A sample of cells from the respiratory tract of the horse can be obtained by the vet using an endoscope that is introduced into the trachea. Saline is flushed via the scope into the windpipe and drawn back up with a syringe and the 'wash', a suspension of cells and mucus is delivered to the laboratory for analysis.
The wash is taken post exercise so that a representative sample of cells are present in the trachea at scoping and the vet can score the horse for the presence of mucus and blood which may appear after exertion.
This mucus and/or blood is scored on a scale from 0-3 in increments of 0.5 and these scores contribute to the interpretation of the wash results.
In the laboratory the cells in the wash are concentrated onto a slide so that they can be stained, identified and quantified. If necessary the wash can be incubated on agar plates overnight so that significant bacteria can be identified and their susceptibility to different antibiotics determined.
The cells in the wash will usually consist of the following:
NEUTROPHILS - present in low numbers in a normal wash ie <10%. Increased numbers are an indicator of infection that can be bacterial or viral, or inflammation. Increased numbers of neutrophils will cause the horse to cough and/or produce excess mucus or bleed and is associated with poor performance. Clumps of these cells are often present post infection even though the overall numbers are normal and probably represent residual areas of inflammation and can be associated with bleeding.
MACROPHAGES - though these cells are present in significant numbers ie <50% in a normal wash excessive numbers especially >60% are associated with 'thick winded' horses, production of excess mucus and bleeding. Increased numbers are usually seen post infection when the airway is in a 'sensitised' state, unable to cope with fast work until the numbers of these cells go back to normal.
EPITHELIAL CELLS - these are the normal cells lining the trachea and would be present in high numbers ie >50% in a normal wash.
HAEMOSIDEROPHAGES - the macrophages are scavenger cells and will mop up the red cells shed into the lungs if the horse bleeds. If a macrophage contains the breakdown products of red cells it is classified as a haemosiderophage. Although it is normal to see some of these in the wash of a horse in fast work excessive numbers are associated with exercise induced pulmonary haemorrhage(EIPH) and the inconsistent performances of a 'bleeder'.
Other Cells that are often present in low numbers are EOSINOPHILS which are associated with allergic conditions or lungworm, SQUAMOUS EPITHELIAL CELLS which indicate that the wash has been contaminated with the cells from the upper respiratory tract because the horse has coughed or resisted when the scope was introduced.
If the cytology of the tracheal wash sample shows significant abnormalities or the vet has scored the horses trachea 1 or above for mucus the wash is plated onto nutrient agar and incubated for up to 48 hours.
If significant numbers of bacteria that are known to cause respiratory infections are grown they are then re incubated against a panel of commonly used antibiotics so that their sensitivity is assessed and the correct antibiotic prescribed.