Final report details

Bovine theileriosis caused by Theileria orientalis appears to be an emerging disease condition in eastern Australia and a sustainable method of control such as a vaccine is urgently needed. There are three variants of this parasite present in this country but only one is associated with disease. The purpose of this study was to determine from the literature how feasible the development of an effective subunit or live vaccine was likely to be. Not enough is known of the immune mechanisms of animals infected with Theileria for a subunit vaccine to be an option at the present time. However, development of a live vaccine based on one or more of the benign variants is worth considering even though significant differences exist between the variants. We recommend that the latter be progressed firstly by looking for evidence of cross-immunity between variants in small-scale pen and field trials. If there is cross-immunity, the technology exists in Australia to develop, register and produce a live vaccine.

The purpose of this study was to determine from the literature how feasible development of a vaccine for bovine theileriosis caused by Theileria orientalis is likely to be. This parasite is very common in Australia and is usually benign. However, major outbreaks of clinical disease have been reported in New South Wales during the past 5 years with mortality rates of up to 30% recorded in some herds.

A Theileriosis Working Group was formed to investigate this apparently emerging disease and rated, amongst others, the need to develop a vaccine as a priority for research. The exact cause of disease is still being investigated as part of MLA-funded project B.AHE.0038 but we already know there are three Theileria variants in Australia identified as ‘Ikeda’, ‘Chitose’ and ‘Buffeli’. Indications are that only Ikeda is associated with disease outbreaks.

Two suggestions for consideration have been put forward regarding development of a theileriosis vaccine:
1. Infection with the benign Buffeli and/or Chitose variants of Theileria will provide adequate cross-protection against Ikeda and the technology is available in Australia to develop, test and produce a live vaccine based on one or both of these variants.
2. An effective recombinant or subunit vaccine can be developed.

In this scoping study, these suggestions are tested by addressing the following six questions.

How sound is the hypothesis that exposure to Chitose/Buffeli might render animals immune to Ikeda•

The hypothesis implies that there is cross-immunity between T. orientalis variants. While the variants cluster together on phylogenetic trees, there are major differences between them at immunological and molecular levels. These differences are based on analysis of, and responses to variable surface antigens but may also apply to actual infections. There is, however, no evidence of any studies done on infection-induced cross-immunity between the variants.

Live Babesia bovis parasites induce strong, lasting heterologous immunity while purified antigens tested only stimulate homologous strain immunity and a similar situation may well exist with T. orientalis. It will be possible to determine the presence or absence of cross-immunity following prior exposure to Buffeli and/or Chitose variants in small scale pen trials and through retrospective and prospective studies of relocated mobs of cattle in the field.

How difficult will it be to immunise animals to Chitose/Buffeli and then challenge them with Ikeda to test the hypothesis•

It will be easy to determine the potency, dosage, route of administration and shelf-life of a live vaccine by measuring the infectivity in vaccinated cattle with the use of molecular, serological and microscopical assays. There is currently no acceptable laboratory challenge model; and it will be necessary to assess vaccine safety and the level of protection in the field.

What are the risks of vaccinated animals being carriers and a source of virulent infection to naïve animals should they be moved into a “clean” area•

The risk is considered acceptable. Both Buffeli and Chitose variants result in life-long infections, are highly infective for tick vectors and most likely endemic wherever the vectors are present and absent where they are not. Their inclusion in a vaccine is therefore unlikely to cause spread of disease. Because of a well developed host/parasite relationship, Buffeli is not associated with disease in cattle but there may be a potential risk of vaccine reactions in naïve, high risk classes of cattle if Chitose is included in the vaccine. In this event, certain precautionary measures will be indicated as is the case with tick fever vaccines.

How much is known of the complete genome sequences of Ikeda, Chitose and Buffeli variants•

Both Ikeda and Chitose variants have reportedly been sequenced but nothing has been published on this work.

What is the feasibility of developing a recombinant subunit Ikeda vaccine•

Development of a subunit vaccine that will protect cattle against Ikeda variant is not considered feasible, at least not in the short to medium term, irrespective of whether it is delivered as a protein in adjuvant or as a DNA construct. Because the harmful effects of T. orientalis are exerted mainly by the intra-erythrocytic piroplasms, the variable piroplasm surface proteins have received most of the attention in research on a subunit vaccine. Despite promising results of work done in the 1990s, progress over the past decade has been negligible. The slow progress being made in the development of vaccines against important vector-borne diseases such as malaria, East Coast fever and bovine babesiosis further suggests that it would be wise to focus on other means of control.

Is Biosecurity Queensland’s Tick Fever Centre interested in developing and producing a Theileriosis vaccine as part of its suite of products•

If Buffeli and/or Chitose variants are shown to stimulate heterologous immunity against Ikeda variant, the Tick Fever Centre (TFC) of Biosecurity Queensland will be interested in developing and producing a live vaccine based on one or both of these variants. It is likely that the procedures and facilities used to produce tick fever vaccines can be applied to prepare a cryopreserved theileriosis vaccine without the need for additional resources. The vaccine will probably be provided as a multidose frozen concentrate to be reconstituted and mixed with diluent before use in the same way as frozen tick fever vaccine (Combavac).

No costings have been done but the cost of production will depend, amongst others, on the number of variants to be included in the vaccine. As TFC is a subsidised, Queensland Government-owned service provider, it does not have access to venture capital for new initiatives and funding will be needed for the development, evaluation and registration of the vaccine.

Conclusions

Development of a subunit vaccine is not feasible at the present time.

Development of a live vaccine is an option worth investigating. It is recommended that this be progressed through the following sequential steps:
1. Provide proof of the principle in controlled pen trials and by monitoring relocated cattle on affected properties retrospectively and prospectively ;
2. Develop, evaluate and register the vaccine.