Could Serum Collectin-11, surfactant protein-A and D be suitable biomarkers for assessing pulmonary damage caused by Rhodococcus equi infection in foals on large farms?

Rhodococcus equi (R. equi) is a primary cause of pyogranulomatous pneumonia of foals between three weeks and five months of age. Early diagnosis of rhodococcal pneumonia has always been considered a preferable approach as it can lead to more successful treatment and better outcomes. Horse stud farms where the disease is common, neonatal foals are subjected to tests such as complete blood count and fibrinogen analysis at certain intervals. However, new biomarkers are needed in addition to blood count and fibrinogen measurement in this field for early diagnosis of diseases. Based on this need, in this study, the diagnostic importance of CL-11, SP-A, SP-D was investigated for the early diagnosis of pneumonia in foals naturally infected with R. equi. In a case-control design, fourteen 1-5 month-old foals with proven R. equi pyogranulomatous pneumonia and 10 healthy 1-5 month-old control foals were enrolled in this study. The median white blood cell count (WBC) and fibrinogen concentration in the case group were significantly higher than in the control group. The median CL-11, SP-A, and SP-D concentrations in the case group were also significantly higher than in the control group. However, there were overlaps in concentrations between groups for SP-A and SP-D. There was less overlap between the groups for the CL-11 concentration. The CL-11 assay was sufficiently accurate but over-diagnosed R. equi infection. The correlation plot between fibrinogen and CL-11 concentrations shows that this problem may be solved if CL-11 is used as the first biomarker and fibrinogen as the second check for those foals with CL-11 serum levels >0.8 and ≤1.5 ng/mL. As a result, in this study, it is recommended to use CL-11 together with fibrinogen to obtain more accurate results in diagnosing pyogranulomatous pneumonia caused by R. equi in foals on large horse stud farms.