There are 2 commonly used methods of sampling airway secretions in horses: bronchoalveolar lavage (BAL) and tracheal wash (TW) (also transtracheal aspirate (TTA) less commonly used). Either can be submitted for bacteriology and/or cytology but there are significant pro’s and con’s of each:
Bacteriology
Sparse bacterial growth in TW fluid is not likely to be relevant to the disease process. If quantitative bacteriology is performed then >105 colony forming units per ml is likely to be relevant although this probably adds little extra information to ‘sparse, medium or heavy’ as a description of bacterial growth - medium being of possible relevance and heavy being of probable relevance. The bacterial species cultured is also an important consideration. The most important agents associated with primary bacterial airway are b-haemolytic streptococci (S. equi subsp. equi and S. equi subsp. zooepidemicus), a-haemolyitc streptococci (S. pneumoniae), Bordetella bronchiseptica, Actinobacillus sp and Pasteurella sp. Other bacterial species are often found but tend to be regarded as contaminants and secondary invaders.
Cytology
TW generally has more neutrophils and less lymphocytes than BAL. The normal cellular constituents of TW and BAL fluid are shown below.
TW | BAL | |
| PMN (%) | <50 | <10 |
| LC (%) | <10 | <50 |
| Monos (%) | 40-80 | 40-80 |
| Eos (%) | <2 | <2 |
| Mast cells (%) | <1 | <5 |
Occasionally increased numbers of eosinophils are seen for example in rare cases of lungworm or eosinophilic interstitial pneumonitis. Some young horses with ‘Inflammatory Airway Disease’ (IAD) may be found to have a high percentage of mast cells. Rarely high lymphocyte percentages are seen possibly as a result of respiratory viral disease?
By far the commonest abnormal finding in cytological analysis of airway secretions is neutrophilia. The wide normal range for TW neutrophils makes this finding difficult to interpret and many normal horses in dusty environments show marked TW neutrophilias in the absence of disease. BAL is far more specific for detecting genuine airway neutrophilia due to the smaller reference range. BAL neutrophils accounting for > 10% total cells confirms lung disease. In adult horses, airway neutrophilia usually arises as a result of allergic lung disease caused by ‘recurrent airway obstruction’ (RAO - the term now preferred for the conditions previously known as chronic obstructive pulmonary disease (COPD) and summer pasture associated obstructive pulmonary disease (SPAOPD)). However, infectious lung disease (bacterial or viral) will usually result in airway neutrophilia also. These disease types cannot always be reliably differentiated simply on BAL although generally the magnitude of the neutrophilia is higher in RAO cases (eg typically 30-70% PMNs and up to 90%+) than infectious diseases (eg. rarely >30% PMNs although can be much higher in some cases) and also the PMNs appear more ‘healthy’ and less degenerate in allergic airway disease compared with infectious airway disease. The results of TW culture, clinical signs and history also add important differential diagnostic information.
© The Liphook Equine Hospital 2009
