Stress conditions do not affect Theileria equi parasitemia levels in sub-clinically infected horses

Stress has been suggested as a risk factor for Theileria equi peracute disease and may lead to relapse in clinical signs in chronically infected horses. The aim of this study was to assess the effect of stress on T. equi parasitemia in sub-clinically infected horses in two settings: horses hospitalized at a veterinary teaching hospital and horses from an endurance farm. Blood samples were collected from the hospitalized horses (n = 32) upon admission (T0) and at discharge (T1) from the hospital, and results were compared between horses that underwent surgery (stress) and other hospitalized horses (control). Blood samples were collected from an endurance farm (n = 20) six weeks before (T0) and two days after (T1) participation in an 80-km endurance event, and results were compared between horses that participated (stress) or did not participate (control) in the event. Theileria equi parasite load was determined using qPCR, and T1/T0 ratio was calculated for each horse. Mean parasite load at both time points did not differ statistically between the stress group and the controls in both settings. Theileria equi genotype was determined based on the 18S rRNA gene, when possible. Parasite genotypes were similar to strains previously characterized in the region and classified as genotypes A and D. The results of this study contradict the common assumption that stress may lead to increased parasite load in horses with a subclinical T. equi infection.