Bacterial infection (pneumonia)

The term pneumonia in simple terms means inflammation of the lungs. The usual course of the disease is for bacteria already in the respiratory attack to increase in numbers or from the inhalation of large amounts of bacteria. The bacteria that are normally in the upper airways are called commensuals and don’t cause pneumonia in normal horses as small finger-like projections and mucus secretions, the muco-ciliary escalator, prevent them entering the lungs themselves. In cases where damage to the respiratory tract has occurred, for example with a viral infection or an allergy, these bacteria are trapped and begin to increase in numbers causing an infection within the lungs. The lungs eventually become consolidated, filled with pus and fluid, rather than being air-filled. The bacteria involved are normally streptococcus species and in general are not contagious, however, multiple horses may be affected. This is most likely because the factors that have caused the disease are the same, e.g. a viral infection (these are very contagious) or poor hay causing allergic airway disease.

The risk factors for pneumonia are as follows

  • Intense work: Racehorses or endurance horses can be at risk due to the physical stresses exerted on them reducing the ability of the white blood cells to fight infection.
  • Long distance transport: When horses are tied up for a long time they are unable to expel material from their lungs effectively causing pooling of bacteria.
  • Overcrowding: Too many horses in a small space can be a major risk as this causes stress and also helps the spread of viruses.
  • General anaesthesia: This is a risk due to the crushing of the lungs during surgery for a long period and increased risk of aspiration pneumonia, breathing in fluid, due to difficulties swallowing.
  • Choke: When the oesophagus is blocked in choke fluid is breathed in, aspiration.
  • No vaccination against influenza

Horses with pneumonia may have an increased temperature, be off their food, be depressed and lethargic, have a cough, have a nasal discharge and have rapid breathing.

Diagnosis

A complete clinical examination should be carried out by the veterinary surgeon including listening to the lungs with a stethoscope. This can often reveal the presence of fluid on the lungs or wheezes. A blood sample is usually taken to measure the extent of the infection the horse is fighting. An ultrasound scan of the horse’s chest can help to determine if there are areas where there is so much infection that the lung has become consolidated or if an abscess has formed. Radiographs are often taken to assess the lung fields. In a normal horse large areas will show up dark on an x-ray as the lungs contain air. In cases of pneumonia the lungs appear lighter in colour. An endoscope can be passed to assess the trachea and bronchii. While doing this a sample of fluid within the airways can be obtained, an aspirate, and then be looked at under a microscope to see the cells present. The sample can be cultured in a lab to see which bacteria is present.

Treatment

The number one treatment for pneumonia is antibiotics. These often have to be given by intra-venous or intra-muscular injections and may be required for a long period.

Other treatments include anti-inflammatory drugs, strict rest (often for a few weeks) and improving the environment (good ventilation and good quality forage).