Co-infection with theileria equi and babesia caballi in a yearling filly

Equine piroplasmosis (EP) is a widely spread tick-borne disease of horses, caused by the hemoparasites Theileria equi and Babesia caballi. Although most horses in endemic areas are subclinical carriers of parasites, acute or peracute disease may occur, especially when naïve horses are infected. This case report presents a characteristic case of peracute and fatal EP in a yearling filly, that was transferred from stable to pasture with first exposure to ticks. The filly was diagnosed with high parasitemia of both T. equi and B. caballi, with coinfection of both parasites within the same erythrocyte. Post mortem findings revealed multi-organ damage (spleen, kidney, lungs and muscles) and systemic bleeding. Quantitative PCR revealed high parasitemia of both parasites, consistent with acute and clinical infection in the sick filly. An epidemiological investigation including molecular analyses of blood samples was conducted on the farm, in which of thirteen horses, two were carriers of T. equi and one was a carrier of B. caballi with similar genotypes to the filly. The described case emphasizes the importance of implementation of preventive measures when transferring naïve horses to tick infected pasture in endemic areas. In addition, identifying the previously undocumented presence of co-infection within the same erythrocytes raises questions regarding the pathogenesis and cell invasion mechanisms of these parasites, with possible future therapeutic implications.