| dc.description.abstract | The understanding of factors contributing to the appearance and maintenance of
trypanosome drug resistance in the field is very limited,. yet this type of information
would be very useful in the design of trypanosomosis control strategies. This thesis
describes a study that was carried out in Mukono County, a peri-urban dairy
production system near Kampala (Uganda). It had the following main objectives:
a) to characterise the drug-sensitivity phenotype(s) of trypanosome field isolates
collected between 1995 and 1996 from cattle; b) to determine the species and
subspecies of trypanosome populations and their prevalence over the same period in
cattle; c) to evaluate an isometamidium-enzyme-linked immunosorbent assay
(ISMM-ELISA) using sera and use the resulting data to monitor the use of
isometamidium and as a tool to indirectly assay for drug resistance; d) to establish
whether the trypanosome isolates from cattle might be of zoonotic importance; and e)
to analyse the factors associated with trypanocide usage and efficacy.
In the first part of this study, 486 cattle from 50 farms in Mukono County were
monitored for trypanosome infections over a two-year period (1995 to 1996).
Eighteen trypanosome isolates were characterised in vivo for their sensitivity to
isometamidium (ISMM), diminazene (DIM) and homidium (HOM). Ten of the
isolates were from animals that had the highest serum ISMM concentrations (ranging
from 0.3-4.5ng/ml). as determined by the ISMM-ELISA. All 18 isolates exhibited low
pathogenicity in cattle and were sensitive to DIM at 3.5-mg/kg-body weight (BW)
and ISMM at 0.5 mg/kg BW. In goats, two of the isolates were highly pathogenic,
producing CNS involvement. All eight populations examined in goats were sensitive
to DIM at 3.5 mg.kg BYV.However. two populations relapsed after treatment with
ISMM at 0.5mg/kg BW. and four relapsed after treatment with HOM at 1.0 mg/kg
BW. In mice. the 50% curative dose (CD50) values for the populations ranged from
OJ to 1.9 mg/kg BW for DIM. from 0.02 to 0.10 rng/kg BW for ISMM and from 0.9
to 3.8 mg/kg BW for HOM. Thus. by comparison to reference drug-sensitive
populations. all the isolates were highly sensitive to DIM and ISMM even though
some expressed moderate levels of resistance to HOM. All the isolates contained only
T brucei and/or T vivax. Twenty five percent of the T brucei populations studied
were human serum resistant, indicating that cattle might be reservoir hosts for the
causative agent of human sleeping sickness in this area.
In the second part of this study, an improved methodology for a competitive enzyme
immunoassay (CEIA) for ISMM was developed. The equilibrium method was
modified to a sequential saturation procedure, because of unacceptably high intersample
variation in the former. For instance. in the equilibrium procedure, the optical
densities (ODs) ranged from 0.208 to 3.332, with a very high coefficient of variation
(CV) of 94.8%, in adult cattle. Using the sequential saturation procedure, the CV for
sera from 20 untreated cattle was 6 %. Four-parameter logistic calibration curves
obtained using the sequential saturation method showed the concentration of
isometamidium required for 50% competition to be approximately 1.0 ng/ml; half that
previously reported for the equilibrium method. This shift in the calibration curve,
together with the reduction in the inter-sample variance for untreated cattle. resulted
in an improved lower limit of detection of the order of 0.1-0.2 ng/m!.
In the third part of the study. the newlv developed ISl\lM-ELISA (sequential
saturation) was used to monitor ISMM concentrations in sera from Mukono cattle.
after block treatment with ISMM (Samorin": 1.0 mg/kg BW). Serum samples
obtained from 474 cattle. distributed among 50 farms were analysed. For each animaL
determinations were available for samples collected from up to four occasions. these
being day 0 (the day of ISMM block-treatment) and three. approximately monthly,
occasions thereafter. The mean serum ISMM concentrations were: at day zero (before
drug administration). 0.67±1.28 ng/ml; at 30 days post-treatment (OPT), 0.67±0.60
ng/ml; at 60 OPT, 0.22±0.27 ng/ml; and by 90 OPT, below the detection limit of the
assay. Mixed logistic regression analysis of mean serum concentrations for the 3
breed categories (exotics. locals and crosses) at 30, 60 and 90 OPT, revealed a higher
probability of detecting ISMM in local than exotic cattle at all time points; suggesting
a possible breed effect on the pharmacokinetics of isometamidium.
Finally, factors affecting trypanocidal drug use and efficacy in Mukono were also
examined. There were no obvious factors associated with drug use in the study area.
Of the 1847 ISMM treatments given during the study, 234 infected cases (failures)
were diagnosed during the post-treatment observation period. Of the 234 (failures),
only 30 (7.4%) occurred between 7-90 days post-treatment, indicating the high
efficacy of ISMM.
In conciusion. the findings in this study provide important information in deciding the
best control option for trypanosornosis in Mukono County. It is suggested that an
integrated disease management strategy may be the best approach for the study area. | en |
