Studies on the Trypanocidal Drug, Homidium: Development and Use of ELISA for Its Detection and Quantification in Cattle

This thesis concerns the development, validation and use of enzyme- linked immunosorbent assays (ELISA) to determine homidium concentrations in sera of treated cattle. Previously published work with particular emphasis on control and specifically, chemotherapy of animal trypanosomiases are reviewed in Chapter One. This includes the development and use of trypanocidal drugs detailing previous analytical techniques used in the determination of drug levels in plasma/serum of treated cattle. Chapter 2 describes the general materials and methods used in the experiments reported in the later Chapters of this thesis. Chapter 3 describes the development of two highly sensitive homidium ELISA methods (detection limit 0.1 ng ml-1); Assay 1 an indirect competition assay and the Assay 2 a direct competition assay. Validation of the assays was carried out on serum samples obtained from treated Friesian calves. Using Assay 2, the serum homidium concentrations obtained following treatment of calves showed less variations between individual animals when compared to Assay L It was thereafter adopted for use in all subsequent experiments. Following the development and validation of the ELISA method, several experiments were carried out in cattle using homidium bromide at 1 mg kg-1 b.w. to establish baseline data on serum homidium concentrations and pharmacokinetics in cattle. Serum homidium concentrations and pharmacokinetics were determined following i.v. treatment of Friesian calves. No drug was detectable after approximately 17 days of treatment in four out of five and in 21 days in the remaining calf showing rapid elimination of the drug. Following the establishment of homidium pharmacokinetics in the Friesian cattle in work carried out in Scotland, the studies were extended to Boran (Bos indicus) cattle, a breed of cattle which is commonly kept in the trypanosomiasis endemic areas, in Kenya. Following i.m. treatment with homidium bromide at 1 mg k-1, serum homidium concentrations were determined. The results showed a wide variation in serum homidium concentrations between individual Boran cattle when compared to the Friesian. However, both groups showed similar the serum homidium concentration-versus-time profiles. The results of investigations into homidium as a chemotherapeutic drug are reported in Chapter 5. Two groups of five animals were inoculated with two different populations of T. congolense; one drug-sensitive (IL 1180) and the other drug-resistant (IL 3330). The animals were treated with homidium bromide at 1 mg kg-1 seven days following detection of trypanosomes. No trypanosomes were detected in the cattle infected with the drug-sensitive trypanosome population within 24 hours in four and 48 hours in of five cattle following treatment. The animals remained aparasitaemic up to the end of the observation period of 90 days post-treatment. Whist trypanosomes did not disappear from the circulation following treatment of cattle infected with drug- resistant trypanosome population, low serum homidium concentrations of between 0.1 and 0.3 ng ml-1 remained in the circulation for over 10 weeks following treatment. Both groups showed an increase in the rate of drug elimination within the first week of treatment which reverted back to normal following disappearance of trypanosomes from circulation of cattle treated with drug-sensitive trypanosome population. This accelerated rate of drug elimination was maintained in the presence of drug-resistant trypanosomes until no drug was detectable within 10 days of treatment. Investigations into homidium as a chemoprophylactic drug under controlled conditions are reported in Chapter 6. Following monthly trypanosome challenge of homidium-treated cattle using the same T. congolense populations mentioned above, trypanosomes were detected in blood of the five cattle challenged with the drug-sensitive trypanosome population after 120, 134, 137, 143 and 144 days following treatment. Homidium concentrations of between 0.1 and 0.3 ng ml-1 were detectable for over 10 weeks in circulation in four out of five cattle challenged with drug-sensitive trypanosome population. However, tiypanosomes were detected in the circulation of all the five cattle challenged with the drug-resistant trypanosome population eight to nine days following the first challenge at 30 days post-treatment. Serum homidium concentrations were undetectable within 13 days of challenge