Bovine respiratory disease (BRD), shipping fever pneumonia, or undifferentiated fever is a respiratory disease of cattle of multifactorial etiology with Mannheimia haemolytica and, less commonly, Pasteurella multocida, Histophilus somni ( see Histophilosis), or Mycoplasma bovis being the important bacterial agents involved. Viral pathogens may also be involved, such as bovine herpesvirus 1, parainfluenza-3 virus, and bovine respiratory syncytial virus.
BRD has a multifactorial etiology and develops as a result of complex interactions between environmental factors, host factors, and pathogens. Environmental factors (eg, weaning, transport, commingling, crowding, inclement weather, dust, and inadequate ventilation) serve as stressors that adversely affect the immune and nonimmune defense mechanisms of the host animal. In addition, certain environmental factors (eg, crowding and inadequate ventilation) can enhance the transmission of infectious agents among animals. Many infectious agents have been associated with BRD. An initial pathogen (eg, a virus) may alter the animal’s defense mechanisms, allowing colonization of the lower respiratory tract by bacteria.
BRD is most commonly associated with the transport and assembly of large groups of recently weaned calves into feedlots. Morbidity in these types of feeder calves often peaks within the first 7–10 days after feedlot arrival. Morbidity can approach 35%–50%, and case fatality is 5%–10%; however, the levels of morbidity and mortality strongly depend on the array of risk factors present in the cattle being fed. The use of broad-spectrum antimicrobials labeled for bovine respiratory disease is the primary treatment, with macrolides and fenicols most commonly used as first-line treatment. Prevention and control are achieved via vaccination programs, preconditioning, identification and treatment of subclinically affected animals, and biosecurity.
Etiology of Bovine Respiratory Disease
The pathogenesis of bovine respiratory disease involves stress factor, sometimes coupled with viral infection, which results in suppressed immune defenses and the proliferation of bacteria in the upper respiratory tract. Subsequently, these bacteria colonize the lower respiratory tract and cause bronchopneumonia with a cranioventral distribution in the lung. Many of the bacteria and viruses involved are commensal organisms of the upper respiratory tract that can be isolated from a proportion of healthy animals.
Multiple stress factors contribute to the suppression of host defense mechanisms in cattle. Weaning is a noteworthy stressor, and the incidence of this disease is highest in recently weaned calves. Transportation over long distances serves as a stressor; it may be associated with exhaustion, starvation, dehydration, chilling, and overheating, depending on weather conditions. Additional important stressors include passage through auction markets; commingling, processing, and surgical procedures on arrival at the feedlot; dusty environmental conditions; and nutritional stress associated with a change to high-energy rations in the feedlot. The individual viral and bacterial etiologies, clinical signs, lesions, and treatment are discussed under Viral Infections Associated with Bovine Respiratory Disease Complex in Cattle and Bacterial Pneumonia in Cattle with Bovine Respiratory Disease Complex.
Viruses associated with BRD include:
bovine herpesvirus 1 (IBR)
bovine adenovirus
bovine coronavirus
Bacteria associated with BRD include:
Mannheimia haemolytica
Pasteurella multocida
Histophilus somni
Mycoplasma bovis
Bibersteinia trehalosi
Control and Prevention of Bovine Respiratory Disease
Prevention of bovine respiratory disease should focus on decreasing the stressors that contribute to development of the disease. Cattle should be assembled rapidly into groups, and new animals should not be introduced to established groups. Mixing of cattle from different sources should be avoided or minimized if possible; however, in the North American beef industry, this risk factor is almost unavoidable for large intensive feedlots. Transport time should be minimized, and rest periods, with access to feed and water, should be provided during prolonged transport. Calves should ideally be weaned 2–3 weeks before shipment, and surgical procedures should be performed in advance of transport; however, the availability of these “preconditioned” calves is quite limited.
Cattle should receive arrival processing, which would include vaccinations and possibly metaphylactic antimicrobials within 48 hours after arrival at the feedlot. Adaptation to high-energy rations should be gradual, because acidosis, indigestion, and anorexia may inhibit the immune response. Vitamin and mineral deficiencies should be corrected. Dust control measures should be used.
Metaphylaxis with long-acting antimicrobials, such as oxytetracycline, tilmicosin, florfenicol, gamithromycin, tildipirosin, or tulathromycin, has been widely adopted as a control measure given “on arrival” to cattle at high risk of developing shipping fever pneumonia. Metaphylaxis on arrival has been shown to substantially decrease morbidity, improve rate of gain, and, in some cases, decrease mortality. Mass medication in feed or water is of limited value because sick animals do not eat or drink enough to achieve inhibitory blood levels of the antimicrobial, and many of these oral antimicrobials are poorly absorbed in ruminants.
On arrival, processing usually involves administration of modified live virus vaccines for viral antigens and for bacterial components of shipping fever pneumonia. Because most cases of pneumonia occur during the first 2 weeks after arrival, these on-arrival vaccines may not have adequate time to stimulate complete immunity in all individuals. When possible, vaccinations for the viral and bacterial components of shipping fever pneumonia should be given 2–3 weeks before transport or earlier and can be repeated on entry to the feedlot.
Key Points
BRD is the most common and costly disease affecting the North American beef cattle industry.
BRD risk factors include weaning, transportation, adverse weather, commingling, and stressful events such as dehorning and castration.
BRD is cause by suppressed immune responses and initial viral infection, which allow colonization of the lung by commensal URT bacteria.
Preconditioning, minimizing mixing, vaccination for BRD pathogens, and metaphylaxis for high-risk animals are major control methods.
Broad-spectrum antimicrobials labeled for bovine respiratory disease are the primary treatment.
