents involved in BRD. The following table provides useful information about tests and samples to submit.

- byAndreaBarnes Cross, Class of 1998

- edited by Lydia Andrews-Jones, DVM