Pneumonia - Cattle

Pneumonia in cattle is often caused by infectious agents, particularly by a combination of bacteria and viruses. Pneumonia in cattle, also known as bovine respiratory disease (BRD), is influenced by a complex causal web involving a range of animal, management, and agent factors. The presence of infectious agents alone may not be sufficient to cause disease. Many potential stressors associated with the export process can predispose cattle to pneumonia including transportation, mixing of cattle from different sources, dietary changes and reduction in feed and/or water intake, changes in weather, dust and other respiratory irritants such as ammonia, and alterations in ventilation. The rapid, deep, open mouth breathing associated with heat stress is believed to facilitate the inhalation of organisms deep into the airways. 

A number of viral, bacterial, and Mycoplasma pathogens may be involved. Many are normal inhabitants of the upper respiratory tract of cattle that can become serious opportunistic pathogens. 

Infectious agents associated with BRD include:

• Viruses: bovine herpesvirus 1 (BHV-1 or IBR), parainfluenza virus type-3 (PI-3), bovine respiratory syncytial virus (BRSV), bovine coronavirus (BCV), bovine viral diarrhoea virus (BVD), and adenoviruses.
• Bacteria: Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, Arcanobacterium pyogenes, Salmonella sp., and Mycoplasma sp.

The most common form of severe BRD is a bacterial bronchopneumonia involving one or more of the bacterial agents identified above. The viruses (and perhaps Mycoplasma) are thought to be initial infective agents that act to compromise respiratory defence mechanisms and allow the bacteria to penetrate into the lower airways and alveoli causing more severe disease. 

Serious outbreaks of pneumonia occur sometimes in the export process, especially on long sea voyages. The full extent of disease is usually only appreciated when the lungs of slaughtered animals are examined at necropsy or processing.

Affected animals may display some or all of the following signs: nasal/ocular/oral discharge, depression, lethargy, emaciated body condition, laboured or rapid shallow breathing, coughing, extended head and neck, and droopy ears. The first observable sign in affected animals is often depression, observed as lethargy, reluctance to move, lowered head, drooped ears, and half-closed eyes. Serous nasal discharge is common in early stages, turning mucopurulent as the condition progresses.

Increased respiratory rates are common in early stages, due in part to elevated temperature. As the disease progresses, dyspnoea becomes more pronounced and animals may adopt a typical stance of extended neck, drooling, open mouth breathing, and a soft cough. 

Affected animals may be found dead without obvious ante-mortem signs, and pneumonia therefore diagnosed at necropsy. 

Under conditions of excessive heat load, cattle that display early signs of distress may be animals with pneumonia. Cattle with pneumonia deteriorate very quickly in hot humid conditions because normally functioning lungs are an important means of shedding heat load. 

If lameness and joint swelling from arthritis is also present, then involvement of Mycoplasma bovis, Histophilus somni, and Chlamydophila sp. should be suspected.

Misdiagnosis is common. Pneumonia may be missed because cattle may show relatively few clinical signs until severely ill, and auscultation of ruminant lungs may be difficult and not very informative. In addition, a diagnosis of pneumonia may be made in cattle suffering from other conditions, such as heat stress, causing increased respiratory rate or dyspnoea. 

At necropsy, anteroventral lobes mostly feel consolidated (firm), rather than spongy, and are discoloured (mostly dark red). The lung surface may be coated with fibrin strands or sheets, which may stick to the parietal pleura (lining of the rib cage). 

Interlobular septa are usually prominent. It is not uncommon at necropsy to find more than 50% of lung consolidated with no ante-mortem signs having been detected. 

Note that discolouration of the lungs on its own is not a sign of pneumonia. Pneumonia should be diagnosed by feel. Normal lungs feel soft and spongy and are pink, but can be severely discoloured by gravitational pooling of blood or congestion. Lungs with pneumonia feel rubbery (some viruses) or consolidated like liver (other viruses and bacteria). 

Laboratory determination of pathogens requires nasal swabs in transport media for virus isolation, and serum from acutely ill and convalescent animals for virus serology. In dead animals, portions of affected lung should be submitted fixed in buffered formalin for histology, and chilled for microbiology and virology. 

The main differential diagnoses clinically are infectious bovine rhinotracheitis (because of nasal discharge), and heat stress and systemic illness (causing increased respiratory rate and elevated body temperature).

Isolate affected animals in a well-ventilated area protected from excessive cold or heat, maintain hydration, and offer good quality cereal hay or roughly chopped chaff.

A range of measures may be considered for animals at their farm of origin such as yard weaning, introduction to trough feeding and watering, dehorning, drenching, castration, and vaccination against a range of diseases including BRD pathogens. Many of these measures may be incorporated into accredited backgrounding or preconditioning programs aimed at ensuring consistent quality of animals arriving at a feedlot or assembly depot. 

Selection of appropriate stock, minimal mixing of animals, low stress handling, minimising exposure to harsh weather prior to loading, optimal stocking rates, good ventilation, and having an ample supply of quality feed and water are all measures that may be used during export voyages to reduce the risk of BRD occurring. 

Vaccines specifically targeting BRD pathogens may reduce morbidity and mortality. In Australia, registered bovine vaccines are available against BVD, infectious bovine rhinotracheitis (BHV-1) and Mannheimia haemolytica. Ideally, vaccination should occur prior to entering assembly points. Other infectious agents may be incorporated into vaccines that are registered in other countries, and these are occasionally used under special licensing arrangements in cattle being exported from Australia.

Metaphylaxis (mass medication) of cattle with antibiotics (ceftiofur sodium, florfenicol, tilmicosin, oxytetracycline or tulathromycin) during pre-embarkation processing may provide increased protection in lines of cattle that may be considered to be at higher risk for some reason, such as time of year, duration of voyage, destination, and animal characteristics (age, class, breed - Bos taurus vs Bos indicus), however this practice will increase the risk of developing antimicrobial resistant pathogens.