USDA, ARS, Animal Disease Research Unit, 3003 ADBF, Washington State University, Pullman, WA 99164, USA.
Parasit Vectors. 2013 Oct 23;6(1):306. doi: 10.1186/1756-3305-6-306.
The apicomplexan hemoprotozoan parasite Theileria equi is one of the etiologic agents causing equine piroplasmosis, a disease of equines that is endemic throughout large parts of the world. Before 2009 the United States had been considered to be free of this parasite. Occasional cases had occurred but there was no evidence for endemic vector-borne transmission in the U.S. until a 2009 outbreak in Texas in which Dermacentor variabilis and Amblyomma cajennense were implicated as vectors. Although D. variabilis has previously been shown to be a competent laboratory vector, studies suggested A. cajennense was not a competent transstadial vector, even though the presence of this tick species on horses in South American is epidemiologicaly correlated with higher a prevalence of infection. In this study we tested the transstadial and intrastadial vector competence of D. variabilis and A. cajennense for T. equi.
A tick passaged T. equi strain from the Texas outbreak and ticks colonized from engorged females collected off horses on the outbreak ranch in Texas were used for these studies. Nymph or adult ticks were fed on infected horses and transmission fed on naïve horses. Infections were tracked with PCR and serology, dissected tick tissues were tested with PCR.
A. cajennense transmitted T. equi intrastadially when adult ticks acquired infection by feeding on an infected horse, and transmitted to a naïve host on subsequent reattachment and feeding. D. variabilis failed to transmit in the same experiment. Transstadial transmission was not successful for either tick species. PCR on DNA isolated from eggs of females that had fed on an infected horse suggests that there is no transovarial passage of this parasite by either tick species.
This work confirms that ticks from the Texas population of A. cajennense are competent intrastadial vectors of T. equi. We propose that the most likely natural mode of transmission for this parasite/vector combination in the Texas outbreak would have been biological transmission resulting from adult male ticks moving between infected and uninfected horses. The intrastadial mode of transmission should be considered as one equally possible scenario whenever implicating vectors of T. equi.
锥虫属血液原生动物寄生虫马媾疫锥虫是引起马梨形虫病的病原体之一，该病是世界各地马属动物的地方性疾病。2009 年以前，美国被认为没有这种寄生虫。虽然偶尔会有病例发生，但在美国没有证据表明存在地方性媒介传播，直到 2009 年德克萨斯州的一次疫情，其中涉及到了璃眼蜱和肩突硬蜱作为传播媒介。尽管璃眼蜱此前已被证明是一种有能力的实验室传播媒介，但研究表明肩突硬蜱不是一种有能力的经卵传递媒介，尽管这种蜱种在南美的马身上存在与更高的感染率有关。在这项研究中，我们测试了璃眼蜱和肩突硬蜱对马媾疫锥虫的经卵传递和经代传递能力。
我们使用了从德克萨斯州疫情中分离出来的、经蜱传代的马媾疫锥虫株，以及从德克萨斯州疫情牧场收集的、从饱血雌蜱分离出来的蜱。若虫或成虫蜱通过感染马的叮咬来获取感染，并在随后的重新附着和叮咬时传播给未感染的马。通过 PCR 和血清学来跟踪感染情况，用 PCR 检测被解剖的蜱组织。
当成年蜱通过叮咬感染马获得感染时，肩突硬蜱可以在体内传递马媾疫锥虫，并且在随后的重新附着和叮咬时可以传递给未感染的宿主。在同一实验中，璃眼蜱未能传递。对于这两种蜱来说，经代传递都不成功。从吸食感染马的雌蜱所产的卵中提取的 DNA 的 PCR 结果表明，这种寄生虫没有通过这两种蜱进行经卵传递。
这项工作证实了来自德克萨斯州肩突硬蜱种群的蜱是马媾疫锥虫的有能力的体内传递媒介。我们提出，在德克萨斯州的疫情中，这种寄生虫/媒介组合最有可能的自然传播方式是通过感染和未感染的马之间移动的成年雄蜱进行生物传播。在暗示马媾疫锥虫的媒介时，应将体内传递方式视为一种同样可能的情况。
