Differentiation between infectious and noninfectious disease and rapid initiation of accurate treatment are essential in managing diseases in the neonatal and young foal. Identification of useful inflammatory markers for these purposes is, therefore, of great importance. The aim of this study was to compare the responses of the acute phase protein serum amyloid A (SAA) with the responses of fibrinogen and total leucocyte and neutrophil counts in infectious diseases encountered in the young foal, and to assess whether SAA measurements give additional information useful in the management of these diseases. In a prospective study, foals (n = 25) showing clinical signs indicative of infectious disease were blood sampled on admission and then daily or every second day during hospitalisation. The main presenting signs were neonatal weakness (n = 9), pneumonia (n = 6) and diarrhoea (n = 10). SAA and fibrinogen concentrations on admission were higher in foals with bacterial infections (n = 8) than in foals with nonbacterial or uncertain diagnoses (n = 17). On admission, weak foals with negative blood cultures (n = 3) had normal SAA and fibrinogen concentrations and varying total leucocyte and neutrophil counts. Foals with positive blood cultures (n = 2) had markedly increased SAA, decreased or increased fibrinogen concentration and leuco- and neutropenia. Those with ambiguous blood cultures (n = 3) had moderate to markedly increased SAA concentrations and normal fibrinogen concentration, leucocyte and neutrophil counts on admission. All foals with negative or ambiguous blood cultures recovered and had normal or decreasing SAA concentration on discharge. Both foals with a positive blood culture were subjected to euthanasia. One foal born with equine herpesvirus-1 infection had moderately increased SAA and normal fibrinogen concentration and leuco- and neutropenia. Foals with Rhodococcus equi pneumonia had increased concentrations of all parameters on admission. On discharge, recovered foals had normal SAA concentrations, whereas fibrinogen and total white blood cell count and neutrophil counts were still increased. There were no consistent inflammatory changes in the parameters measured in diarrhoeic foals and there was no statistical difference between rotavirus-positive (n = 4) and -negative (n = 6) foals in this respect.
The results of this investigation suggest that SAA might be an aid in the differential diagnostic procedure of neonatally weak foals and in foals with diarrhoea as the main presenting clinical sign and that SAA measurements could add information in the monitoring of treatment in Rhodococcus equi pneumonia by responding more rapidly than the markers used to date.