8.2.3 Lower respiratory tract disease
Lower respiratory tract disease (LRTD) is a common cause of poor athletic performance. While it is well-recognised that respiratory tract infections result in poor performance, lower respiratory tract disease is still under-diagnosed. Reactive small airway disease, which commonly affects animals following respiratory infections, but which is exacerbated by poor air hygiene, is being recognised with increasing frequency.
When investigating any case presented for poor athletic performance, careful questioning about incidents of infectious respiratory disease in the individual animal and in other in-contact horses is important. It is also important to ask detailed questions about the stable management and, if necessary, to conduct a review of the stable environment. The type of bedding, feed and ventilation are key factors in the likelihood of allergic small airway disease (COPD) affecting an animal. Even details of the surrounding stables and air hygiene during transare important.
Details of how horses perform at exercise are also an important clue to the diagnosis of LRTD. Usually, animals gradually fade, although if the condition results in abnormal pressures within the upper airways it may induce dynamic obstruction with the result that horses suddenly stop. Exercise induced pulhaemorrhage (EIPH) is also a common cause of poor performance. This may be evident from the history, although many more horses bleed during a race without blood being seen at the nostrils.
Few animals are presented for investigation of potential cardiac disease when there is evidence of active respiratory infection. However, many are examined for this reason when they have clinical evidence of allergic small airway disease. Auscultation of lung sounds is an important part of the examination of horses which perform poorly and is greatly aided by the use of a re-breathing bag. A plastic bag such as a large dustbin liner is placed over the nostrils and held tight to produce an airtight seal. The bag should be prevented from occluding the nosThis makes the animal breath more deeply and consequently auscultation of abnormal respiratory sounds is easier. Fluid is often heard most clearly over the trachea at the level of the thoracic inlet. Deep breaths are often taken just as the bag is removed and may provide the clearest sounds. Some horses with marked COPD will cough during re-breathing. However, occasionally animals with significant LRTD have no abnormal respiratory sounds.
Clinical pathology Viral and bacterial respiratory infection may be suspected on the grounds of haematological findings. Paired viral titres and virus isolation are sometimes useful. Readers are referred to the reading list for further details. Culture of bacteria from lower airways is best performed by trans-tracheal lavage or using guarded endoscopic techniques.
Endoscopy Endoscopy is a very helpful diagnostic aid in cases of COPD. Long endoscopes (> 1.5 m) are particularly useful as they can be advanced to the carina. Exudate may be most easily seen after exercise, transportation, or the use of a re-breathing bag. Cytology can be performed from tracheal washes taken via an endoscope, but if bacterial culture is performed, the results should be interin the light of likely contamination by upper airway tract commensals.
Cytology Cytology is best performed on samples obtained by broncholavage, although tracheal wash samples may be sufficient for a diagnosis to be made. The cells should be examined as soon as possible; if this cannot be performed within a few hours the sample should be spun down and a smear made. COPD results in an inflammatory response. Haemosiderin filled macroare suggestive of a recent episode of EIPH. Detailed descriptions of cytological findings are found in relevant articles in the reading list.
Bacteriology Bacteriological culture of a tracheal wash should be performed as soon as possible after sampling. BAL samples are unsuitable. The significance of bacterial isolates from wash samples is a matter of some debate at the present time. It appears that, in some horses, LRT bacterial infection may be a more important cause of poor athletic performance than was previously thought. The preferred method of storage and preparation of cytological specimens depends on the laboratory and their advice should be sought prior to submitting samples.
Radiology The utility of radiology for detection of changes associated with COPD is a matter of some debate. Good quality radiographs are essential for meaningful interpretation. A bronchial and interstitial pattem has been regarded as a sign of small airway disease by some clinicians; however, a wide range of radiographic changes are seen in normal animals. If COPD is severe, it may result in small pulmonary vessels due to the raised intra-pulmonary pressure and an increase in interstitial and bronchial markings. Evaluation of these changes is extremely subjective except in severe cases. Exercise induced pulmonary haemay be evident as an area of increased density in the dorso-caudal extremity of the lung fields.
Nuclear scintigraphy Perfusion and ventilation scans are a useful method of evaluating the distribution of blood and air within the lungs. Both techniques require the use of a large-field gamma camera and a computer. Ventilation scans need additional specialised equipment in order that radionuclides can be delivto the lung and cleared without contamination of the environment. Aerosols of Technetium 99m are suitable. The technique is primarily a research tool.
Perfusion scans are more easily performed. They may be particularly useful in demonstrating the changes in perfusion of the dorso-caudal lung fields associated with EIPH. However, the technique is probably insufficiently sensitive to be used as a screening technique to detect cases of mild COPD.
Lung function tests Pulmonary function tests require expensive equipment and an expertise in exercise physiology. They have been used for many years as a research tool, primarily to attempt to quantify the level of fitness of a horse. More recently they have become a useful clinical aid.
The tests are performed on a treadmill. Most horses are able to gallop on a treadmill after two to three days of training. Pulmonary function tests and blood lactate measurement may be helpful in cases in which a diagnosis has not been reached using other diagnostic methods. They seldom provide a definitive diagnosis, but can be helpful to guide the diagnosis and to quantify the approximate level of athletic performance which an animal is capable of producing.