Plasma C-reactive protein and interleukin-6 concentrations in foals during health and respiratory disease.

Background: Early and specific diagnosis of bronchopneumonia in foals is important to prevent severe disease. In human medicine, C-reactive protein (CRP) and interleukin-6 (IL-6) are important diagnostic and prognostic biomarkers in neonatal pneumonia in other species. Evaluation of these markers in foals with naturally occurring respiratory diseases is lacking.

Objectives: To determine if CRP and IL-6 were useful predictors of respiratory disease in foals from birth to weaning.

Study design: Prospective cohort study.

Methods: Periodic blood samples from 200 initially healthy foals were collected from birth to weaning on a farm with endemic Rhodococcus equi and Streptococcus equi pneumonia. The foals were examined weekly by physical examination and trans-thoracic ultrasonography to determine the presence or absence of pulmonary consolidation and were divided into three groups after weaning: (1) foals that remained healthy; (2) foals that developed subclinical, mild, self-limiting pulmonary lesions; and (3) foals that developed severe pulmonary lesions and clinical pneumonia that required antimicrobial treatment. Thirty foals from each health group (N = 90 total foals) were randomly selected from the 200 initially enrolled for assessment of associations between CRP and IL-6 concentrations and health status. Data were analysed using linear mixed models, with p-values < 0.05 considered statistically significant.

Results: Age-related changes were found in both plasma CRP and IL-6 concentrations. Circulating concentrations of CRP were increased through weaning, while plasma IL-6 concentrations decreased through weaning. Respiratory disease did not significantly impact concentrations of CRP or IL-6 at any age.

Main limitations: Timing of sample collection, small sample size.

Conclusions: Neither IL-6 nor CRP concentrations were suitable predictors of subclinical or clinical bronchopneumonia in foals in this study. Further studies are needed to determine if more frequent measurement of these markers in foals at the time of pneumonia diagnosis provides helpful diagnostic or prognostic information.