The role of viruses in respiratory disease and poor performance

Dr Brett Tennent-Brown BVSc, MS, DACVIM, DACVECC, Senior Lecturer in Equine Medicine As seen in Racing Victoria's Inside Racing Magazine.

Respiratory disease is the second most common cause of lost training days after lameness for racehorses. Both infectious (i.e., caused by viruses or bacteria) and non-infectious respiratory diseases occur commonly in young horses entering training stables for the first time and both can be important causes of poor performance.

Inflammatory Airway Disease (IAD) is a non-infectious disease of horses that shares similarities with human asthma. The condition can be difficult to detect because affected horses often appear completely normal and effective management is challenging.

Viral respiratory diseases are pervasive amongst young horses and complete elimination of disease is probably unrealistic; however, prompt recognition of an infection and appropriate management can reduce the impact on animal health and stable operations.

The viruses most often implicated in equine respiratory disease are the Herpes and Rhinitis viruses. Equine Herpes viruses are particularly common; they are ubiquitous in horse populations and most horses are probably infected at a young age. There are 4 members of the equine herpes virus family that seem to be important. Historically, equine herpes virus 4 (EHV4) is the member most often involved in respiratory disease with EHV1 involved less commonly. Recent studies have examined the role of EHV2 and EHV5 in disease; EHV2 is frequently identified in horses with poor performance issues related to the respiratory tract. However, both EHV2 and EHV5 can be found in normal horse so the exact role of these viruses is currently not well understood. It might be that EHV2 and 5 weaken the horse’s immune system allowing infections by other viruses or bacteria.

A key feature of herpes virus infections is the development of ‘latent’ infections. Horses latently infected with herpes viruses are clinically normal and the presence of latent virus doesn’t appear to have any ill effects on horse health. However, latent infections can be re-activated causing recrudescence of disease. Re-activated infections are often relatively mild with less severe clinical signs, but these animals serve as a source of infection for others. The reasons for re-activation of latent infections are not well understood but are probably related to events that the horse finds stressful and which subsequently impair their immune system. Events that lower a horse’s immune system might include increased training load or introduction into a new environment. Young horses coming into training seem to be especially susceptible.

While it is almost impossible to completely prevent viral infections, some basic precautions can reduce their impact. To decrease the chances of introducing horses with an active viral infection into a barn and infecting resident horses, new arrivals should be quarantined for 3 weeks. Quarantined animals should have their temperature taken frequently (at least twice daily) and should be carefully assessed for signs like nasal discharge and loss of appetite. However, because racehorses are continuously mixed with other horses and because of latent viral infections, even the most rigorous of quarantine procedures can’t always prevent infection within a population. Therefore, all horses should be carefully monitored for the signs of respiratory tract infection – these include inappetence, quieter than normal behavior, coughing and nasal discharge. Horses showing any of these signs should have their temperature checked and a veterinarian should be consulted.

Contact between horses showing clinical signs of infection and other animals should be prevented as much as possible and basic hygiene (hand washing!) is essential for barn staff. Because young animals are more likely to become sick and more likely to spread viruses to other animals, it is often a good idea to house younger animals separately from older animals. Improving air quality can improve respiratory function and is beneficial in preventing infection.

Vaccination against the herpes viruses has been suggested to reduce the severity of clinical signs and might reduce the amount of virus that an infected animal sheds. This might mean that infected animals recover more quickly and might reduce the spread of disease from horse to horse.

Although anti-viral agents have been used to treat horses with viral infections, the efficacy of those drugs is questionable and treatment of viral disease is largely symptomatic just as it is for humans (antibiotics are ineffective against viruses). Judicious administration of non-steroidal anti-inflammatories (e.g., phenylbutazone) will make some horses feel better and improve their appetite, but strict rest from exercise is most important.

It is hard to know how long horses need to be rested, but some authorities have suggested 1 week for every day of fever. While it is tempting to get recovering horses back to work as quickly as possible; this risks prolonging disease and increases the risk of a more serious infection (including bacterial infections). Most horses should recover from viral infections over 3 to 7 days but should have 2 or more weeks off from structured exercise.

For further advice and to make an appointment, contact us.

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