BRD is defined as a "disease complex" or a complex multi-factorial disease as it involves an interaction between several factors -

• Host factors, or characteristics that make the animal more susceptible to the disease: age, immune status, prior exposure to the pathogens, injury and genetics.
• Environmental factors: transport, handling, dust, temperature fluctuations, crowding and ventilation.
• Infectious agents (disease-causing organisms or pathogens) such as
  • Viruses, including the Infectious Bovine Rhinotracheitis (IBR) which often lead to secondary bacterial infections.
  • Bacteria, in particular the three most common; Mannheimia haemolytica, Pasteurella multocida and Histophilus (Haemophilus) somnus. While you may not know exactly which is causing the disease, many BRD cases are multi-organism infections, with more bacteria invading as the disease advances.
  • Parasites, including lungworm.

Any one risk factor may not cause BRD, but in conjunction they can predispose the animal to BRD.

Also, BRD can re-occur due to:

• The animal not being completely cleared of bacteria during earlier treatments.
• An on-going infection with resistant population of bacteria.

Learn more: What is Bovine Respiratory Disease? Part 1: Symptoms