One of the mysteries of the English language finally explained.
Any of several cattle ticks of the genus Rhipicephalus and subgenus Boophilus (family Ixodidae); specifically R. decoloratus, which is found mainly in sub-Saharan Africa and transmits babesiosis (redwater), anaplasmosis, and other parasitic diseases in wild and domesticated mammals.
Mid 19th century; earliest use found in Journal Royal Asiatic Society. From blue + tick, so called because of the blue appearance of the body of the engorged female.
blue tick/ˌbluː ˈtɪk/
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