Distribution of Theileria orientalis in Virginia Market Cattle, 2018-2020

Abstract

Theileria orientalis, genotype Ikeda, was recently detected in North America. Determining the emerging distribution of this pathogen is critical for understanding spread and developing management strategies. Whole blood samples were collected from cattle at Virginia livestock markets from September 2018 through December 2020. Animals were tested for T. orientalis using a universal and then genotype specific real-time PCR based on the MPSP gene. Prevalence for each genotype was analyzed for temporal trends and mapped by county. Spatial patterns were compared between genotypes and assessed for associations with habitat features, cattle movements through cattle markets and county proximity. Overall, 212 of 1980 samples tested positive for T. orientalis with an overall prevalence of 8.7% (172/1980) for genotype Ikeda, 1.8% (36/1980) for genotype Chitose, 0.2% (3/1980) for genotype Buffeli. The Ikeda genotype increased over time in northern and southwestern Virginia markets. The Ikeda and Chitose genotypes occurred in different regions, with little overlap, but for each genotype, spatial distribution was associated with a combination of cattle movements and environmental factors. Genotype specific qPCR testing and surveillance of cattle from across a wide area of Virginia are providing information on temporal, spatial, and other patterns for this emerging disease.

Keywords: Chitose genotype; Ikeda genotype; Theileria orientalis; prevalence; spatial distribution.

Figure 1

T. orientalis genotypes detected across Virginia and neighbors. Counties are colored as red if Ikeda was detected; yellow if Chitose was detected; orange if both genotypes were detected; blue if neither genotype was detected; grey for Virginian counties with no samples. Positive counties outside of Virginia are lighter shades of the same color scheme. Cattle markets are shown as triangles (northern region), circles (central region), and diamonds (southwestern region).

Figure 2

The predicted suitability map for the Ikeda genotype of T. orientalis. Orange and brown areas represent optimal habitat, while green areas are predicted to be less suitable habitat.

Figure 3

The predicted suitability map for the Chitose genotype of T. orientalis. Orange and brown areas represent optimal habitat, while green areas are predicted to be less suitable habitat.

Figure 4

Ternary graphs showing the weighted contribution of market sales, adjacency, and environmental similarity to county-level clustering of Ikeda (a) and Chitose genotypes (b). The color at each point represents the join count statistic, measuring the spatial autocorrelation for the combination of factors. The brightest peaks represent the optimal contribution that best explained the observed spatial association for each genotype.

Figure 5

Three month moving average of sample positivity by genotype for the northern (a), central (b) and southwestern (c) market regions. The total volume of testing is shown with grey dashed line on each graph.