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| Viral Respiratory Disease In Horses |
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 In Australia, infection with viruses such as the Equine Herpes Virus (EHV-1 and EHV-4) are a common cause of respiratory disease in horses.
The 'stable virus' is essentially an upper respiratory tract viral disease, similar in this respect to symptoms associated with the flu or head cold in humans.
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Infection process
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The virus particles are inhaled in aerosol droplets or on contaminated dust, or taken in on feed. Infected horses carry the virus for up to one week, and it can persist in nasal secretions for up to 20 days. It has an incubation period of 3-4 days during which the viral organisms enter and multiply inside the cells lining the nasal passages and throat.
Viral organisms are also taken into the large pharyngeal lymphoid or tonsil-like area that covers the rear of the throat. There they iniate an inflammatory response, with release of inflammatory mediators that trigger an increase in body temperature and an initial immune reaction with accumulation of white blood cells. This helps to localise the viral infection to the upper respiratory tract.
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Symptoms
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| First | 12-36 hours | Reduced appetite
Reduced work capacity
Temp. above 39°C |
| 36-72 hours | Depression
Slight moist nasal discharge
Poor appetite
Temp. 39.5-40°C |
| 72-96 hours | Copious nasal discharge
Cough after exercise
Clinical signs of stable virus |
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The key to avoiding long term complications is to recognise these early signs. During the 3-4 day incubation period, the horse may be depressed, off its food, and lack its normal enthusiasm and stamina when worked. Most horses in this early stage will have an elevation of body temperature to about 39-39.5°C, with no signs of coughing or nasal discharge at this stage.
At this early stage, your vet may suggest injections of immunostimulants, to rapidly boost immunity and speed recovery. These preparations also help to stimulate lung immunity and reduce the risk of long term lung infection.
In most cases the course of the upper respiratory tract disease will be completed in 5-7 days, and the symptoms will clear up. If symptoms are not recognised early, and the horse is continued in fast or hard work, the virus particles erupt from the lining of the upper respiratory tract, and can be inhaled deep into the lower airways of the lungs. Two-thirds of EHV-1 infections produce inflammation in the lower airways and this can be seen on scoping as mucopus 2-12 weeks after the initial infection. Once the lower respiratory tract is infected, it can take up to 6-8 weeks for full recovery, usually requiring antibiotics to limit secondary bacterial infection, bronchodilators and other airway medications to assist drainage, together with extended paddock rest.
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'Carrier' state
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Once the disease becomes established, horses often become 'carriers' of the Stable Virus. This causes them to occasionally cough before and after work, and in many cases put in one or two good runs, then start coughing again and pulling up distressed after a training workout. Thses horses can be a source of infection to other horses, particularly in humid environments with poor airflow.
Once a horse develops this type of performance problem, a thorough check of the respiratory tract should be carried out by 'scoping' to check for inflammation and accumulation of respiratory discharge in the lower airways and windpipe. These conditions are often aggravated by dust and allergens inhaled from feed, bedding and dusty track surfaces. Consult your vet for advice.
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Management and therapy
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It is important to recognise the early symptoms, prior to the obvious signs of the virus, and cut back on hard or fast work for 3-4 days to minimise risk of lung involvement following simple Herpes virus infection. There are a number of other management procedures you can adopt to assist recovery:
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- Keep the horse in a sheltered area, rug to keep it warm, and provide appetising dampened feeds (if the horse loses its appetite, give 5ml Oralject Megavite-B paste every second day over the tongue or 60ml Pulse-8 daily over the tongue or in the feed).
- Dampen all feed and hay to reduce dust, and feed on the ground or at floor level in a safe tub or trough (so that the head is below chest height to assist drainage of the airways). Alternatively, turn the horse out onto green pasture to encourage it to graze with its head down.
- If the infected horses is one of a group in a stable, take care not to share buckets, bits or other equipment.
- Work the horse lightly by walking or lunging in the warmer part of the day. Do not use swimming as a form of light exercise.
- If the horse develops a snotty nose, seek advice on a course of mucolytic therapy from your vet. An antibiotic cover to prevent or clear up a mild secondary bacterial infection may also be prescribed.
- If the horse develops a chronic cough, a cough relief preparation may be beneficial and an oral bronchodilating agent prescribed to help open up the lower airways and relieve coughing and noisy respiration. See your vet for advice.
- A vaccine against EHV-1 and EHV-4 is available. Consult your vet for advice about vaccinating your horse.
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Article courtesy of Dr John Kohnke from ‘Health Care and problems of Horses, 9th edition’ published by Virbac-Vetsearch.
Dr John Kohnke has over 20 years of experience in the health care and management of horses. He is well known for his ability to give sound, practical and up-to-date advice, which is sought by trainers and horse owners worldwide. As Technical Director of Vetsearch for 20 years, John had an opportunity to pursue research in equine nutrition, parasite control, lameness and respiratory problems.
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