Viruses generally initiate a pneumonia outbreak, but a secondary infection of bacteria results in more severe lung damage.

The cause of an outbreak of pneumonia is usually multifactorial and may be due to a number of the following: –

• Poor colostrum quality (may be due to poor dry cow nutrition, including vitamin E and selenium)
• Mixing of age groups of calves
• Shared air space with older cattle
• Poor cleaning routine
• Early weaning (before 5 weeks)
• An outbreak of scours can lead to increased risk of pneumonia
• Poor ventilation in calf housing
• Cold, humid conditions, sudden changes in air temperature
• Stress due to different causes and change in the environment have also been associated with outbreaks of pneumonia
• Poor nutrition including poor quality calf milk replacer