A Genomic Comparison of Theileria equi to the Novel Parasite Theileria haneyi to Identify Candidate Virulence Genes

Alexandria Wexler

doi:10.7273/000003784

Theileria equi (T. equi) is a parasite that belongs to the phylum apicomplexa and is an obligate intracellular parasite that infects horses and other equids. It is transmitted via tick vectors and first enters host lymphocytes, or immune cells, where it undergoes a transformation from the infective sporozoite stage to the asexually reproducing merozoite stage. These merozoites are released into the blood stream where they enter host red blood cells (RBCs) and replicate within them. The invaded RBCs eventually burst or lyse and release progeny merozoites into the blood stream, which enter more RBCs and continue the cycle of invasion and cell lysis. The lysing of RBCs eventually leads to RBC deficiency, or anemia, and clinical symptoms in the host known as the disease equine piroplasmosis. Some horses that develop piroplasmosis from a T. equi infection will die from severe anemia while a majority become chronically infected and are life-long carriers and reservoirs for transmission. For horses that live in non-endemic regions, where the parasite is not common, a T. equi infection is more likely to be fatal. For this reason, the trafficking of horses from endemic to non-endemic regions is highly regulated and surveilled by the United States Department of Agriculture (USDA). There is an abundance of research on the life-cycle and clinical outcomes of T. equi but very little is known about how this parasite induces the lysis of RBCs it infects. Furthermore, it is known that T. equi can persist in a life-long chronic infection, but how it evades the immune system for this duration of time is also not well understood. These factors contribute to the high virulence, or level of disease, which T. equi causes but lack of knowledge around their exact mechanisms makes it difficult to treat or prevent T. equi caused equine piroplasmosis. Recently, a new member of the Theileria genus was serendipitously discovered in a horse that had borderline positive tests for T. equi. Upon further investigation, this parasite was classified as a separate species but closely related relative to T. equi and was named Theileria haneyi (T. haneyi). Studies demonstrated that T. haneyi had a genome most similar to T. equi, compared to other well-known Theileria species, but that its genome was significantly smaller. Furthermore, horses infected with T. haneyi appeared to have much less severe clinical symptoms, indicating that the virulence of T. haneyi is much lower than that of T. equi. These two observations led to the hypothesis that the difference in genome size between the two parasites is due to genes in T. equi, not in T. haneyi, that code for virulence proteins. To test this hypothesis an in silico, or computer based, genomic comparison was carried out.

A Genomic Comparison of Theileria equi to the Novel Parasite Theileria haneyi to Identify Candidate Virulence Genes

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