RESPIRATORY disease is common in young thoroughbred racehorses in training, as it is in most young domestic animal species.

The occurrence of the disease in racehorses varies widely between yards and between years. The reasons underlying this variation, including the causes of the disease, are not well-defined.

Inflammation that is detected in the upper respiratory tract, trachea and bronchi is highly significant in young racehorses in terms of the impact on training days missed and racecourse performances.

Inflammatory airway disease is usually associated with increased amounts of mucus visible in the trachea, coughing, nasal discharge and poor performance, although not all of these clinical signs are visible in all cases.

Inflammatory airway disease is characterised by neutrophilic (a type of inflammatory cell) inflammation, mild clinical signs, and accumulation of mucus in the trachea. It is a multifactorial disease involving infectious, environmental and immunological factors. Tracheal infections with viruses (Equine Herpes Virus 1 and 4, and equine influenza) are often implicated in inflammatory airway disease. Equine influenza destroys the mucociliary apparatus; these are a group of cells that clear mucus from the airways, wafting the mucus along a blanket of hair-like structures. Vaccination against equine influenza is commonly practised among most equine populations within Britain. However, it is mandatory that all horses racing in Britain be vaccinated for equine influenza under BHA regulations.

Sadly, although vaccination reduces the prevalence and severity of disease, even vaccinated horses can become infected with equine influenza and therefore it can contribute to airway infection and inflammation. Influenza vaccination involves a primary course of three vaccinations followed by yearly boosters.

Immunity to influenza can be enhanced by giving vaccinations at more regular intervals, increasing and maintaining circulating antibody levels. Antibodies are markers created by inflammatory cells in the body.

They stick to foreign agents, viruses and bacteria, allowing them to be identified and killed by other inflammatory cells.

Equine Herpes Virus (EHV) has two strains, 1 and 4, that cause respiratory disease in horses. The virus is endemic within the British equine population and affects many horses as foals and yearlings. It destroys bronchial-associated lymphoid tissue, a tissue that is involved in the body’s immune defence of the respiratory tract.

Herpes virus hides within the bodies cells, re-appearing when animals are stressed or when their immune system is compromised. Herpes virus causes cold sores in humans which, as some of you will be aware, tend to act in a similar way and appear at times of stress. Vaccination against EHV is possible. However, due to the endemic status of herpes and the constant viral challenge horses face, it acts only to minimise clinical signs and reduce the spread of the virus between horses.

Bacterial respiratory infections tend to be primarily initiated by viral disease. This is because viral respiratory infections impair or destroy respiratory defence mechanisms, (ie, influenza destroys the mucociliary apparatus, EHV destroys bronchial-associated lymphoid tissue). Bacteria (Streptococcus pneumoniae, Streptococcus zooepidemicus, Actinobacillus, and Mycoplasma equirhinis) undertake a key role in the development and progression of inflammatory airway disease, colonising damaged or immune-compromised tissue and causing further damage.

In some cases, bacteria of certain types can act as a primary infectious agent not requiring any prior viral infection. Bacterial infection causes mucoid discharge to become thick and purulent which further exacerbates difficulties in clearing excess mucus by damaged respiratory tract tissue. This adds another perspective to the goals for treating inflammatory airway disease.

External factors contribute to the onset, severity and duration of the disease in racehorses.

It is often more common and severe in younger horses, yearlings and two-year-olds. These horses are adapting to the increased disease challenge. The stress of the training environment and increased exercise load involved in training makes them potentially immuno-suppressed and therefore more susceptible to infection.

Another immune factor contributing to inflammatory airway disease is the almost hyper-responsive reaction of inflammatory cells secreting excessive mucus and causing pronounced inflammation in response to minor challenges.

GOOD MANAGEMENT ESSENTIAL

As with many diseases of the racehorse, management and husbandry have paramount importance in the development and progression of inflammatory airway disease. Poorly ventilated, dirty, dusty and humid environments can initiate and exacerbate airway inflammation. This in turn can aid infection or prolong the recovery from viral or bacterial infections.

As previously discussed, the stress of high-exercise levels and mixing of young horses from multiple sources creates the perfect environment for inflammatory airway disease to develop within racing yards.

Diagnosis of the disease involves recognition of the clinical signs; respiratory tract endoscopy and sampling. Endoscopy allows us to look inside horses’ airways and visualise any inflammation and accumulation of mucopururent discharge.

Samples can be obtained by flushing sterile fluid into the airway, then drawing the fluid back into sample pots. These samples can then be evaluated by specialist laboratories to identify the inflammatory cells present and any bacteria.

In modern racing yards, the total prevention of inflammatory airway disease and infections is an almost impossible task. This is because many of the risk factors previously stated are unavoidable. The challenge is to reduce the incidence of disease, and decrease the severity and chronicity of disease, thereby minimising the negative effects of underperformance and training days missed. As indicated earlier, strategically planned vaccination programmes should be initiated to control viral influence on disease.

It is vital that yards have well-ventilated, light and airy boxes that are mucked out and cleaned thoroughly to reduce the chance of environmental factors enhancing or predisposing airway inflammation. Adequate rest and turnout also relieves the stress of intensive training regimes and allows horses access to fresh air outside their boxes.

The speed of recovery will be aided by adjusting training regimes to a horse’s disease status, and resting clinically affected or recovering horses.

Any bacterial infections are treated with appropriate antibiotics, and mucolytic agents are administered to help to break down the thick mucopurulent discharge that builds up within the respiratory tract.

In some severe cases, where hypersensitive immune responses are present, courses of inhaled steroids may be indicated to help reduce inflammation in the airways.

Inflammatory airway disease is where all the above factors merge to create the commonly presented syndrome in young horses. Alone, many of these infectious agents or management factors can cause coughing and inflammation of the airways.

Fungal and allergic airway disease can cause inflammation within the respiratory tract. Accurate sampling and clinical examination are paramount in identifying and treating airway inflammation appropriately. Here at Mark Johnston Racing, as at every other thoroughbred racing yard in Britain, we are constantly at risk of having horses contract inflammatory airway conditions.

Every possible measure is taken to prevent, identify and treat appropriately all cases of suspected airway inflammation.

Neil Mechie is a graduate of the University of London and is in practice in Mark Johnston Racing. This article first appeared in the Kingsley Klarion.