Applied research of an emerging condition: Theileria associated bovine anaemia

This issue of the New Zealand Veterinary Journal is a special issue highlighting some of the significant work carried out by veterinarians, laboratory scientists and veterinary parasitologists responding to the emergence of bovine anaemia associated with Theileria orientalis infection (TABA) in New Zealand.

The articles represent a collection of knowledge at a time early in the course of the epidemic, when it was evident that the impacts on dairy and beef farms were substantial, but the level of understanding of the disease's aetiology, pathogenesis and epidemiology was limited. Such information is critical for mounting an effective response. The articles demonstrate that veterinary science is fundamentally an applied science taking, at times, limited information and using this to inform disease management and drive required research.

The focus of the collected papers is on understanding the parasite and tick vector and on descriptive epidemiology of the disease including development of surveillance tools. The papers reviewing the protozoan parasite T. orientalis (Watts et al. 2016), and the biology, ecology and distribution of the tick vector Haemophysalis longicornis in New Zealand (Heath 2016), highlight the substantial complexity of the disease. There are interesting papers describing the epidemiology of the first 18 months of the TABA epidemic (Lawrence et al. 2016) and the farm-level impact of the outbreak (Vink et al. 2016), together with an examination of differences in prevalence by geographical region (McFadden et al. 2016a). Three papers describe diagnostic investigation and test development (Gias et al. 2016; Pulford et al. 2016a, 2016b) and a further paper attempts to determine the pathogenicity of a new-to-New Zealand haemoparasite diagnosed as a result of the increased surveillance effort. The key authors behind most of these papers are based within the Ministry for Primary Industries (MPI), reflecting the breadth of knowledge underpinning New Zealand's capability to investigate and respond to new and emerging animal health issues.

An epidemic is occurrence of disease to a level in excess of the expected (endemic) level (Thrusfield 2005). It was in this context that reports from the Northland region of disease, attributable to anaemia and associated with piroplasms, were investigated by the MPI veterinarians in 2012. Pulford et al. (2016a) describes the TABA outbreak on the index farm and the diagnostic investigative effort which led to the finding of T. orientalis Ikeda type by molecular testing. These findings led MPI to mount a biosecurity response.

Disease occurrence is the result of complex interactions between the host, the agent and the environment (Noordhuizen et al. 1997). Researching current understanding about the natural history of T. orientalis and H. longicornis and the specific environmental conditions, including management actions, which lead to outbreaks of disease was a key first step. This is reviewed nicely in two papers by Heath (2016) and Watts et al. (2016). The science reviews logically lead to identification of likely risk areas and times, potential risk factors and spread mechanisms. For instance, animal movements are likely to be a typical feature of new outbreaks as they introduce infected cattle and tick vectors into hitherto disease free areas, or conversely, susceptible cattle into infected areas. The complex effects of season and climate on the tick life stages and husbandry practice on occurrence and progression of theileriosis are also examined. This knowledge was the cornerstone of the entire response.

The ongoing collection and analysis of data from case farms provided epidemic-specific information. An epidemiological descriptive analysis of the patterns observed over the first 18 months of the epidemic by spatial location is summarised by Lawrence et al. (2016). This work adds weight to the existing knowledge base and describes how the disease behaves under New Zealand conditions. The methods used helped to answer the questions: “how is the epidemic spreading?” and “what is happening at this point in time?”, and were critical for situation reporting to industry stakeholders and decision makers. The mixed methods approach used by Vink et al. (2016) resulted in a much clearer understanding of how TABA presented at the farm level. They also hypothesised putative risk factors using a causal web that led, at the time, to development of practical screening tools for high-risk periods (Vink et al. 2013).

An epidemic can result from introduction of a new pathogen to a susceptible population. Depending upon the outcome of infection this can be followed by a variable period of stability as an endemic level of infection is reached. This is a dynamic process and driven by many factors including the ongoing availability of susceptible animals and the environmental conditions determining tick abundance over time (Norval et al. 1992; Deem et al. 1996; Jonsson et al. 2012). The methods described by McFadden et al. (2016a) look at regional distribution of potential tick activity versus presence of T. orientalis Ikeda type, explore the degree of endemic stability which exists, and report on areas most at risk of new outbreaks as well as those areas most risky to inward movement of naïve animals.

The demand from surveillance testing and additional studies required the application of quantitative PCR, which provided the capability to screen large numbers of samples rapidly and with precision (Gias et al. 2016), and drove development of new and more specific high-throughput molecular tests as well as defining their operating characteristics under New Zealand conditions, as described by Pulford et al. (2016b). Analysis of anomalies in test results lead to the finding of Candidatus Mycoplasma haemobos (McFadden et al. 2016b), a previously unrecognised haemoplasm, and attempts to define its pathogenicity.

Work reported in the articles making up this special issue increased the understanding of bovine haemoparasites in New Zealand. It also allowed science and timely response-specific information to be combined and communicated to large animal veterinarians dealing with outbreaks and with concerned clients, and informed diagnostic screening and management decisions. The work was done in consultation with industry but also done rapidly and, although substantial, represents knowledge at that early stage in the epidemic. The information was communicated in real-time through an industry working group to veterinarians and farmers; allowing the research outputs to be operationalised during these early stages. Future research is required to further understand the patterns of disease by area, season and farm type and the optimum management methods. The full spectrum of risk factors that lead to subclinical and clinical disease are not understood, in particular why some farms are so severely affected. Further work is also required to understand the production effects and economic impacts of infection with T. orientalis.

The diagnosis of T. orientalis Ikeda type in New Zealand, and its emergence as an important animal health issue, highlights the importance of the veterinary profession to national animal health surveillance, and in particular the early reporting of unusual disease events. Bovine anaemia associated with T. orientalis infection is having an impact on the dairy and beef farming landscape, the extent of which is not yet fully understood. What is certain is that the huge number of movements of cattle that typify the New Zealand cattle industry at certain times of the year are no longer without risk, especially if they involve movement of naïve cattle into infected areas (Lawrence et al. 2016).