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Abstract
BackgroundTheileria equi is a tick-borne apicomplexan hemoparasite that causes equine piroplasmosis. This parasite has a worldwide distribution but the United States was considered to be free of this disease until recently.MethodsWe used samples from 37 horses to determine genetic relationships among North American T. equi using the 18S rRNA gene and microsatellites. We developed a DNA fingerprinting panel of 18 microsatellite markers using the first complete genome sequence of T. equi.ResultsA maximum parsimony analysis of 18S rRNA sequences grouped the samples into two major clades. The first clade (n = 36) revealed a high degree of nucleotide similarity in U.S. T. equi, with just 0–2 single nucleotide polymorphisms (SNPs) among samples. The remaining sample fell into a second clade that was genetically divergent (48 SNPs) from the other U.S. samples. This sample was collected at the Texas border, but may have originated in Mexico. We genotyped T. equi from the U.S. using microsatellite markers and found a moderate amount of genetic diversity (2–8 alleles per locus). The field samples were mostly from a 2009 Texas outbreak (n = 22) although samples from five other states were also included in this study. Using Weir and Cockerham’s FST estimator (θ) we found strong population differentiation of the Texas and Georgia subpopulations (θ = 0.414), which was supported by a neighbor-joining tree created with predominant single haplotypes. Single-clone infections were found in 27 of the 37 samples (73%), allowing us to identify 15 unique genotypes.ConclusionsThe placement of most T. equi into one monophyletic clade by 18S is suggestive of a limited source of introduction into the U.S. When applied to a broader cross section of worldwide samples, these molecular tools should improve source tracking of T. equi outbreaks and may help prevent the spread of this tick-borne parasite.
Highlights
Theileria equi is a tick-borne apicomplexan hemoparasite that causes equine piroplasmosis
Phylogenetics of the T. equi 18S rRNA gene All T. equi samples from the U.S grouped together into a single monophyletic clade based on 18S rRNA gene sequences
One of the 387 maximum parsimony (MP) trees based on 18S rRNA gene sequences is shown in Figure 1
Summary
Theileria equi is a tick-borne apicomplexan hemoparasite that causes equine piroplasmosis This parasite has a worldwide distribution but the United States was considered to be free of this disease until recently. When naïve horses are parasitized by T. equi the infection can cause a range of disease symptoms up to and including death This parasite is less prevalent in countries such as the U.S, Australia, England and Canada, even with transport regulations there is a potential for spread from infected horses or ticks from endemic regions [10]. Since 2008 numerous cases of T. equi infection have been identified in the U.S in California, Colorado, Florida, Georgia, Oklahoma and Texas These are most likely the outcome of importing asymptomatic horses that produced negative results during mandatory screening procedures [6,11,12]. DNA-based methods have been developed for screening horses and these may be more sensitive for detecting early infection, or very lowlevel parasitemia [15]
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