D.L. Berkvens, D.M. Geysen and G.M. Lynen
Belgian Animal Disease Control Project
Provincial Veterinary Office
P.O. Box 510016
Chipata, Zambia
The Belgian Animal Disease Control Project started its activities in October 1982. Monitoring sentinel herds along an east-west transect and surveying the Eastern Province of Zambia indicated that East Coast fever (ECF) was spreading westward together with its vector (the Rhipicephalus appendiculatus/R. zambeziensis complex), largely through cattle being moved from epidemic areas to clean areas further west. In 1982/83 a dip-tank barrier was erected in Katete District and compulsory dipping was enforced in the district to stop this spread and to protect Petauke District. Simultaneously a start was made with the isolation of local Theileria parva strains in an attempt to control the disease in the area east of the barrier through immunization.
By mid-1987 the disease and its vector had invaded Petauke District despite the control measures, putting another estimated 40000 head of cattle at risk. Currently ECF appears to have reached some form of endemic stability in Chipata, Chadiza and Lundazi districts and parts of Katete District. The disease is mainly confined to calves, and the majority of calves have been in contact with T. parva by the age of 30 months, the mean age at first contact being 10 months. ECF kills about 50% of the calves. The area is considered marginal for the vector, resulting in a relatively low tick challenge. An anomalous epidemiological feature is the occurrence of a second generation of adult ticks at the start of the dry season in May/June, increasing the incidence of ECF at this time of the year. The western part of Katete District and the eastern half of Petauke District make up the epidemic area. Large-scale epidemics usually erupt at the start of the dry season, when cattle are allowed to graze on free range in the seasonally waterlogged grasslands called "dambos".
In October 1987 a bulk stabilate was produced and tested in titration trials. A training course for the veterinary assistants to be involved in the immunization campaign was organized. A calf census was undertaken and a representative sample of the calves (about 2000) was ear-tagged to allow an assessment of the immunization campaign. Between November 1987 and January 1988 a total of 4800 calves between 2 months and 1 year old were immunized in Chipata District. The start of the trial was delayed due to the late arrival of essential supplies. The rains had started and a natural ECF challenge was present during most of the campaign. The overall turnout of calves was 78% and co-operation from the farmers was generally satisfactory. A certain suspicion about the whole exercise was discernible, and it was obvious that the immunization would be blamed for all fatalities afterwards, even those due to totally unrelated causes. In April and May 1988 a second campaign was undertaken in Chipata District, aimed at the calves over 2 months old. A total of 1100 calves were immunized during this campaign.
The veterinary assistants were asked to make regular follow-up visits to farmers, elicit reports from the farmers on sick and unthrifty animals, submit samples for laboratory diagnosis and provide extra treatment with parvaquone and/or tetracyclines where deemed necessary. Sera from 600 immunized animals were tested for antibodies to T. parva one to two months after the immunization to provide further information. Although it was practically impossible to maintain control animals throughout the area, a fairly accurate idea of the natural ECF challenge was obtained. A few farmers who had agreed to have their calves ear-tagged later refused to have them immunized. When they realized the benefits of the immunization, their calves were closely monitored for ECF incidence and treated where required. In other areas, co-operation of the farmers was very poor. Once again, a great deal of information was gathered on the incidence of ECF when they approached project staff for assistance with drugs. The information thus gathered indicated a normal incidence of ECF, with about 70-80% of the calves under 1 year old becoming infected and an estimated 30-40% of fatal cases.
Apart from logistic problems during the actual campaign, which were solved as they cropped up, the main problem was the evaluation of the results achieved. The consensus of the veterinary staff and the farmers was that the immunizations had drastically reduced the calf mortality in the area. It proved impossible, however, to obtain accurate information on the number of fatal cases directly attributable to the immunization and the number of fatal ECF cases in immunized animals. Cases requiring further treatment were most frequently encountered in herds where the general condition of the animals was poor (nutritional status of calves, worm burden, management). It therefore appears dangerous to extrapolate results from well-managed experimental animals directly to a field situation where numerous other factors interfere with the immunization programme.
Too few calves were ear-tagged and the recollections of farmers as to which animals had or had not been immunized were unreliable. Of all the fatalities reported and unequivocally substantiated as being due to ECF, three involved definitely immunized calves. Even in these cases it could not be determined whether the calves harboured an already active ECF infection at the time of immunization or whether their deaths were due to the immunizing infection. A total of about 150 calves received further treatment after the immunization, but again it was not possible to determine which animals would have succumbed eventually to ECF and which would have survived without treatment. Therefore systematic record-keeping by the veterinary assistants should be established, the records to include herd size and composition and numbers of births, deaths (plus causes of deaths), sales, purchases and slaughters. This should be done for all farmers and villages involved in the programme and the record-keeping should be monitored regularly.
In one particular area (Chikando-Sairi) other tick-borne diseases were sporadically recorded in immunized calves. Of a total 800 immunized calves, 3 confirmed cases of babesiosis were reported and an estimated 45 calves subsequently died during an outbreak of sweating sickness in April 1988. The latter was confirmed clinically and Hyalomma truncatum adults were present in abnormally high numbers with an average of 10-15 per sick calf examined, compared to less than 1 per animal in previous years. This might have been the result of the unusually heavy rains during March and April, but was more likely due to the fact that farmers had stopped dipping their animals completely after the introduction of a dipping fee. These outbreaks emphasize the necessity of maintaining some form of strategic tick control, even when the incidence of ECF has been reduced or eliminated.
Finally, the question of financing the immunization programme remains. The total cost of such a programme includes the actual production cost of the stabilate, including the purchase of chemicals, and the cost of the drugs to be used during the campaign. Furthermore, transport and facilities and equipment needed for the immunization have to be budgeted for. Lastly, the veterinary assistants and their associates need vehicles (motorcycles and fuel or bicycles) to enable them to prepare for the campaign, assist researchers during the campaign and monitor the immunized calves afterwards, especially during the first years of an immunization programme, when most problems arise. The importance of this aspect was underlined in the second campaign, when a considerable number of farmers refused to present their calves for immunization because they feared they would be charged for the drugs in the same way that they were charged for dipping their animals.
An attempt was made in July and August 1988 to collect detailed information on the effect of the immunization campaign on the overall calf mortality in Chipata District. The results from the veterinary camps for which sufficient information was gathered are summarized in Table 1. This table indicates that a significant reduction in mortality (about 80%) occurred in the immunized calves, and the results confirm the general impressions of the veterinary assistants and the cattle owners.
Table 1. Percentage mortality in immunized and non-immunized calves in Chipata District, Zambia
|
Veterinary camp |
Total no. of calves |
Percentage immunized |
Mortality |
|
|
No. immunized |
No. non-immunized |
|||
|
Chiwale Zule |
651 |
75 |
8 |
49 |
|
Mtenguleni |
855 |
62 |
3 |
14 |
|
Macro |
504 |
94 |
2 |
10 |
|
Msandile |
164 |
76 |
0 |
14 |
|
Chiparamba |
304 |
84 |
5 |
60* |
|
Kalichero |
377 |
83 |
3 |
Unknown |
|
Kanyanja |
706 |
37 |
18** |
50 |
* Mortality of non-immunized calves: percentage of calf mortality in the sentinel herd between December 1982 and November 1986. (Nine calves were immunized in this herd; all are still alive.)** The mortality figures have not yet been verified.
