Search

Fifty-nine Hereford cattle susceptible to tick-borne diseases were used as tracer animals to assess the tick challenge and pathogenicity of Theileria parva under field conditions in Zimbabwe. They were moved periodically in groups of five to three commercial farms (one group consisted of four) during seasons of Rhipicephalus appendiculatus nymphal and adult activity. All tracer cattle were herded together with the farm cattle but were not dipped. The nymphal tick counts were high on two of the farms (up to 2000 per animal) but were very low on the third farm (less than ten per animal). On the three farms, 19 out of 24 (76%) tracers had patent Theileria schizonts. There was a range of clinical manifestations of theileriosis with acute and fatal infections occurring on one farm. The adult R. appendiculatus infestations during the wet season numbered 120-800 per animal on the three farms. The disease transmitted by the adults was very pathogenic on the three farms; 30 out of 35 (86%) had severe theileriosis infections. Cattle, which survived the nymphal diseases challenge, showed various degrees of immunity to subsequent T. parva challenge transmitted by adult ticks. Therefore, 13 out of 18 (72%) of these cattle had a second disease episode and the case fatality rate on the three farms was 46%. The factors which determined the epidemiological status of Theileria challenge on the farms, such as the farming systems and presence of wild animals, are discussed.

Five hundred and ten cattle were immunised using the Theileria parva (Boleni) stock without concurrent chemotherapy with tetracycline on 2 farms in Zimbabwe, both of which had a history of theileriosis. The stabilate had been titrated in Friesian calves to determine a 50% protective dose (PD50) and 2 or 3 (PD50s) were used to immunise the cattle. None of the cattle showed a clinical reaction following the immunisation procedure. However, the cattle were shown to have responded immunologically on testing for antibodies to a T. parva antigen in an indirect fluorescent antibody test. The immunised cattle were then exposed to a natural field challenge causing severe theileriosis in control cattle. Immunisation against theileriosis without the need for concurrent chemotherapy is much less expensive than the infection and treatment method (US $2.72) compared to US $10.23 in the first year) and would be much more attractive to commercial and traditional farmers.

Over 500 cattle of all age groups on 2 farms in Zimbabwe were immunised against theileriosis using the "infection and treatment" method and disease prevalence and their productivity assessed during a period of 18 months. The immunising stock, Theileria parva (Boleni) was isolated in Zimbabwe. None of the immunised cattle suffered from theileriosis upon natural exposure whereas 22 unimmunised cattle died of theileriosis and a further 48 required treatment for theileriosis. In the first year, some immunised cattle were maintained with minimal threshold dipping (once or twice during the rainy season). During periods of very high tick challenge of 100 to 1,000 Rhipicephalus appendiculatus per animal from January to March, a transient decrease in liveweight gain was observed particularly in cows. However, by the end of the period of observation, the weights had recovered so that intensively dipped and immunised and threshold or strategically dipped groups of cattle showed no significant differences. From the results it was estimated that each engorging female R. appendiculatus caused a temporary depression in weight gain of 8 grams. In young stock the weight loss was exacerbated by the presence of screw worm (Chrysomya bezziana) infestation. It was then possible to define an economically attractive integrated tick and theileriosis control strategy based on these findings, whereby immunised cattle were dipped 6 times between mid-December and mid-March. In this regimen, no weight loss occurred and no cases of screw worm were observed. For each of 3 herd sizes of 250, 500 and 1,000 cattle, comparisons were made of the costs of 4 different control options: (i) Intensive dipping (40 times/year) (ii) Intensive pour-on acaricide treatments (18 times/year) (iii) Theileria immunisation with strategic dipping (6 times/year) (iv) Theileria immunisation with pour-on treatment (4 times/year) It is concluded from these studies that, on farms where theileriosis is a serious problem, immunisation coupled with a strategic dipping programme is economically very attractive. In the year in which immunisation is carried out, costs will be higher than for intensive dipping, but from the second year on, the costs are decreased to approximately 50% of those for intensive dipping.

Theileria parva (Boleni) stock from Zimbabwe was used to immunise 24 susceptible Friesian calves by infection and treatment with oxytetracycline. Twenty-eight days after immunisation the animals in groups of 6 plus 2 susceptible controls were subjected to challenge: 3 groups with heterologous stocks and one group with the immunising stock. Theileria parva (Boleni) gave good protection against homologous challenge, the T. parva (Muguga, Kenya) and buffalo-derived T. parva (Serengeti transformed, Tanzania) parasite stocks. It did not protect against the T. parva (Kasoba, Malawi) stock and 3 out of 6 immunised cattle died and the remaining 3 had to be treated with parvaquone. In a second experiment, the 6 T. parva (Boleni) immunised animals which had received homologous challenge, together with the 2 controls which had recovered without treatment from T. parva (Boleni) infection, were challenged with the T. parva (Kasoba) stock. Four out of 6 of the immunised animals resisted the challenge with mild to moderate reactions. The other 2 animals had severe reactions and one died. The 2 control animals which recovered from T. parva (Boleni) infection resisted the T. parva (Kasoba) challenge and both had mild reactions. It is suggested that oxytetracycline used in the first experiment may have interfered with the expression of the full protective capacity against the virulent T. parva (Kasoba) stock. Further studies on the use of the T. parva (Boleni) stock without oxytetracycline treatment could identify a more broadly immunising effect and a more economical vaccination method.

Theileria mutans (Chisamba) was isolated from a steer at Chisamba, Central Province, Zambia by inoculation of blood into a susceptible unsplenectomised calf. The parasite was then transmitted on three occasions by nymphs and once by adult Amblyomma variegatum ticks. Macroschizonts, typical for T mutans, were detected in two calves for short periods. The parasite caused varying degrees of anaemia in all experimental calves, whose sera showed high antibody titres to T mutans in the indirect fluorescent antibody test.