What are the causes of calf pneumonia and what to do to prevent it?

Over two million¹ calves in Ireland every year, but unfortunately far too many fail to reach adulthood because of disease.

The incidence rate of bovine respiratory disease (BRD) or calf pneumonia, as it is more commonly known, is approximately 20%². It is the most common reason for death and poor performance in calves and young, growing cattle.

Calf pneumonia is the result of a complex interaction between viral and bacterial pathogens, environmental stress factors and the animal’s own resilience to disease.

Calf pneumonia causes inflammation and damage of the lung tissue and airways – compromising lung function. In severe cases, the damage is irreversible and can result in death. However, even mild cases of pneumonia can significantly increase the cost of production.

Cause of calf pneumonia

Calf pneumonia results from a complex interaction between disease-causing viruses and bacteria and the animal’s immune system that is also influenced by external stress factors.

The important viral causes of respiratory disease are bovine respiratory syncytial virus (BRSV), parainfluenza-3 virus (PI-3) and bovine herpes virus-1 (BHV-1); the latter is the cause of infectious bovine rhinotracheitis (IBR).

Bovine viral diarrhoea (BVD) may also be associated with pneumonia in some herds, due to the negative effects it has on an animal’s immune status.

These viruses can cause disease by themselves or damage the defence mechanisms of the respiratory tract and predispose the calf to secondary-bacterial infections of the lungs. Click here for more information on calf pneumonia


There are a large number of bacteria that can cause disease in their own right or act as secondary invaders following viral damage to the lung defence mechanisms.

The important bacteria associated with calf pneumonia are mannheimia haemolytica, pasteurella multocida, histophilus somni and mycoplasma bovis.

An animal’s susceptibility to calf pneumonia will be influenced by the strength of its immune system. In young calves, the single-biggest factor affecting the immune system is colostrum.

Ensuring calves receive good-quality colostrum as quickly as possible after birth will give them the best possible start in life and help reduce the risk of disease.

A calf’s immunity will also be affected by nutrition, management practices, stress and the environment. Issues in any of these areas will have a negative impact on the calf’s ability to fight disease.

Mixing animals of different ages or from different sources within the same airspace will increase the risk of respiratory disease. Similarly – if animals are placed in sheds that are poorly ventilated or humid – even high levels of immunity will not prevent disease outbreaks.

Click here to be proactive on your farm

How does calf pneumonia affect calves?

A number of factors increase susceptibility to calf pneumonia. This include: stress resulting from management practices; sub-optimal nutrition; and in young calves, poor immunity as a result of inadequate colostrum intake.

Viruses are often the initial invader. They cause lung damage that soon allows secondary-bacterial infections to take hold.

Clinical signs of calf pneumonia include:
  • A temperature of greater that 39.5º Celcius;
  • Increased breathing rate and effort;
  • Coughing;
  • A nasal discharge which is initially clear and watery, but becomes thick and pus-like as the disease progresses.

During the early stages of infection, calf pneumonia can be difficult to detect. An elevated temperature is the first sign of disease and typically occurs 12-72 hours before detectable clinical signs appear3.

Early detection

Developing monitoring strategies to identify cases early in the course of disease is critical.

Early detection, diagnosis and treatment of calf pneumonia have recognised benefits:4
  • Improved response to treatment and quicker resolution of clinical signs;
  • Reduction in lung damage and the risk of developing chronic disease;
  • Reduction in re-treatment rate;
  • Reduction of pathogen spread and new cases of disease;
  • Reduction in the impact of pneumonia on future growth and productivity.


Given the multi-factorial nature of the disease, good calf pneumonia control programmes rely on implementing an appropriate vaccine strategy, alongside improvements to management practices to reduce the risk of disease.

Calf, Calves, BVD

Vaccines are available that will enhance protection against the common viral and some of the bacterial causes of pneumonia.

It is important that vaccines are administered prior to high-risk periods to ensure that there is sufficient time for animals to develop effective immunity.


#CALFMATTERS is a vaccination campaign from Merial Animal Health, which supports veterinary practices in encouraging farmers that have not vaccinated against calf pneumonia to vaccinate their calves by helping to reduce the overall cost of vaccination. Click here for more information on #CALFMATTERS


  1. Irish Department of Agriculture, Food and the Marine (2016) AIM Bovine Statistics Report 2016;
  2. Atkinson, 0. (2016) Welsh Dairy Heifer Report. Hybu Cig Cymru;
  3. Wolfger et al. (2015) A Systematic Review of Bovine Respiratory Disease Diagnosis Focused on Diagnostic Confirmation, Early Detection, and Prediction of Unfavorable Outcomes in Feedlot Cattle. Veterinary Clinics of North America Food Animal 31, 351–365
  4. McGuirk & Peek (2014) Timely diagnosis of dairy calf respiratory disease using a standardised scoring system. Animal Health Research Reviews 15, 145–147.