H.T. Koch,* L. Kambeva,* J.G.R. Ocama,* F.C. Munatswa,* T.T. Dolan+ and R.A.I. Norval+
* Veterinary Research Laboratory
P.O. Box 8101
Causeway, Zimbabwe
+ International Laboratory for Research on Animal Diseases
P.O. Box 30709
Nairobi, Kenya
East Coast fever caused by Theileria parva parva was eradicated from Zimbabwe in the 1950s, but a less virulent form of disease, caused by T. p. bovis and known as January disease or Zimbabwe malignant theileriosis, has persisted. Theileria p. bovis is of considerable economic importance to Zimbabwe, not because of the number of cattle deaths which it causes (1000-2000 each year), but because of the cost of its control by intensive dipping. Immunization offers an alternate and probably more economic means of control. This paper reports an immunization trial that was carried out in Zimbabwe in the 1986/87 and 1987/88 rainy seasons, using local isolates of T. p. bovis.
METHODS
The trial was carried out at Willsbridge Farm, approximately 100 km north of Harare. The farm was selected because it had severe outbreaks of theileriosis in the two rainy seasons prior to the trial. In the 1985/86 season four of five susceptible "bait" cattle exposed on the farm died of theileriosis.
Cattle for immunization were obtained from a farm without a history of pathogenic theileriosis. All were Sussex (Bos taurus) weaners, approximately 14 months old. Immunization using tick-derived stabilates was carried out at the laboratory and the cattle were moved to the farm and exposed to natural challenge at the start of the first rainy season (November 1986). Fifteen animals were immunized with the Boleni stock (Lawrence and Mackenzie, 1980) and 15 with a mixture of three isolates from the Willsbridge Farm. There were 10 unimmunized controls. The stabilates were prepared at a concentration of 10 ticks per ml. Doses of 0.1 ml, which had been shown in laboratory titration experiments to cause mild but immunizing reactions in cattle, were used to infect the cattle. No chemoprophylactic treatment was given.
In the 1987/88 rainy season eight susceptible weaners were exposed at the farm to compare their reactions to those of the cattle immunized and exposed in the previous year.
RESULTS
Severe reactions occurred in four of the Boleni- and two of the Willsbridge-infected animals during immunization and these were treated with parvaquone (Clexon, Coopers). One Boleni-infected animal died because treatment was administered too late in the course of the disease.
The maximum challenge with adults of Rhipicephalus appendiculatus in the 1986/87 season was approximately 100 per animal and occurred in late January. It was approximately 70 per weaner in the 1987/88 season, also in late January.
Reactions to Theileria infection are summarized in Table 1. In the 1986/87 season both the immunized and control groups became infected with a Theileria parasite between days 10 and 23 after initial exposure at the farm. The reactions were all mild and temperatures seldom rose above 39.5 °C. A second episode of infection, in which only the control group was affected, occurred between days 25 and 51 after initial exposure. Seven out of 10 had severe reactions with high temperatures and two died of theileriosis. There were no theileriosis reactions in the immunized cattle introduced in 1986/87.
Table 1. Results of exposure of immunized and control cattle at Willsbridge Farm, Zimbabwe
|
Treatment |
Number of animals |
First infection |
Second infection |
|||
|
Severe reactions |
Deaths |
Severe reactions |
Deaths |
|||
|
1986/87 |
||||||
|
|
Boleni stabilate |
14 |
0 |
0 |
0 |
0 |
|
|
Willsbridge stabilate |
15 |
0 |
0 |
0 |
0 |
|
|
Control |
10 |
0 |
0 |
7 |
2 |
|
1987/88 |
||||||
|
|
Control |
8 |
4 |
1 |
|
|
In the 1987/88 season four of the eight introduced controls had mild reactions and the other four had severe reactions, and one of these latter died of theileriosis.
DISCUSSION
It was concluded that the first infection detected in the 1986/87 season, in which both the immunized and control groups were affected, was caused by a mild Theileria species, possibly T. taurotragi. The second infection, which resulted in severe and fatal reactions in the controls, was almost certainly caused by T. p. bovis.
The trial showed that both the Boleni and the Willsbridge stocks protected cattle against field challenge. In unpublished laboratory experiments it has also been shown that the two stocks are cross-protective and that the Boleni stock protects against three other pathogenic stocks from different parts of Zimbabwe. The prospects of a vaccine against T. p. bovis based on a single stock thus appear to be good.
An important problem in this trial was the severity of some of the reactions to immunization, in which chemotherapy was not used. Severe reactions had not occurred when the stabilates had been tested previously at the immunization dilution in Bos taurus cattle. Further work is required to establish a method that is safe in all circumstances.
ACKNOWLEDGEMENT
This project was supported by the Food and Agriculture Organization of the United Nations/Danish Agency for International Development Projects GCP/ZIM/004/DEN and GCP/ZIM/012/DEN.
REFERENCE
Lawrence, J.A. and Mackenzie, P.K.I. (1980). Isolation of a non-pathogenic Theileria of cattle transmitted by Rhipicephalus appendiculatus. Zimbabwe Veterinary Journal 11: 27-36.
