Pneumonia in cattle a complex story

Bovine respiratory disease, more commonly known as pneumonia, is the most significant health issue affecting the beef industry and it is also a significant health issue for the dairy industry. While pneumonia can affect animals of any age, the weaning and sales/transport period poses 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 animal’s routine gradually. For example, weaning animals several weeks before sale, introducing one management change at a time, eg introduce concentrates a few weeks before weaning and introduce 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, eg in 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 in Ireland 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 are 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.

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 lose appetite and 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 in Ireland. 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 and depressed with a loss of appetite.

The size of the infectious dose of bacteria required to cause disease is significantly less where a viral infection is already present.

Mycoplasma is 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.

These cases are 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.

While the clinical signs of pneumonia, including fever, dullness, depression and increased respiratory rate are well recognised among farmers and vets, there are only a few studies of the effect of pneumonia at herd level. One of the main reasons for this is the difficulty in gathering suitable data for analysis. The following analysis is based on a case referred to Sligo Regional Veterinary Laboratory. Data from 40 bulls which were treated for pneumonia was compared with data from 149 bulls that had no history of pneumonia in an Irish feedlot. Cattle received a vaccine for IBR and a wormer, approximately one week after arrival.

The cattle were purchased between August and November 2014, from a variety of cattle marts. It was found that the odds of pneumonia occurring in November-purchased cattle were twice that of cattle purchased in October, which in turn was twice the odds of pneumonia in September-purchased animals. This peak in pneumonia incidence coincided with the seasonal peaks in the detection of RSV and PI3 reported in the All-Island Animal Disease Surveillance Report.

Bulls purchased at a younger age were found to have a greater risk of pneumonia. The odds of pneumonia doubled for each 50-day decrease in age. This probably relates to the length of time the bulls were weaned prior to sale.

There was no difference in risk between the 15 marts in which the bulls were purchased, although only one of the 36 bulls which had been purchased directly from the farm of origin got pneumonia. There was no difference in susceptibility among the nine breeds purchased.

The average daily gain of bulls affected by pneumonia during their time in the feedlot was 0.17kg per day less than that of animals not affected by pneumonia. The unaffected bulls (84.1% Es and Us) tended to grade better at slaughter than the bulls affected by pneumonia (76.9% Es and Us), and the liveweight at slaughter of non-affected bulls was, on average, 13kg heavier than bulls affected by pneumonia.

Conclusion

Younger bulls purchased later in the year had the greatest risk of pneumonia. The performance of animals affected by pneumonia has significantly diminished. This loss in performance is in addition to the increased costs of treatment (antibiotics, anti-inflamatories and labour).

Damien Barrett is a research officer at Sligo Regional Veterinary Laboratory, which is part of the Department of Agriculture, Food and Marine’s network of laboratories, (DAFM Laboratories).

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