Asian long-horned tick distributions in Virginia pastures and evaluating the horn fly as a possible vector of Theileria orientalis Ikeda

Abstract

The Asian longhorned tick (Haemaphysalis longicornis) is an invasive ectoparasite of growing concern in the U.S. due to its role as the primary vector of the emerging cattle parasite, Theileria orientalis Ikeda. To better understand the transmission landscape in Virginia, this study first evaluated the fine-scale environmental drivers of H. longicornis density. Across two field seasons (2023–2024), 25,929 ticks were collected and analyzed using negative binomial generalized linear mixed models. Results indicated that vegetation height was the most consistent predictor of density, with short vegetation supporting significantly fewer ticks than medium-height vegetation. While relative humidity was positively associated with nymphal density, landscape features like distance to trees or water sources were not significant predictors. These findings suggest that fine-scale pasture management may be a viable tool for reducing tick populations. However, because T. orientalis outbreaks have been observed in regions with no or low tick density, investigating alternative transmission pathways is essential. To address this, the second phase of this study evaluated the potential for the horn fly (Haematobia irritans), the most economically significant fly pest of U.S. beef cattle, to act as a vector. A total of 2,365 horn flies (254 pools) were collected from nine Virginia counties and screened using conventional and quantitative PCR. T. orientalis Ikeda DNA was confirmed in flies from four counties (Augusta, Bland, Culpeper, and Nottoway), with remaining samples either negative or below the limit of detection. Collectively, these findings provide a comprehensive look at the ecology of T. orientalis in Virginia. While vegetation management remains a key strategy for controlling the primary tick vector, the detection of Ikeda DNA in horn flies provides the first evidence of this pathogen in a novel arthropod species. This suggests that horn flies may serve as vectors, highlighting a critical need for further experimental studies to determine their epidemiological significance in cattle health management.