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Tick Salivary Cholinesterase: A Probable Immunomodulator of Host–parasite Interactions

Temeyer, Kevin B., Tuckow, Alexander P.
Journal of medical entomology 2016 v.53 no.3 pp. 500-504
Culicidae, Ixodes scapularis, Rhipicephalus annulatus, Rhipicephalus microplus, acaricides, acetylcholine, acetylcholinesterase, amino acid sequences, antigens, blood meal, cattle, central nervous system, cholinesterase, dipping, engorgement, enzyme activity, genes, host-parasite relationships, human diseases, immune response, immunomodulators, malaria, pathogens, quarantine, saliva, ticks, toxicity, United States
The southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), is the most economically important cattle ectoparasite in the world. Rhipicephalus microplus and Rhipicephalus annulatus (Say) continue to threaten U.S. cattle producers despite eradication and an importation barrier based on inspection, dipping of imported cattle in organophosphate (OP) acaricide, and quarantine of infested premises. OP acaricides inhibit acetylcholinesterase (AChE), essential to tick central nervous system function. Unlike vertebrates, ticks possess at least three genes encoding AChEs, differing in amino acid sequence and biochemical properties. Genomic analyses of R. microplus and the related tick, Ixodes scapularis, suggest that ticks contain many genes encoding different AChEs. This work is the first report of a salivary cholinesterase (ChE) activity in R. microplus, and discusses complexity of the cholinergic system in ticks and significance of tick salivary ChE at the tick–host interface. It further provides three hypotheses that the salivary ChE plausibly functions 1) to reduce presence of potentially toxic acetylcholine present in the large bloodmeal imbibed during rapid engorgement, 2) to modulate the immune response (innate and/or acquired) of the host to tick antigens, and 3) to influence transmission and establishment of pathogens within the host animal. Ticks are vectors for a greater number and variety of pathogens than any other parasite, and are second only to mosquitoes (owing to malaria) as vectors of serious human disease. Saliva-assisted transmission (SAT) of pathogens is well-known; however, the salivary components participating in the SAT process remain to be elucidated.