Bovine respiratory disease (BRD)

Bovine respiratory disease (BRD) is one of the most important disease complexes in beef and dairy production. The name sounds like a single condition, but BRD is never the result of one agent: it emerges when stress, viral infection and bacterial infection stack on top of each other.

The three-way interaction

1. Stress weakens the defences. Weaning, transport, regrouping, overcrowding, sudden weather changes and procedures such as disbudding all raise cortisol. One consequence is that mucociliary clearance — the escalator of cilia lining the airway that carries foreign particles upward — slows down.

2. A virus opens the door. Agents such as BRSV, IBR (BoHV-1), PI-3 and BVDV damage the respiratory epithelium directly. BVDV additionally suppresses immunity. At this point it becomes far easier for bacteria normally resident in the upper airway to reach the lung.

3. Bacteria make it severe. Mannheimia haemolytica, Pasteurella multocida, Histophilus somni and Mycoplasma bovis are already present in the upper respiratory tract of most healthy animals. When defences fall they multiply in the lung, and this stage causes most of the tissue damage.

This structure explains why BRD control cannot be reduced to a single intervention: addressing the bacteria alone leaves the stress and the viral pressure exactly where they were.

Why risk concentrates in particular periods

Most BRD cases appear within 2–4 weeks of specific events:

• Weaning — dietary change and social stress together.
• Transport — journey duration, feed and water withdrawal, temperature swings.
• Mixing animals from different sources — each group brings its own flora, and their immune histories differ.
• Housing changes and higher density — where ventilation is inadequate, pathogen and ammonia loads rise.

The predictability of these periods is BRD’s greatest weakness as a disease: because the risk window is known, prevention can be planned in advance.

Spotting it early

Success in treating BRD is tightly linked to how early it is detected. Cattle conceal illness for a long time; by the time obvious respiratory distress is visible, lung damage has usually progressed.

Signs to look for, roughly in the order they appear:

• Behaviour — separating from the group, head held low, indifference to surroundings. Usually the first sign.
• Feed intake — an animal not coming to the bunk.
• Breathing — increased rate, abdominal effort, coughing.
• Nasal and ocular discharge.
• Drooped ear — particularly notable with Mycoplasma.
• Fever — often rises before other signs are visible.

Observing the group calmly, at the same time each day, is the most practical way to catch these early.

The principles behind control

Spread stressors out. Compressing weaning, vaccination, disbudding and transport into the same week multiplies risk. Separating them makes a measurable difference on its own.

Prepare before transport. Animals accustomed to feed and water before a journey recover faster on arrival.

Ventilation. Ammonia and humidity accumulating in a closed building damage respiratory epithelium directly. Ventilation is the most frequently overlooked component of BRD control.

Vaccination. Vaccines exist against the viral agents and some of the bacteria. The critical point is timing: because immunity takes time to build, the vaccine must be given far enough before the stress event. Programme selection is region- and farm-specific.

Group management. Sourcing from a single origin where possible, keeping age groups apart, and giving new arrivals a separate adaptation period all limit the mixing of agents.

Treatment decisions — which product, which animal, when — belong to the veterinarian examining the animal. The purpose of this article is to show why BRD has to be approached as a management problem.