S.P. Morzaria
International Laboratory for Research on Animal Diseases
P.O. Box 30709
Nairobi, Kenya
Since the development of the infection-and-treatment method of immunization against East Coast fever (ECF), Theileria parva infection of cattle (Radley, 1981), several field immunization trials have been performed in different countries of eastern, central and southern Africa. Most of these trials, which were carefully planned with appropriate controls and adequate supervision, have demonstrated that over 90% of the immunized cattle could be protected against natural T. parva challenge (e.g., Radley et al., 1975; Robson et al., 1977; Uilenberg et al., 1977; Paling and Geysen, 1981).
Immunization strategies and field-trial designs have varied in the different reports. One of the central strategy issues has been the choice of immunizing parasite(s), whether to use local stocks isolated in the area in which the trial is to be conducted or the "Muguga cocktail", a centrally prepared combination of stocks, in the infection-and-treatment schedule (Dolan, 1987). In this paper we report a systematic approach to immunization against ECF, using locally isolated stocks of T. parva in a geographically defined area in the Coast Province of Kenya. In this approach, a series of stepwise studies were performed, which involved selection of the trial site, a preliminary survey of the ticks and tick-borne diseases, isolation and laboratory characterization of T. parva stocks, selection of appropriate immunizing stocks, field immunization and productivity trials and the final production of bulk stabilate for a large-scale immunization in the chosen area.
THE STUDY AREA
The study area was the Kilifi District, which lies in the Coast Province of Kenya and covers an area of approximately 3000 sq. km. The area represents the northern coastal belt of Kenya and stretches 30 km inland. The annual mean rainfall is 1077 mm and the daily mean temperature varies between 24 and 32°C. Because of the high rainfall, agricultural potential is high. The cattle population at the time of the survey was approximately 150000, most of which were of the Bos indicus type. Ten percent of the cattle population was classified as grade, which consisted mainly of Bos indicus crossed with Bos taurus and pure milking breeds such as Jerseys, Ayrshires and Guernseys.
The main reasons for selecting the area were the following:
a) ECF was present in the areab) the disease was delineated by geographical and ecological boundaries
c) the coastal strip has a high potential for supporting intensive dairy and beef production systems
d) ECF is not complicated by T. p. lawrencei because the buffalo, which is a reservoir host of this parasite, is absent in this area
e) there was an increasing number of ECF-susceptible cattle due to regular dipping in the newly constructed dip tanks in the area
f) there was a good local infrastructure, with a newly built veterinary investigation laboratory at Kilifi
SURVEY OF TICKS AND THEILERIA PARASITES
Ninety-two farms in the regions of Malindi, Marikebuni, Kiswani, Kilifi, Bamba, Kaloleni and Mariakani were visited and 100 herds comprising altogether about 18000 cattle and representing approximately 12% of the Kilifi District's cattle population were sampled. Ticks were collected from 531 cattle for the identification and isolation of parasites and sera were collected for an examination for antibodies to T. parva and T. mutans.
The tick survey revealed the presence of Amblyomma gemma, A. lepidum, A. variegatum, Boophilus decoloratus, B. microplus, Hyalomma marginatum rufipes, Rhipicephalus appendiculatus, R. evertsi, R. parvus and R. pulchellus (A.D. Irvin, R.S. Chumo and P.R. Spooner, unpublished).
The serological investigations (Goddeeris et al., 1982) demonstrated a high prevalence of T. parva and T. mutans antibodies at all sites sampled. There was a good correlation between the distribution of R. appendiculatus and Amblyomma ticks and that of the animals serologically positive to T. parva and T. mutans, respectively.
ISOLATION OF THEILERIA STOCKS
Isolation of Theileria parasites was attempted either by the application of field-collected ticks to susceptible cattle in the laboratory followed by nymphal pickup with the laboratory stock of R. appendiculatus, or by exposure of susceptible naive cattle in the field followed by nymphal pick-up from the infected bait animals.
Seven Theileria isolates were made, of which five were identified as T. parva, one as T. mutans and one as T. taurotragi. The T. parva stocks were designated Marikebuni, Mariakani, Kilifi, Junju and Mavueni. The T. mutans and T. taurotragi stocks were referred to as Mitangoni and Malindi, respectively.
CHARACTERIZATION OF THE T. PARVA ISOLATES
Stocks of the T. parva field isolates were prepared from sporozoite suspensions derived from the laboratory infected ticks and cryopreserved as reference stabilates. They were characterized using in vitro and in vivo infectivity tests, cross-immunity trials, sensitivity to antitheilerial drugs and monoclonal antibody profiles (Minami et al., 1983). Initial cross-immunity studies showed that the mixture of Marikebuni and Kilifi stocks protected cattle against all the other stocks isolated. Therefore, a combination of these stocks was selected for field trials. Later cross-immunity studies showed that the Marikebuni stock protected cattle against all the stocks from the Kenya coast and several other stocks isolated from different parts of Kenya, and therefore the remaining field trials in Kilifi District were performed using only the Marikebuni stock.
FIELD TRIALS
Three sites were selected for immunization trials. These sites were selected on the basis of the presence of R. appendiculatus and ECF, accessibility, presence of adequate infrastructure (such as water, grazing and sampling facilities) and the potential for intensification of the cattle industry. The main objectives of the trials were to test (a) if different breeds (Bos taurus, Bos indicus and crosses) can be immunized with the selected parasite stocks and withstand unlimited tick challenge, (b) whether different cattle age groups can be successfully immunized and (c) whether 12- to 18-month-old cattle, immunized at 4 to 6 weeks of age and kept in a tick-free environment, could withstand field challenge. The three sites selected were the following:
a) Coast Agricultural Research Station, Mtwapa
b) Kibarani Farm, Kilifi
c) ADC Farm, Kiswani
In the first trial, at Mtwapa, young Jersey calves less than 1 year old were immunized against the Kilifi and Marikebuni stocks and all survived the field challenge, while the controls died of ECF. In the second trial, at Kibarani, 12- to 18-month-old Boran (Bos indicus) cattle from Kapiti were used. These were immunized with the Marikebuni stock when they were between 4 and 6 weeks old and kept tick-free before exposure. All the immunized cattle survived the challenge, while the susceptible controls died of ECF.
At the same site, another group of Boran cattle less than one year old were immunized with the Marikebuni stock and 35 days later exposed to natural challenge. All survived the challenge, while 77% of the controls died. Similar trials were carried out on a beef production system at Kiswani, where the cattle used were Sahiwal/Red Poll crosses under 6 months old and the immunizing stock was T. parva (Marikebuni). All the immunized cattle survived the challenge, while 71% of the controls died of ECF.
Several animals (not in the trials) became infected with ECF despite being sprayed twice weekly, suggesting that either the spraying was inefficient or the ticks were transmitting the parasite soon after attachment. To investigate this further, a pilot trial was carried out in which the effect of reducing the acaricide spraying frequency on immunized and unimmunized cattle was studied. One immunized group was sprayed twice weekly while the other was sprayed once every three weeks. Susceptible control cattle were included in each group. At the end of the exposure period, all the immunized cattle survived, while 83% and 67% of the susceptible cattle sprayed once every 3 weeks and 2 times weekly, respectively, died of ECF. The detailed results of these trials are presented in Morzaria et al. (1987).
During these trials several T. parva stocks isolated from the control cattle were characterized. All these isolates were found to be of T. p. parva type and belonged to one of the three monoclonal antibody groups as defined by Minami et al. (1983).
PRODUCTIVITY
The immunization trials showed that the Marikebuni stock provided immunity against a wide spectrum of T. parva challenge, suggesting that this stock would be appropriate for use in the Kilifi District. However, before wide-scale immunization could be recommended it was important to study the effect of immunization on productivity. Further, for planning rational tick control programmes it was necessary to assess the effect of different tick-control regimens on ECF immunity and the productivity of ECF-immunized cattle. With these objectives in mind, a productivity trial was carried out using beef cattle at Kiswani in which appropriate parameters for cost/benefit analyses were measured in different treatment groups.
The detailed results have been reported by Morzaria et al. (1988). Briefly, the immunized cattle showed better weight gains than the unimmunized controls. Among the immunized groups, cattle under twice-weekly spraying had the highest weight gains, followed by the group with acaricidal ear-tags, the group sprayed once every 3 weeks and the group without any tick control. The unimmunized animals, without tick control or with limited tick control, could not be maintained without significant losses due to ECF. A summary of the cost/benefit analyses of the various treatments are presented by Mukhebi et al. in these proceedings.
DISCUSSION AND CONCLUSIONS
A systematic approach to ECF immunization in the Kilifi District, on the Kenya Coast, was adopted which culminated in the identification of the Marikebuni stock of T. parva as providing wide protection against several stocks isolated from the same area. Immunization with this stock protected all cattle exposed to natural challenge at three widely separate sites within the district. The quality of immunity engendered was similar in Bos taurus, Bos indicus and their crosses. Cattle as young as 6 weeks old were immunized without any side effects. In addition, it was observed that ECF was endemic in the Kilifi District and challenge occurred throughout the year despite seasonal variations in tick challenge. Based on the in vivo and in vitro characterization of the field isolates, it was concluded that only T. p. parva parasites existed in that area and only limited immunogenic types might be present.
During this work several shortcomings in the design of the trials were identified. In all trials, cattle were exposed to unlimited tick challenge without acaricidal intervention. Consequently, other tick-borne diseases, especially babesiosis, anaplasmosis, benign theileriosis and cowdriosis, were diagnosed in the trial animals. In many instances multiple infections (e.g., with Trypanosoma spp., Babesia bigemina, Anaplasma marginale and T. mutans), together with heavy tick burdens, caused chronic anaemia and severe losses in productivity. Heartwater was responsible for several deaths. In addition, intercurrent diseases often caused problems interpreting the results of the trials. If cattle are exposed to unlimited tick challenge, it is recommended that very close supervision of the animals be undertaken and other diseases be treated promptly. Alternatively, the trial cattle should be immunized against other tick-borne diseases in the trial area or the tick exposure should be limited.
One problem encountered with the Marikebuni stock was that when some Bos taurus cattle or their crosses were immunized using one dose of long-acting oxytetracycline (Terramycin LA, Pfizer), the parasite caused severe ECF and animals had to be treated with parvaquone (Clexon, Wellcome) to prevent losses. The parasite was originally characterized in Boran (Bos indicus) cattle, which were safely immunized using one dose of long-acting tetracycline. Because of differences in susceptibility of cattle to T. parva infection, it is recommended that all immunizing stocks for field use be characterized in highly susceptible breeds.
The Marikebuni stock has been prepared as a bulk stabilate and further characterized (see Mutugi et al., this meeting). The systematic approach described has provided sufficient information on its safety and wide immunogenic properties to justify its use on a wide scale in the Kilifi District.
REFERENCES
Dolan, T.T. (1987). Immunization to control East Coast fever. Parasitology Today 3: 4-10.
Goddeeris, B.M., Katende, J.M., Irvin, A.D. and Chumo, R.S.C. (1982). Indirect fluorescent antibody test for experimental and epizootiological studies on East Coast fever (Theileria parva infection in cattle): evaluation of a cell culture schizont antigen fixed and stored in suspension. Research in Veterinary Science 33: 360-365.
Minami, T., Spooner, P.R., Irvin, A.D., Ocama, J.G.R., Dobbelaere, D.A.E. and Fujinaga, T. (1983). Characterization of stocks of Theileria parva by monoclonal antibody profiles. Research in Veterinary Science 35: 334-340.
Morzaria S.P., Irvin, A.D., Taracha, E., Spooner, P.R., Voigt, W.P. and Katende, J. (1987). Immunization against East Coast fever: the use of selected stocks of Theileria parva for immunization of cattle exposed to field challenge. Veterinary Parasitology 23: 23-41.
Morzaria, S.P., Irvin, A.D., Wathanga, J., D'Souza, D., Katende, J., Young, A.S., Scott, J. and Gettinby, G. (1988). The effect of East Coast fever immunization and different acaricidal treatments on the productivity of beef cattle. Veterinary Record 123: 313-320.
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