Epidemiology and Vectors of Theileria orientalis on the Northern Tablelands of New South Wales, Australia

Abstract

<p>Bovine theileriosis, caused by the <i>Theileria orientalis</i> complex, causes a mild persistent infection with the severity dependent on the infecting genotype and host exposure status. Clinical theileriosis was first detected on the Northern Tablelands of NSW in 2009 and a high prevalence of infection in cattle reported in 2013 but the parasite was not genotyped and likely vectors not investigated. In response to the ongoing clinical cases, a cross-sectional (December 2017 - April 2018) genotypic study on eight farms within 150 km of Armidale on the Northern Tablelands of NSW and a longitudinal (March 2018 - May 2019) vector study on subset of six farms shown to be<i> T. orientalis</i> positive was conducted. As part of investigation for potential mechanical vectors in the region, <i>Culicoides</i> species which were trapped previously were analysed and subsets of species known to take blood meal from cattle were used for the PCR screening of<i> T. orientalis</i>.</p> <p>Genotype-specific multiplex qPCR on ninety blood samples for screening of <i>T. orientalis</i> revealed a 100% cattle prevalence with concurrent infection of the genotypes (Ikeda, Chitose and Buffeli) present in 73% of cases. Mixed infections with Chitose and Buffeli were common compared to Ikeda and Chitose, while Ikeda only infection accounted for the least. The prevalence of the pathogenic genotype (Ikeda) differed significantly between farms; however, the level of parasitemia was not affected by genotype nor associated with clinical disease. A retrospective analysis of the incidence of clinical cases associated with Ikeda infection between 2007 and 2019 on properties in the Northern Tablelands and the neighbouring North Coast local land services regions revealed that the first detections of Ikeda associated theileriosis occurred in 2009 in both regions. Interestingly, cases occurring on the Northern Tablelands were more often linked to cattle introductions up until 2016, after which time the majority of cases occurred in homebred cattle or on properties with no history of recent introductions. This is indicative of the parasite becoming endemic on the Northern Tablelands after 2016. Moreover, the high prevalence of co-infection with the <i>Theileria</i> genotypes confirms the lack of cross-protection between genotypes.</p> <p>Drag-sampling for the collection of questing ticks on the six farms resulted in morphological detection of <i>Haemaphysalis bancrofti</i> in only one of the farms. Larvae accounted for 59% of the ticks followed by nymphs (34%) and adults (7%). The phenology of tick capture revealed that the majority of larvae (90.5%) were collected in autumn, nymphs (89.4%) in spring and adults (84%) in summer. However, no ticks of any lifecycle stage were detected during the winter months. Screening of tick sections pooled depending on developmental stages for <i>T. orientalis</i> using multiplex qPCR resulted in detection of the Ikeda and Buffeli genotypes in only nymphs.</p> <p>As part of the seasonal dynamic study of potential vectors of<i> T. orientalis</i> in the region, the same farms for the tick survey were also used for the collection of biting insects. A total of eight genera of biting flies comprising of eleven species were collected using unbaited Nzi traps. The tabanid or march flies were detected in all six farms. <i>Dasybasis oculata</i> (43.6%) and <i>D. circumdata</i> (27.6%) were the most abundant and widespread species while the Stomoxyini flies were less common. The abundance and diversity of species was higher in summer (38%, eight species caught) and autumn (36.7%, seven species caught) than spring (25.3%, six species caught). In addition to the biting flies, sucking (<i>Linognathus vituli</i> and<i>Haematopinus eurysternus</i>) and biting (<i>Bovicola bovis</i>) cattle lice were collected during the cooler months (July to November 2018) of the year and from all farms except the later species which was detected in only one of the farms.</p> <p>The retrospective data on <i>Culicoides</i> between 1990 and 2018 provided a comprehensive list of species trapped across the three subregions (Tablelands, Slopes and Plains) from the New England region of NSW. Nineteen species were identified with eight of the most abundant species made up 99% of the total trap catch. <i>Culicoides marksi</i> and <i>C. austropalpalis</i> were the most abundant and widespread species. A trend towards higher abundance during the warmer seasons and at lower altitudes was observed for<i> C. marksi</i> and <i>C. austropalpalis</i> indicating dispersal of these species in the New England is primarily dependent on temperature and rainfall. Subsets of <i>Culicoides</i> species known to feed on cattle (<i>C. brevitarsis</i>, <i>C. dycei</i>, <i>C. nattaiensis</i>, <i>C. victoriae</i>,<i> C. marksi</i> and <i>C. bundyensis</i>) were subjected to the qPCR testing for the presence of<i> T. orientalis</i>. </p> <p>Longitudinal half-sections from tabanids, <i>Stomoxys</i> spp., <i>Haematobia</i> and pools made from longitudinal half-sections of five lice, and a pool of ten <i>Culicoides</i> were all screened for the presence of<i> T. orientalis</i> using PCR. As for the tabanids, subsamples of approximately half-longitudinal section from a pool of five were also screened for<i> T. orientalis</i>. PCR screening for<i> T. orientalis</i> of these hematophagous insects resulted in parasite detection in the tabanid and Stomoxyini flies, biting midges and sucking lice. The highest proportion of positive samples were for<i>Haematopinus eurysternus</i> (4/4 pools) and <i>H. irritans exigua</i> (6/15 individuals) with detection in these species the first in Australia. The same also goes to the detection of the parasite in <i>Culicoides brevitarsis, C. victoriae</i>, <i>D. circumdata</i> and in the <i>Stomoxys</i> species. To conclude, detection of the parasite in these potential vectors indicates a possible role in mechanical transmission of<i> T. orientalis</i> and may partly explain the ubiquitous presence of <i>Theileria</i> in areas where ticks are absent. Transmission through transplacental means and husbandary practices also cannot be ruled out.</p>