J.D. Woodford,* J.G. Gayer,* T.T. Dolan+ and S.P. Morzaria+
*Smallholder Livestock Development Project
P.O. Box 368
Pemba, Tanzania
+ International Laboratory for Research on Animal Diseases
P.O. Box 30709
Nairobi, Kenya
Present address of J.D. Woodford:
Tana Beles Project
P.O. Box 101463
Addis Ababa, Ethiopia
Pemba Island has a population of approximately 50000 short-horned East African Zebu cattle, whose productive performance is extremely low. A major priority of the Ministry of Agriculture and Livestock Development (MALD), Zanzibar, is to improve milk and beef production on the islands of Unguja and Pemba by upgrading indigenous Zebu cattle using artificial insemination from exotic Bos taurus and Bos indicus semen.
East Coast fever (ECF), caused by Theileria parva, has been one of the main causes of the high mortality in first cross Bos taurus and Bos indicus calves on Pemba. Immunization against ECF using the infection-and-treatment method seems an attractive means of controlling this disease, as recent evidence suggests enzootic stability for other tick-borne diseases, particularly babesiosis, which are known to occur on Pemba. For this reason Rhipicephalus appendiculatus ticks were collected from Zebu cattle at two widely separate locations on Pemba and T. parva isolates were prepared and characterized. Stabilate 2913, designated T. parva Pemba/Mnarani, was chosen for use in an immunization trial to be conducted on Pemba (Morzaria, Irvin and Dolan, 1986).
IMMUNIZATION TRIAL
The characterization of T. parva (Pemba/Mnarani) stabilate 2913 involved drug sensitivity tests and homologous and heterologous isolate challenge using Boran (Bos indicus) steers. Mild immunization reactions were recorded when a single dose of long-acting oxytetracycline (Terramycin LA, Pfizer) at 20 mg/kg was administered along with an immunization dose of 1 ml of stabilate 2913 at a 1:10 dilution.
Nineteen pure-bred Jersey (Bos taurus) calves and 2 Jersey × New Zealand milking Zebu (NZMZ) cross-bred calves aged 13-15 months were immunized on Pemba as described above. Nine pure-bred Jersey calves and 1 NZMZ calf (Friesian [Bos taurus] 75% × Sahiwal [Bos indicus] 25%), all of the same age, were used as controls and were given long-acting oxytetracycline at 20 mg/kg only.
Of the 21 immunized cattle, 2 died of clinical ECF following immunization and 1 calf, which was in poor condition at the time of immunization, suffered clinical ECF and was killed by dogs. Nine immunized calves suffered severe clinical reactions but recovered following repeated treatments (in some cases as many as six) with parvaquone (Clexon, Wellcome) at 10 mg/kg. The nine remaining calves suffered mild immunization reactions and recovered without treatment.
Prior to field exposure all calves were treated with a prophylactic dose of imidocarb dipropionate (Imizol, Wellcome) at 2 mg/kg. Field exposure was carried out on rough grazing where indigenous Zebu cattle were known to have grazed recently. Tick challenge was extremely heavy, especially of R. appendiculatus and Boophilus microplus. Fourteen days after the beginning of field exposure a mean tick count of 107 adult ticks was recorded on the head and ears.
All calves were found to have T. parva schizonts in lymph gland biopsy smears from day 15 following exposure. On day 23, 22 of the 28 calves were found to have B. bovis and B. bigemina piroplasms in blood smears. In addition, piroplasms of T. mutans were seen in the blood smears of 25 of the calves and a few had Anaplasma marginale. Imidocarb dipropionate treatment was repeated, but in spite of this and parvaquone therapy, three immunized and three control animals died of ECF, complicated by concurrent tick-borne diseases.
DISCUSSION
The 18 calves that survived immunization fell into two distinct categories: 9 calves suffered severe clinical reactions and required parvaquone therapy and 9 calves suffered mild reactions and made spontaneous recoveries. During field exposure 7 of the 9 calves that suffered severe immunization reactions required repeated parvaquone therapy and 2 died of ECF complicated by the other tick-borne diseases. In contrast, of the 9 calves that were immunized and did not require parvaquone treatment, only one suffered a severe clinical reaction during field exposure and this calf died from a combination of babesiosis, anaplasmosis, ECF and T. mutans (see Figure 1). It is suggested that most, if not all, of the immunized calves that suffered severe reactions following immunization were harbouring latent, chronic theileriosis and were probably immunosuppressed at the time of field exposure. Unfavourable reactions to immunization were almost certainly due to the greater susceptibility of Bos taurus cattle to T. parva With hindsight it would have been more appropriate to have characterized the stabilate using pure Bos taurus calves. Babesiosis and anaplasmosis were known to be present on Pemba but it was believed that the imidocarb dipropionate prophylaxis should have controlled these diseases. However, the tick burdens were very heavy and the disease challenge complicated and severe, while the animals were further compromised by the immunization reactions. These complications highlight also the risk of exposure of cattle to unlimited tick challenge and suggest that some level of tick control is justified in trials when certain threshold burdens are reached.
Figure 1. Summary of disease responses of immunized and control cattle on Pemba Island, Zanzibar.
REFERENCE
Morzaria, S.P., Irvin, A.D. and Dolan, T.T. (1986). Immunization against East Coast fever: isolation and characterisation of Theileria parva stocks from Unguju and Pemba for use in infection and treatment. ILRAD Technical Report. Nairobi: International Laboratory for Research on Animal Diseases.
