Asthma is a chronic, allergic, inflammatory disease of the airways. Equine asthma is the term given to include three interrelated allergic respiratory conditions, namely, inflammatory airway disease (IAD), recurrent airway obstruction (RAO) and summer pasture associated recurrent airway obstruction (SPRAO). In this article, we will concentrate on the latter.
What are the signs of equine asthma?
Clinical signs include coughing, wheezing, nasal discharge, chest flaring, loss of performance or exercise intolerance. hese signs are caused by inflammation in the airways which leads to a build-up of mucus and narrowing of the airways. There is no cure, but it can be managed with appropriate drug therapy and environmental management.
What is different about summer associated equine asthma?
Typical RAO (or heaves) is caused by mould spores in dusty hay and is usually seen in the winter or when horses are stabled. Summer associated asthma typically flares up during late spring to early autumn as the name suggests. Although in terms of clinical signs, it is very similar to RAO or heaves, it has distinct seasonal triggers and needs slightly different management. Grass pollens or fungal spores are thought to be the main pathogens in SPRAO. It is also important to realise that horses with SPRAO may also be quite sensitive to dust, due to inflammation in their airways.
How is SPRAO diagnosed?
Your vet may suspect SPRAO based on history and clinical examination alone, particularly in chronic or severe cases. In some horses, examination and response to treatment and appropriate management can be enough to give a presumptive diagnosis.
However, testing can be carried to confirm SPRAO and to rule out other causes of airway disease, such as viral infections, leading to a more accurate diagnosis to allow targeted therapy. Testing is most frequently used in horses that are affected with a milder form of the disease, or for whom this is a new condition. It should also be noted that because this is a progressive illness, early diagnosis and treatment is important to maximise the chances of successful management of the condition.
- Clinical examination
The first phase of diagnosis starts with a thorough clinical examination. Focussing on the lungs, your vet may use exercise or a re-breathing bag (where the horse has to breath in and out into bag) to increase the respiratory rate and depth while listening to the chest.
- Endoscopy
Following a clinical exam, endoscopy is used to examine the inside of the respiratory tract and to collect samples of the cells and material present. Horses are often sedated for the procedure, although it is generally well tolerated. There is a small risk of a nosebleed, which might be distressing, but usually stops within a short time.
Endoscopy can be carried out at your yard or at a clinic setting and the whole procedure should take no more than 30-40 minutes. A narrow, flexible camera is passed up the horse’s nostril into the upper airway and the vet will be able to visualise the larynx and into the trachea, or windpipe.
- Taking samples for testing
A trans tracheal wash (TTW) is carried out by instilling a small volume of sterile saline into the lungs through a small tube, and a sample of mucus is collected for analysis. This is most useful if the pathology is more prevalent in the upper airways.
Your vet may also choose to perform another test called a bronchoalveolar lavage (BAL). This test is, in some ways, more accurate, as it involves sampling fluid from the small bronchioles and alveoli which are at the very end of the airways, which is the area most affected by asthma. The sample is also less likely to be contaminated with cells from the trachea or nasal passages which may confuse the result.
A BAL is more invasive than a TTW, as a larger tube is used, which is passed more deeply into the lungs. It will cause some coughing, particularly in more severe cases where the lungs are very sensitive. In some of these cases, it may not be appropriate to carry out a BAL, as it may be too stressful for the horse, in which case a TTW will be used, or the test may be delayed while some medication is used to reduce inflammation and improve clinical signs.
Whichever test is used, the sample will be sent to a lab to identify and quantify the cells present in the mucus. This will help in reaching a diagnosis.
How do we treat the disease?
As asthma is an allergic, inflammatory disease, no definitive cure exists, but with appropriate medical and environmental management, we can control the symptoms and reduce the speed of progression of the illness.
Management
In cases of SPRAO, as the horse is allergic to pollens in grass and foliage, it can be helpful to stable them in the summer months, when these pollens are most prevalent.
In some cases, where the horse is kept next to arable land, or in areas where the yard is surrounded by flowering trees or thick shrubbery, it may even be worth considering moving them for these months. Stabling will help, but as the pathogens are airborne, it can be challenging to avoid them, in much the same way that humans struggle to avoid pollen that causes hay fever.
As these horses may also be sensitive to dusty environments, it is advisable to make sure that bedding is as low in dust as possible, and hay nets are soaked.
Medications
As the disease is characterised by inflammation and narrowing of the airways, two main types of drugs are used, steroids to reduce the inflammatory response and bronchodilators to open the airways and help the horse to breathe.
In very severe cases, as part of immediate, first aid treatment, your horse may be given an intravenous injection of Atropine or Buscopan to open the airways. However, these are not suitable for long term use.
Steroids can be given by injection, tablets, powder or in an inhaled form. In many cases, treatment will be started with injectable or oral steroids, with the horse maintained on inhaled steroids. Inhalers are often preferred, as we can use the smallest possible dose to manage the case. Another useful way of administering drugs such as steroids in by way of a nebuliser. This converts a liquid into an aerosol. Saline a is sometimes used with varying success, either alone or in addition to other drug therapies.
There is a small risk of laminitis associated with steroid use, particularly in overweight horses, or those with metabolic syndrome or a history of laminitis. While these conditions would not preclude us from using steroids, using the lowest possible dose reduces the risk. With clicker training and positive reinforcement, most horses will learn to tolerate inhaler use.
Bronchodilators, drugs which open the airways (such as clenbuterol), can be given orally, by injection or, in the case of salbutamol, inhaled. Oral clenbuterol is often used for maintenance or long-term management, but it is more expensive than inhalers and some horses will need a short break from it occasionally, as tolerance develops.
Mucolytic drugs are sometimes used to break down mucus, however, their use is limited in horses.
In summary, Equine Summer Associated Asthma is a chronic, allergic disease of adult horses. While there is no cure, early diagnosis and appropriate management and drug therapy can help to ensure a good level of control for your horse. If you have any concerns or are worried that your horse may be suffering from asthma, please call the clinic where our vets would be delighted to help.
