Viral infection

Viral infections of the respiratory tract are seen very commonly in horses and, although often relatively mild, spread rapidly between horses. Outbreaks of infections are often seen when horses are mixed together, for example on stable yards and often involve more than one virus. The viruses that can be involved include equine influenza, equine herpes-virus 1 and 4, equine rhinovirus and equine viral arteritis. The virus particles are inhaled in aerosol droplets or on contaminated dust. Initially the virus multiplies within the upper respiratory tract (nasal passages, throat and trachea) and no clinical signs are evident, however, after a few days the virus causes inflammation of the cells lining the respiratory tract and the signs of infection can be seen. When this damage to the airways has occurred secondary bacterial infection can occur, see section on pneumonia. Most affected horse will have a cough, increased lymph node size, nasal discharge and be lethargic or show reduced performance. Some horses, especially if bacterial infection has occurred will have a high temperature and be off their food.

The risk factors for viral infection 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.
  • Overcrowding: Too many horses in a small space can be a major risk as this causes stress and also helps the spread of viruses.
  • No vaccination against influenza

Diagnosis

Usually clinical signs are enough for a diagnosis to be reached, however, other tools can also be used. A blood sample can be taken which can often aid in diagnosis and assess whether bacterial infection has also occurred. 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. Radiographs and ultrasound scans are usually only used if bacterial infection has occurred.

Treatment

In most cases the disease will resolve itself in 5-7 days with rest.

Antibiotics are not routinely used for a viral infection unless the risk of secondary pneumonia is considered to be high. During recovery horses should be kept in a sheltered area with good ventilation away from other horses. Feed and hay should be dampened.