![]() ![]() ![]() Thus, the search to identify pathogenic mechanisms and virulence factors for both viral and bacterial contributors to disease continues, and as a result the importance of certain defense mechanisms in protection of the respiratory tract is being investigated. Those developed in the past decade or so have shown enhanced efficacy but at best protect only 75% of vaccinated animals and provide no protection in outbreaks where the causative organisms are not those most commonly targeted by vaccines. Prevention would be the preferred intervention, and many vaccines targeting the various bacteria and viruses implicated in BRD have been developed over the last 70 or more years. Treatment with broad-spectrum antibiotics may assist recovery in many animals, though feed conversion, weight gain, and the resulting economic return can be seriously compromised in those that recover. Clinical diagnosis, based on the presence of lethargy or depression, reduced feed intake, fever, increased respiratory rate, and dyspnea, with or without nasal discharge, is often made without attempt to identify the offending viruses or bacteria, leaving the diagnosis as undifferentiated bovine respiratory disease (UBRD). ![]() Catastrophic outbreaks or "wrecks" involving large numbers of animals typically follow a week to 10 days after shipment of calves to feedlots, hence the alternative name "shipping fever," but isolated cases in home-reared calves and dairy animals are also recognized. Often called pneumonic pasteurellosis, this disease syndrome has a multifactorial nature that is better captured in its designation as the bovine respiratory disease complex (BRD) ( 47). Viral infection and the host's response to it further compromise defense and facilitate colonization of deeper pulmonary tissues by bacteria normally carried in the nasopharynx, especially members of the family Pasteurellaceae ( 54). The pathogenesis typically involves some combination of predisposing stress which compromises respiratory defense mechanisms and coincidental primary infection with one or more respiratory viruses. Bovine respiratory disease is the principal source of economic loss for the North American beef industry and a significant health problem in the dairy industry as well ( 151). ![]()
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