Bovine Respiratory Disease Complex (BRD)

Early signs are general: reduced appetite, standing apart from the group, depression and fever, with drooped ears and sunken eyes in feedlot cattle. These progress to nasal and eye discharge, salivation, cough, rapid breathing and laboured breathing. Auscultation may reveal crackles or wheezes, though it misses much lung consolidation that thoracic ultrasound detects. Fever patterns vary with the agent. Many subclinical cases go unnoticed. At necropsy, fibrinous or fibrinosuppurative bronchopneumonia with fibrinous pleurisy and lung necrosis is typical of bacterial involvement.

Bovine respiratory disease (BRD) is not one infection but a multifactorial pneumonia of cattle arising from viruses, bacteria, host factors and stress acting together. The usual sequence is a primary viral infection, by bovine herpesvirus 1, bovine respiratory syncytial virus, parainfluenza virus 3, bovine viral diarrhoea virus or bovine coronavirus, that damages the respiratory lining; bovine viral diarrhoea virus also suppresses immunity. This opens the lower airways to bacteria that normally live harmlessly in the upper tract, chiefly Mannheimia haemolytica, Pasteurella multocida, Histophilus somni and Mycoplasma bovis. M. haemolytica relies on its leukotoxin and fills bronchioles and alveoli with fibrin and neutrophils, giving acute fibrinous pleuropneumonia. H. somni can enter the blood and cause septicaemia, encephalitis or myocarditis. Stress-induced immunosuppression makes every step easier.

BRD occurs in cattle worldwide and is among the costliest cattle diseases, estimated at over USD 1 billion a year to the US beef industry alone. It is the leading cause of death in pre-weaned dairy calves and is most severe in young stock. Beef calves meet a cluster of stressors (weaning, transport and mixing with unfamiliar cattle) long known as shipping fever, which temporarily suppress immunity; handling for dehorning and feed changes add to it. Poor ventilation, wet dirty bedding, dust, overcrowding, cold damp weather and inadequate colostrum raise disease rates, and indoor intensive systems generally show more disease than pasture. Reported prevalence varies widely with diagnostic method, for example 8% in summer and 15% in winter on Quebec dairy farms by thoracic ultrasound.

Signs are non-specific, so the main task is identifying which agents are involved rather than naming a look-alike. Lungworm (Dictyocaulus viviparus) infection produces and complicates respiratory disease and must be ruled out, and Aspergillus fungi can also trigger respiratory disease. Separating viral from bacterial causes, and upper from lower tract disease, needs laboratory testing of respiratory samples, bacterial culture, thoracic ultrasound or necropsy.

Upper respiratory tract, trachea, bronchi and bronchioles, lungs (alveoli), pleura; bloodstream, brain and heart in Histophilus somni septicaemia

Vaccinate against key viruses and bacteria ahead of stress, ensure good colostrum, reduce transport, weaning and handling stress, rest cattle after shipping, ventilate well, avoid crowding and mixing ages, and isolate new or sick animals.

Leading killer of pre-weaned dairy calves; transport raises deaths when bacteria follow viral infection. Antibiotics are the usual first treatment, amid rising resistance. Affected cattle grow more slowly, give lighter carcasses and may keep lung damage.