Studies Involving Three Species of Ticks (Acari: Ixodidae) Arising From an Outbreak of Equine Piroplasmosis (Theileria equi) in South Texas
Abstract
Equine piroplasmosis (EP) is a tick-borne disease of equines caused by Theileria equi. In 2009, an outbreak occurred in Texas, USA. Known intrastadial vectors of EP include Amblyomma mixtum and Dermacentor variabilis. The status of A. tenellum, a common tick on equines in Texas remains unknown. Male ticks remain on hosts to blood feed and mate but may transfer from one host to another during close contact and mutual grooming. The research presented here arose from this outbreak, and included four lines of investigation: 1) a cross-mating and development study of two sympatric and morphologically similar ticks, A. mixtum and A. tenellum, 2) the use of phylogenetic techniques (parsimony, maximum-likelihood, and Bayesian) to parse their relationships with A. maculatum and A. americanum using four genes 12S, 16S, COI, and ITS2 and two concatenated datasets, 3) the role of host-to-host transfer of male D. variabilis might have in the transmission and maintenance of T. equi using agent-based modeling (ABM), and 4) the creation of a population matrix model (PMM) for D. variabilis that incorporated the life history of male ticks. The cross-mating study showed that one female of the A. mixtum × A. tenellum cross produced larvae likely due to parthenogenesis than hybridization. Overall differences occurred among crosses for all comparisons of life history events. For the phylogenetic analyses, all gene topologies for the four Amblyomma species were similar expect for the COI which showed poor resolution in branch support.
Though the COI differed from the other genes, the concatenated datasets showed that it had little influence. Analyses revealed that A. americanum and A. tenellum are closely related. The ABM showed that the number of infected horses were influenced by the infection probability, and horses and ticks could maintain T. equi at low levels of infection and male transfer. The PMM showed a low abundance of transferring males and that the off-host stages to on-host stages were most the sensitive to changes in the transition parameter. These stages may be targeted for management and control of tick populations.
Subject
Ticksequin piroplasmosis
Theileria equi
Amblyomma imitator
Amblyomma tenellum
Amblyomma mixtum
Citation
Donaldson, Taylor G (2019). Studies Involving Three Species of Ticks (Acari: Ixodidae) Arising From an Outbreak of Equine Piroplasmosis (Theileria equi) in South Texas. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /187955.