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Genetic characterization of Theileria equi infecting horses in North America: evidence for a limited source of U.S. introductions

Hall, Carina M. and Busch, Joseph D. and Scoles, Glen A. and Palma-Cagle, Kristina A. and Ueti, Massaro W. and Kappmeyer, Lowell S. and Wagner, David M. (2013) Genetic characterization of Theileria equi infecting horses in North America: evidence for a limited source of U.S. introductions. Parasites & Vectors, 6 (35). pp. 1-12. ISSN 1756-3305

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Publisher’s or external URL: http://dx.doi.org/10.1186/1756-3305-6-35


Background: 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. Methods: We 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. Results: A 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. Conclusions: The 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.

Item Type: Article
ID number or DOI: 10.1186/1756-3305-6-35
Keywords: Babesia equi, Theileria equi, Equine piroplasmosis, 18S rRNA gene, Microsatellite, Population genetics
Subjects: Q Science > QH Natural history > QH301 Biology
NAU Depositing Author Academic Status: Faculty/Staff
Department/Unit: Research Centers > Center for Microbial Genetics and Genomics
Date Deposited: 22 Dec 2015 21:27
URI: http://openknowledge.nau.edu/id/eprint/1797

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