Theileria microti is a parasitic blood-borne piroplasm transmitted by deer ticks. It was previously in the taxonomic genus Babesia, as Babesia microti, until ribosomal RNA comparisons placed it in the sister genus Theileria. T. microti is responsible for the disease human theileriosis, similar to babesiosis, a malaria-like disease which also causes fever and hemolysis.
Within the medical community, as of 2012, this parasitic agent is still classified as Babesia microti.
The genome of Theileria microti (aka Babesia microti) has been sequenced. The sequence shows that this species does not belong to the either of the established genera - Babesia and Theileria - but instead belongs to a separate genus.
The mitochondrial genome is linear.
An important difference from malaria is that T. microti does not infect liver cells. Additionally, the piroplasm is spread by tick bites (Ixodes scapularis, the same tick that spreads Lyme disease), while the malaria protozoans are spread via mosquito. Finally, under the microscope, the merozoite form of the T. microti lifecycle in red blood cells forms a cross-shaped structure, often referred to as a "Maltese cross", whereas malaria forms more of a diamond ring structure in red blood cells. Living in red blood cells, T. microti is an important transfusion-transmitted infectious organism. Between 2010 and 2014 it causing four out of fifteen (27%) of transfusion-transmitted microbial infections (the highest of any single organism).
In May 2010, it was reported that a vaccine to protect cattle against East Coast fever had been approved and registered by the governments of Kenya, Malawi and Tanzania.
A vaccine to protect humans has yet to be approved.
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