Research on the schizont cell culture vaccine against Theileria annulata infection in Xinjiang, China

Theileria annulata: Its Propagation in Rabbits for the Attenuation of Piroplasms in Cross-Bred Calves

Tropical theileriosis caused by the protozoan; Theileria annulata is a tick-borne disease (TBD) transmitted by ticks of genus Hyalomma; is clinically characterized by fever, anemia, and lymphadenopathy; and is responsible for heavy economic losses in terms of high morbidity and mortality rates with reduced production. Infected red blood cells of T. annulata were inoculated into rabbits intraperitoneally, and propagation of T. annulata has been investigated. The current study has shown an association between induced tropical theileriosis and variation of body temperature in rabbits. A significant rise in temperature (39.92 ± 0.33 °C) was seen on day 8 onwards, with the maximum temperature (40.27 ± 0.44 °C) on day 14 post-inoculation. In the current study, in vivo trials in susceptible cross-bred calves to investigate the attenuation and comparison with the infected group were also conducted. All the infected calves (n = 5) showed a significant rise in temperature (40.26 ± 0.05 °C) on day 10 onwards, with the maximum temperature (40.88 ± 0.05 °C) on day 16. The temperature of inoculated calves increased gradually post-inoculation, but the difference was not significant. A maximum parasitemia of 20% was observed in infected calves, but no piroplasm parasitemia was observed in inoculated calves. The prescapular lymph nodes of infected calves were enlarged, while the lymph nodes of inoculated calves remained normal throughout the trial. Analysis of clinical and parasitological responses of infected and inoculated calves showed a significant difference (p ≤ 0.05) in terms of temperature, parasitemia, and lymph node scoring between two groups. The current study was primarily aimed to attenuate T. annulata in rabbit and to check its virulence in susceptible calves. It is concluded that propagation of Theileria annulata in rabbits made it attenuated. Rabbit can be used as an in vivo model to weaken the virulence of T. annulata.

Tick control: Thoughts on a research agenda

Tick control is critical to the control of tick borne disease, while the direct impact of ticks on livestock productivity is also well known. For livestock, tick control today rests overwhelmingly on the twin approaches of genetics and chemical acaricides, although the disadvantages and limitations of both are recognized. The achievement of the full potential of vaccination, the application of biocontrol agents and the coordinated management of the existing technologies all pose challenging research problems. Progress in many areas has been steady over the last decade, while the acquisition of molecular information has now reached a revolutionary stage. This is likely to have immediate impact on the identification of potential antigens for improved vaccines and novel targets for acaricide action. In many circumstances, the rate limiting step in making scientific progress will remain unchanged, namely the resource constraint on evaluating these appropriately in large animals. For other approaches, such as the use of biocontrol agents, the limitation is likely to be less in the identification of suitable agents than in their delivery in an efficient and cost effective way. Our scientific understanding of the molecular basis for the tick vector-tick borne disease interaction is in its infancy but the area is both challenging and, in the long term, likely to be of great practical importance. What is arguably the most difficult problem of all remains: the translation of laboratory research into the extremely diverse parasite control requirements of farming systems in a way that is practically useful.

Vaccination against tropical theileriosis

Theileria annulata, the cause of tropical theileriosis is propagated in cattle with stage-to-stage transmission by Hyalomma ticks. Three stages in the life cycle of the parasite--tick-derived sporozoites, intramononuclear schizonts, and erythrocytic merozoites--infect cattle. When cattle are inoculated with schizont-infected cells, the parasite is transferred from the donor cell to the recipient. The main pathological damage in cattle is induced by the schizont stage. Each development stage of T. annulata elicits a specific immune response. Schizont-infected lymphoid cells can be grown indefinitely in culture and prolonged cultivation results in loss of virulence. Blood-derived schizonts induce stronger immunity than culture-derived schizonts, which suggests that restrictions on the parasite population or antigenic variation occur during prolonged cultivation. The duration of immunity following sporozoite or schizont infections has not yet been determined, but does not appear to be lifelong. The attenuated, culture-derived anti-theileria vaccine proved to be safe and effective in prevention of field theileriosis in large enzootic areas.

Culture-derived parasites in vaccination of cattle against tick-borne diseases

The major economically important tick-borne diseases of cattle are theileriosis, babesiosis, anaplasmosis, and cowdriosis. Culture-derived attenuated schizonts of Theileria annulata have proved to be safe for all types of cattle and they protect against tick-borne theileriosis. T. parva was also successfully grown in vitro; however, inoculation of cattle with allogeneic schizont-infected cells resulted in rejection and destruction of the parasites together with the host cells. The number of schizont-infected cells needed for immunization is greater than for T. annulata theileriosis. Culture-propagated Babesia bovis and B. bigemina were used for large scale vaccination in the field. An avirulent population of Babesia spp. was obtained by in vitro cloning; inoculation of cattle did not induce clinical babesiosis, but produced specific antibodies. Culture-derived exoantigens of Babesia spp. proved to be completely safe for cattle, however, they conferred less protection than live parasites. Cell-cultured Cowdria ruminantium was highly infective for susceptible animals but, attenuated in vitro, could offer a potential source for vaccination. Anaplasma marginale, successfully grown in tick cell culture, may be developed for vaccines. Factors that should be considered in the developing of vaccines against tick-borne diseases include: the protective immune response to the pathogenic parasite developmental stages, virulence, immunological strain differences, and antigenic variations in cattle and in culture.

Immunity and vaccine development in the bovine theilerioses

There are three economically important bovine Theileria species: Theileria annulata, which causes tropical theileriosis and occurs across north Africa and most of central Asia; Theileria parva, which causes East Coast fever and is found in East and Central Africa; and Theileria sergenti, which is predominantly a problem in Japan and Korea. Theileria annulata preferentially infects macrophages in vivo. It is controlled largely by means of live, attenuated vaccines, which are produced by prolonged tissue culture of the schizont-infected cells. The immunity induced in animals, which have either recovered from an infection or have been vaccinated (with an attenuated vaccine), is broad, solid and cell mediated. It is considered that the main effector cells are cytostatic macrophages that produce nitric oxide. Subsidiary roles for bovine leucocyte antigen (BoLA)-restricted, transiently appearing, cytotoxic T cells, and possibly also natural killer (NK) cells, have been identified. Cytokines such as tumour necrosis factor alpha (TNF-alpha) may have important roles, particularly in the induction of pathology. Matrix metalloproteinases have been implicated in the metastatic behaviour of schizont-infected cells. The nature of the protective schizont target antigens remains unknown. Attempts to develop a subunit vaccine have focused upon a sporozoite antigen (SPAG-1) and a merozoite antigen (Tams1). Both SPAG-1 and Tams1 have given partial protection using different delivery systems and adjuvants, but further vaccine development will probably require identification of a range of other antigens, especially from the schizont stage. Theileria parva has a tropism for T cells. Vaccination is currently by the 'infection and treatment' method, which involves challenging with a controlled dose of sporozoite stabilate and the simultaneous administration of long-acting tetracyclines. The immunity thus induced is mediated by BoLA-restricted cytotoxic T cells, which recognize polymorphic schizont antigens. These antigens have not been characterized at the molecular level. However, the polymorphic nature of the target antigens underlies the fact that the immunity is very strain specific--a situation that distinguishes T. parva from T. annulata. Interestingly, it is not possible to produce an attenuated vaccine to T. parva, as T. parva requires up to two orders of magnitude more schizonts in order to achieve transfer to the new host. A suggested reason for this is that the macrophage targets of T. annulata are phagocytes and thus the schizont has a natural, efficient route of entry whilst the preferred host of T. parva is the non-phagocytic T cell. Analysis of the cytotoxic T-cell response has revealed evidence of BoLA haplotype dominance plus competition between parasite epitopes. Subunit vaccination using a recombinant sporozoite antigen (p67) has proved very promising, with levels of protection of the order of 70% being achieved. A proportion of the protected calves exhibits complete sterile immunity. Interestingly, the basis for this immunity is not clear, since there is no correlation between the titre of antibodies that inhibit sporozoite penetration of lymphocytes and protection. Similarly, there is no significant T-cell response that distinguishes the protected and susceptible animals. These data are very encouraging, but other components, particularly those derived from the schizont, need to be identified and characterized. The mild Theileria species of Japan and Korea (termed T. sergenti in the literature) cause fever and severe chronic anaemia. The schizont stage of the life cycle is very rare and the host cell type is not known. The pathology is associated with chronic piroplasm infection. Immunity can be induced by immunizing with crude piroplasm extracts. Serological analysis of immune sera reveals that the immunodominant antigen is a polypeptide of 30-33 kDa, which corresponds to the protective T. annulata polypeptide Tams1. (ABSTRACT T