Bovine respiratory disease, more commonly known as pneumonia, is the most significant health issue affecting the beef industry and is also a significant health issue for the dairy industry. While pneumonia can affect animals of any age, the weaning and sale/transport periods pose the greatest risk. The interaction between the animal, its environment and the pathogens (viruses, bacteria and parasites) are key to the occurrence and prevention of pneumonia.

Certain stress-related events and management practices, such as weaning and transport, increase the susceptibility to pneumonia. Decreasing the susceptibility to pneumonia to a large extent is achieved by decreasing animal stress levels. In practice, this means introducing changes to the animals’ routine gradually. For example, weaning animals several weeks before sale, introducing one management change at a time and introducing dietary changes over several days rather than suddenly.

Vaccination can be used to reduce susceptibility to pneumonia by increasing resistance to infection.

The key risk from an environmental perspective is the presence of stale air containing viruses, aerosols and waste gases where ventilation is poor.

This is managed by keeping the air in cattle sheds fresh by optimising ventilation. In most sheds this involves the exploitation of the natural stack effect or warm air rising.

Insufficient air outlet areas tend to be the most common issue and this can be addressed by space sheeting in the roof, or by raising roof sheets at intervals throughout the shed. However, air movements at animal level should be avoided, as these create draughts, and such draughts actually predispose to pneumonia by chilling animals.

Disease occurs when the pathogens overcome the defences of the animals. Undesirable environmental conditions may increase the pathogen challenge. For example, a poorly ventilated shed or the environment may increase susceptibility to pneumonia by way of a draught.

Bovine respiratory complex

The pathogens involved in bovine respiratory disease are often referred to as a complex. This is because there is a variety of viruses and bacteria involved in pneumonia in cattle. Each of these pathogens can cause damage to the respiratory system individually, but when in combination work together at a lower infection threshold, creating greater damage to the lungs and airways. It is usually a combination of viruses and bacteria that give rise to pneumonia. However, on a pasture-based system with significant rainfall, lungworm infestation can also be a significant predisposing factor.

Lungworm

Hoose pneumonia or lungworm is a disease of pasture-based cattle husbandry. It is associated with mild temperate climates with plenty of rainfall. The most characteristic manifestation is widespread coughing, often initiated after mild exercise. It is more common in dairy-bred animals. It becomes more of an issue in beef calves after they have been weaned, although once the grass intake of their diet is increased they are at increased risk because this is where they pick up the worm larvae that infect them.

There is a greater prevalence at higher stocking densities, and heavily contaminated pastures pose an increased risk. In dry summers the prevalence decreases, but this can increase suddenly when the rain eventually comes.

Affected animals cough, and have an increased breathing rate. They may become inappetent and lose weight.

Most of the clinical signs occur when the immature lungworm larvae are migrating through the lungs, during the pre-patent phase. At this stage, no eggs or larvae are being passed in the dung and therefore dung samples are not useful to confirm the presence of the lungworm. Affected animals cough, and have an increased breathing rate. They may become inappetent and lose weight. Fever occurs when there is a secondary infection.

While the immunity to lungworm is strong, it is relatively short-lived, lasting about three months after infection. Therefore, continual exposure to the parasite is required to maintain immunity. The advent of long-lasting anthelmintic regimes, and aggressive worming regimes in the first grazing season may hinder the development of this natural immunity, and give rise to lungworm in animals in their second or subsequent grazing seasons. While lungworm can cause pneumonia on its own, it also facilitates infection by viruses and bacteria.

Viral pneumonia

There are three main viruses associated with pneumonia in Irish cattle – infectious bovine rhinotracheitis (IBR), respiratory synctial virus (RSV) and parainfluenza 3 virus (PI3). BVD through its immunosuppressive effects is also an important part of the pneumonia complex, but the advent of the BVD eradication scheme has reduced its prevalence. These viruses interfere with the natural defences in the respiratory system, predisposing to secondary bacterial infections. Clinical signs include fever, coughing and nasal/ocular discharges. These signs are more severe where more than one virus is present.

The viruses are usually transmitted by aerosol spread (inhalation). The detection of RSV and PI3 is greatest among weanlings and peaks in the late autumn and early winter coinciding with the peak in weanling sales and housing. IBR on the other hand is predominately found among young adult cattle, and while it peaks at the time of housing, the seasonal peak in detections is not as great as is the case for the RSV and PI3.

Bacterial pneumonia

Bacterial infection with Mannheimia haemolytica and Pasteurella multocida can lead to severe damage to the lungs, and give rise to the more severe clinical signs. These bacteria produce toxins which cause further damage, allowing more bacteria to grow in the damaged tissue. Affected animals will have fever and are likely to be dull, depressed and inappetant. The size of the infectious dose of bacteria required to cause disease is significantly less where a viral infection is already present.

Mycoplasma are emerging as a significant cause of pneumonia in Irish feedlots. Suspicions of mycoplasma involvement may be raised when affected animals develop an arthritis following a bout of pneumonia. Such cases of mycoplasma arthritis respond poorly to treatment and the prognosis is generally poor. These cases are very difficult to cure and those that survive take some time to recover, but the prognosis is usually poor. Middle ear infections, with associated loss of balance, can be seen on occasion as well. Experiences from US feedlots indicate that robust vaccination programmes for viral pneumonia and antibiotic treatment regimes for pneumonic cattle of up to 10 days are required to manage mycoplasma outbreaks.

Conclusion

Pneumonia is a multifactorial disease. Management of the animal to minimise stress and enhance immunity and environmental management have a key role in dealing with pneumonia.

Several pathogens are involved in the pneumonia complex, and these often interact together augmenting the damage done by the others to the lungs. The control of the respiratory viruses and lungworm are key to the prevention of pneumonia this autumn.