Factors associated with seroconversion to the major piroplasm surface protein of the bovine haemoparasite Theileria orientalis

Jenkins and Bogema Parasites & Vectors (2016) 9:106 DOI 10.1186/s13071-016-1395-7 RESEARCH Open Access Factors associated with seroconversion to the major piroplasm surface protein of the bovine haemoparasite Theileria orientalis Cheryl Jenkins1* and Daniel R. Bogema1,2 Abstract Background: Bovine theileriosis caused by Theileria orientalis is an emerging disease of cattle in the Asia-Pacific region where it causes a significant economic burden to meat and milk production. While host immunological responses to the lymphocyte-transforming species of Theileria, T. parva and T. annulata, have been well studied, little is known about the immune response to this non-transforming species. Methods: We developed a recombinant antigen ELISA based on the major piroplasm surface protein (MPSP) of T. orientalis and investigated whether seroconversion to the MPSP was associated with clinical factors (anaemia), parasite burden and parasite genotype. We also examined the dynamics of seroconversion in animals acutely infected with T. orientalis. Results: In cattle testing qPCR positive for T. orientalis, seroconversion was more frequent in anaemic compared to normal cattle (P < 0.0001). The ELISA ratio (ER) was highly correlated with total parasite burden as measured by qPCR (r = 0.69; P < 0.0001); however when loads of individual genotypes of the parasite were examined, only the pathogenic Ikeda genotype was highly correlated with ER. Conversely, seroconversion was less frequently detected in the presence of benign T. orientalis genotypes. Temporal measurement of the serological response, parasite burden and packed cell volume (PCV) in acutely infected animals revealed that seroconversion to the MPSP occurs within 2-3 weeks of the initial qPCR detection of the parasite and coincides with a peak in infection intensity and a declining PCV. Conclusion: Whether the serological response to the MPSP is immunoprotective against re-infection or recrudescence requires further investigation; however the MPSP represents a promising target for a subunit vaccine given that genetic variability within the MPSP results in differential pathogenicity of T. orientalis. Keywords: Theileria orientalis, Major piroplasm surface protein (MPSP), Seroconversion, Anaemia, Genotype Background Theileria orientalis is an emerging apicomplexan pathogen of cattle in the Asia-Pacific region. Previously described as benign, this haemoprotozoan is now recognised as a disease of production cattle causing anaemia and ill-thrift. Mortalities of up to 5 %, particularly in pregnant heifers and calves, have been reported and the disease is also commonly associated with late term abortion. The bush tick Haemaphysalis longicornis is * Correspondence: 1 NSW Department of Primary Industries, Elizabeth Macarthur Agricultural Institute, Menangle, NSW 2568, Australia Full list of author information is available at the end of the article recognised as the vector for disease transmission [1, 2], and the geographic distribution of recent bovine theileriosis cases in Australia and New Zealand closely follow the known range of this species [2, 3]. Many recent studies have focussed on identification and differentiation of various genotypes of T. orientalis. These genotypes are defined based on sequence variations in the gene encoding the major piroplasm surface protein (MPSP), an immunodominant antigen expressed during both sporozoite and piroplasm phases of the T. orientalis life-cycle [4]. While eleven genotypes of T. orientalis have been identified globally (Types 1-8 and N1-N3) [5], disease outbreaks in cattle have been largely limited to Type © 2016 Jenkins and Bogema. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Jenkins and Bogema Parasites & Vectors (2016) 9:106 2 (Ikeda genotype) [6–9], with only occasional reports of clinical cases linked to other genotypes [10, 11]. Genotype 3 (Buffeli) [6, 7, 12] and its phylogenetic relative, Type 5 [7] have also been identified in Australian cattle, but these genotypes have not been associated with clinical disease and are considered benign. Like other apicomplexans [13, 14], T. orientalis infection most frequently presents as a mixture of genotypes, which likely facilitates evasion of the host immune system [15–17]. Indeed, infected cattle appear to retain the parasite for lengthy periods, perhaps for life [15]. Subclinical infections with T. orientalis including the Ikeda genotype are common [18] however; the immune mechanisms responsible for disease resistance are poorly understood. Naïve cattle introduced to areas where the disease is enzootic, as well as stressed, immunocompromised, pregnant or lactating animals are most at risk of developing clinical disease [6, 12, 19], while cattle in T. orientalis-endemic areas appear to develop a degree of resistance to disease. It is unclear whether the host develops a humoral response against the parasite prior to the intra-erythrocytic phase of the parasite’s life-cycle, or whether the immune response is largely cellmediated. Prior studies on T. annulata and T. parva suggest that responses against these parasites are largely cell-mediated [20, 21], however these organisms represent transforming theilerias which cause a tumour-like lymphocytic proliferation [22] not observed in T. orientalis infection. In T. orientalis, the MPSP is highly expressed during both the sporozoite [4] and piroplasm [23] phases of the parasite’s life-cycle and is believed to mediate entry into bovine erythrocytes via interactions with heparin-like compounds on the host cell surface [24]. Immunoblots using sera from infected animals indicate that the MPSP is strongly recognised by host IgG and that immunisation of cattle with MPSP is at least partially protective against T. orientalis [25]. Nonetheless, immune response to this antigen has not yet been quantified in cattle using ELISA, although this method was found to be a sensitive means of detecting T. orientalis infection in water buffalo [26]. In this study, we developed a recombinant MPSP ELISA to measure bovine IgG response to this major surface antigen and correlate this response with clinical disease, parasite genotype and infection intensity. Methods Samples A total of 430 EDTA blood samples and their matching sera were analysed in this study. Of these, 280 pairs of samples were collected by private and district veterinarians from 21cattle herds from the states of N (...truncated)