# Plasma C‐reactive protein and interleukin‐6 concentrations in foals during health and respiratory disease

## TL;DR

This study found that CRP and IL-6 levels in foals are not reliable indicators of respiratory disease during health or illness.

## Contribution

The study evaluates CRP and IL-6 as potential biomarkers for respiratory disease in foals, finding they are not predictive.

## Key findings

- CRP concentrations increased with age in foals, while IL-6 concentrations decreased.
- Respiratory disease did not significantly affect CRP or IL-6 levels at any age.
- Neither CRP nor IL-6 were suitable predictors of bronchopneumonia in foals.

## Abstract

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.

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

Prospective cohort study.

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.

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.

Timing of sample collection, small sample size.

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.