1. Immunization procedure and constraints
2. Problems of trial management and data collection
3. Data collection - Design and analysis
4. Impact of immunization on tick control
5. Future proposals and recommendations
The infection and treatment method of immunization against ECF is described in detail in the Food and Agriculture Organisation Handbook on "Ticks and Tick-borne Disease Control" (FAO, Rome, 1984 pp 457-507). This method is now being used in a number of field situations (see Summary of Field Reports), but in order to maximise its effectiveness and impact the current Meeting identified a number of areas where particular attention is required as the immunization method becomes more extensively applied in the field. These are described below:
Infection and treatment is an effective method of immunization against ECF but as it becomes applied more extensively in the field its use must be closely controlled and monitored, at least in the initial stages of its use.
It is essential that proper planning takes place before immunization is carried out, whether it is an experiment, a field trial with appropriate controls, a pre-extension trial with limited controls, or the application of a vaccine in a national disease control scheme. Planning must include the capacity for data collection and analysis. Before undertaking immunization adequate funding, equipment, consumables, facilities and staffing must be available.
A decision has to be made whether to use local isolates, a centrally prepared isolate(s) or a combination of isolates originating from another country.
The age at which immunization is applied and the need to re-immunize will be dictated by management and experience, but it has been shown that calves can be immunized as early as one month. Reduction in tick control frequency may be used to boost immunity. Seasonal variation in challenge may influence the timing of immunization.
Recommendations
i) Planning
Planning of immunization should involve consultation with statisticians and economists on the design and data recording for analysis.
ii) Parasite stocks for immunization
Well-characterized stabilates (for example, Muguga cocktail) should be used for immunization in the field. The extent of the area in which the stabilate(s) will be used should be decided by the national government based upon epidemiological information. Isolation of local stocks, especially breakthrough strains, should be encouraged for further characterization, determination of their immunological relationship to the immunizing stock(s) and their potential use for immunization.
Original isolates should be preserved as reference stabilates. These stabilates should be used to prepare sequential bulk working stabilates with minimal manipulation to avoid parasite selection.
Immunizing stabilates must be characterized by testing for infectivity and drug sensitivity. Their monoclonal antibody profile should be determined.
A standard procedure for safety testing of stabilates for immunization should be adopted. It should include screening for viruses and other pathogens.
Infection rates in ticks used for preparation of bulk stabilates should be determined.
Studies should be carried out on the loss of infectivity of stabilates after thawing and on their long-term deterioration in storage. All stabilate information should be properly recorded.
iii) Immunization procedure
Only healthy animals should be immunized and, if practical, animals should be tested for antibodies to T parva and other tickborne diseases before immunization.
The extent of the area in which local isolates will be used should be determined.
Initially, immunization should be carried out to protect high producing cattle in risk areas. This should include cattle belonging to small holder dairy farmers. As immunization becomes more widely available, cattle in other sectors can be immunized.
The effects of immunization on fertility, pregnancy and milk production should be investigated taking into account the effects of both stabilates and drugs.
Successful trials and immunization programs depend on careful planning, constant monitoring during the application phase, effective reporting and analysis of results, and a follow-up phase to assess the long-term effects of immunization.
Recommendations
i) Planning and sampling
The frequency of sampling should be determined by the data required, the costs of data recording and the stresses caused to cattle through excessive sampling. Once established, the sampling protocol should be strictly adhered to as the omission of data causes serious problems in analysis.
Data should be accurately recorded on data sheets following a set sampling procedure. Duplicates should be made to insure against loss. It is better to monitor a limited number of cattle well than a large number badly.
The reliability of data recording should be closely monitored. A high degree of precision is required with the recording of data such as liveweight. Cattle scales should be regularly calibrated and serviced. If possible, electronic scales, which are portable and accurate, should be used.
ii) Management
It is important to establish effective working relationships between the trial manager and project staff, and to ensure the motivation of the staff. Effective communication between the laboratory and the trial site is essential, particularly with regard to disease diagnosis.
Unanticipated problems can be caused by drought and concurrent infections, especially with other tickborne diseases. Cattle in ECF immunization field trials may need to be immunized against other tickborne diseases, such as babesiosis, anaplasmosis and heartwater (cowdriosis).
In southern Africa where there is a seasonality in the lifecycle of the vector it is important to ensure that there is adequate disease challenge during the trial period.
iii) Data sheets
A set of data sheets was designed by a sub-committee appointed by the Meeting. Examples of these sheets are shown in Appendix II. In order to obtain uniformity in recording and analysis of data it is recommended that these sheets be used in immunization trials.
The collection and analysis of data are of great importance. The data can be used to evaluate the immunization procedure scientifically and to assess the productivity associated with field immunization programmes. If an integrated approach to control of tick-transmitted diseases is to be adopted, more extensive data on tick challenge will be needed.
The information priorities for economic analysis are: the production effects of immunization using different techniques, and the production effects of tick control policies in immunized cattle. These will need to be estimated for each production system under consideration. If these effects are estimated in terms of their impact on production parameters, then the effect on production efficiency can be calculated from a productivity index which can be considered, in economic terms, from the point of view of the farmer or the nation.
Recommendations
i) Data collection and analysis
Researchers preparing experimental designs are recommended to seek professional advice on preparing a suitable protocol before experiments are conducted, and so avoid problems of generating data which cannot be analysed.
Livestock productivity indices should be evaluated with a view to selecting a standard for use in economic assessments of ECF immunization.
Where trials of the effects of ECF immunization or tick control on production are performed, estimates of all production parameters used by the productivity index should be obtained, not just those that might be affected by the tick or tickborne disease constraint. Where estimates of other production parameters are not already available, sensitivity analysis with a productivity index can identify the most important parameters.
Preliminary evaluations using 'worst' and 'best' case assumptions can be used to identify production systems in which immunization might be worthwhile. Experimental work and trials should be directed on the basis of such preliminary evaluations.
The design of experiments and trials should be kept as simple as possible; for example, if the effect of immunization per se is not of interest in a trial of tick control policies in immunized cattle, then non-immunized controls are not required. If designs are too complex there is a danger that the whole experiment may fail.
Excessive handling of experimental animals is likely to affect their performance, making them atypical of the population under study.
Large-scale immunization schemes should have a component designed to monitor productivity and detect problems which could arise during the program. These can then be investigated by further experimental work.
Frequent interim analyses should be conducted so that early recognition of unexpected events can be recorded and any necessary interventions carried out.
ii) Data Handling and processing
ILRAD should become the centre for computerized storage and software development of data records during the development stages of immunization programs. This would require due consideration to the reliability and confidentiality of the data sets. Once suitable software systems are available to handle ECF immunization databases, computer training should be considered.
As immunization programs develop, tick control practices can be modified but, initially, careful monitoring of tick levels and their impact on the host, is required. As appropriate data is obtained, recommendations can be made regarding alteration of tick control practices.
Recommendations
i) Tick identification and counting
Experiments require the accurate identification, by a reliable worker, of the tick species involved, as the easily-confused rhipicephalid species may be present. The data should come from initial half-body collections from 5 to 10 animals.
Tick counts should be made from half the host body, with separate counts of the ear ticks, thus taking account of ECF and non-ECF vector species which could affect overall productivity.
If possible, male/female/standard female data should be obtained; this can be obtained from periodic half-body collections from the same animals chosen initially from a random sample of the cattle.
Tick counts should be made at a maximum of once per week or a minimum of once per month.
The host sample size should be determined following a statistician's advice based on local conditions.
Tick numbers should be assessed by a rapid rating by a trained observer on half the body. A possible rating for total ticks would be: 1 = 0-10; 2 = 11-50; 3 = 51-250; 4 = 251-750; 5 = greater than 750. A similar rating on a generic basis could be devised.
Ticks should be rated monthly if possible (this is a minimum in Central/Southern African countries with strict seasonality of tick infestation).
ii) Tick control
Tick control regimens in ECF immunization trials should take into account the likelihood that tick numbers tolerated by cattle owners will increase greatly when the tickborne disease problem has diminished after immunization programs.
In principle, control should be at three levels of intensity, that is, complete, moderate or none. In practice, this might mean control twice weekly, once every 3 to 4 weeks, or never. Under strict seasonality conditions, moderate control should be at 2 to 3 week-intervals for the season of adult tick abundance. This can be modified such that the moderate intensity control is based on a predetermined tick infestation threshold. The threshold should be a group mean infestation likely to cause slight damage, i.e. at least a total of 100 adults per side of the host. A comparison of results obtained from the 'moderate' and 'never' tick-controlled groups should allow acceptable minimal tick control regimens to be determined.
If past experience is favourable the 'complete' regimen can be omitted. Alternatively, if other tickborne diseases are feared, the existing tick control intervals should be progressively doubled with conservative time steps, provided there is no unacceptable increase in tickborne disease incidence.
Novel methods of tick control can be included in trial procedures where appropriate.
Immunization against ECF must form part of integrated management strategies. Before tick control policies can be radically changed, immunization against anaplasmosis, babesiosis and cowdriosis must be considered where these diseases are a problem. Some immediate reduction in tick control intensity should be possible; further reduction would be dependent upon the establishment of the economic threshold for each particular situation.
Epidemiological data, as well as long-term animal production data, is needed to develop and test models for disease and production modelling. Important features of both the epidemiology of theileriosis and immunization are (1) occurrence of antigenically distinct strains, (2) duration of immunity in different situations and (3) age susceptibility and immuno-competence.
Recommendations
i) Application and impact of immunization
Immunization against ECF should form part of an integrated program which includes tick control, chemotherapy and management.
The possible effect of immunization in modifying the epidemiology of ECF and other tick-borne diseases, should be closely monitored.
Large-scale pre-extension immunization trials at the farm/ranch level should be implemented coupled with long-term monitoring of immunized animals.
Improved methods of immunization against all tickborne diseases should be developed.
ii) Collaboration
ILRAD should play a major role in strain identification and collaborate in the development of models for epidemiology and monitoring animal production, as well as in data processing and analysis.
Close collaboration between all disciplines should be maintained with full participation of national staff in all aspects of immunization programs. Provision should be made for formal and inservice training of national staff.
At the Second Annual Review of the FAO/DANIDA program for tick and tickborne disease control in East and Central Africa it was proposed that a meeting be held in October 1986 in Addis Ababa, to examine in detail the economic aspects and the effects of ticks and tickborne diseases on productivity of cattle. This proposal is supported.
