Abstract
Mixed species infections of Theileria spp. are common in nature. Experimental and epidemiological data suggest that mixed species infections elicit cross-immunity that can modulate pathogenicity and disease burden at the population level. The present study examined within-host interactions, over a period of 13 months during natural infections with two Theileria spp., pathogenic (T. lestoquardi) and non-pathogenic (T. ovis), amongst a cohort of naive sheep in Oman. In the first two months after exposure to infection, a high rate of mortality was seen among sheep infected with T. lestoquardi alone. However, subsequently mixed-infections of T. lestoquardi and T. ovis prevailed, and no further death occurred. The overall densities of both parasite species were significantly higher as single infection vs mixed infection and the higher relative density of pathogenic T. lestoquardi indicated a competitive advantage over T. ovis in mixed infection. The density of both species fluctuated significantly over time, with no difference in density between the very hot (May to August) and warm season (September to April). A high degree of genotype multiplicity was seen among T. lestoquardi infections, which increased with rising parasite density. Our results illustrate a potential competitive interaction between the two ovine Theileria spp., and a substantial reduction in the risk of mortality in mixed parasite infections, indicating that T. ovis confers heterologous protection against lethal T. lestoquardi infection.
Highlights
Mixed species infections of Theileria spp. are common in nature
The more pathogenic T. lestoquardi was associated with higher density and mortality when present as a single infection relative to mixed infection with the nonpathogenic T. ovis
Co-infection with the two parasites species correlated with reduced mortality of Malignant Ovine Theileriosis (MOT) in sheep, supporting the hypothesis of heterologous protection proposed for other combinations of Theileria spp. mixed infection 9
Summary
Mixed species infections of Theileria spp. are common in nature. Experimental and epidemiological data suggest that mixed species infections elicit cross-immunity that can modulate pathogenicity and disease burden at the population level. Our results illustrate a potential competitive interaction between the two ovine Theileria spp., and a substantial reduction in the risk of mortality in mixed parasite infections, indicating that T. ovis confers heterologous protection against lethal T. lestoquardi infection. This study reports the first detailed longitudinal investigation of the dynamics (within-host interactions) of coinfection of two Theileria species, the pathogenic parasite, T. lestoquardi, the causative agent of MOT and the non-pathogenic, T. ovis, in a naïve cohort of sheep in Oman. The data suggest the occurrence of interactions between the pathogenic and less pathogenic species of Theileria and supports the hypothesis that concurrent co-infection can lead to a reduction in parasitaemia and mortality associated with the pathogenic s pecies[9] This premise was tested further in the current study
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