Response of Trypanosoma congolense in goats to single and double treatment with diminazene aceturate

Detection of multiple drug-resistant Trypanosoma congolense populations in village cattle of south-east Mali

Background

Tsetse fly-transmitted African animal trypanosomosis causes annual losses that run into billions of dollars. The disease is assumed to cause hunger and poverty in most sub-Saharan countries since it represents a serious impediment to sustainable livestock production. Both a cross-sectional and a longitudinal study were carried out from November to December 2007 to evaluate trypanosomosis risk and susceptibility of trypanosomes to trypanocidal drug treatment in village cattle populations in south-east Mali.

Methods

Eight purposively selected villages participated in the study. In each village, eight traps deployed along drainage lines over 24hour duration were used to catch tsetse. One hundred systematically selected cattle in the study villages were examined for trypanosomes. All trypanosome-positive cattle were randomly allocated into two treatment groups: a group treated with 0.5 mg/kg bw. isometamidium chloride (ISMM) and a group treated with 3.5 mg/kg bw. diminazene aceturate (DIM). The cattle were monitored for trypanosomes at day 14 and 28 post-treatment.

Results

Of the 796 cattle examined, 125 (15.7%) were trypanosome-positive. Village trypanosome prevalences ranged between 11% and 19%. There were no significant (p > 0.05) differences in the village trypanosome prevalences. Trypanosoma congolense was the dominant trypanosome species accounting for 73% (91/125) of the infections and T. vivax the remainder. Twenty (31.7%) of the 63 cattle on 0.5 mg/kg bw. ISMM treatment were still positive14 days post-treatment. Of the 43 aparasitaemic cattle monitored to day 28, 25.6% (11) became parasitaemic, resulting in a cumulative failure rate of 49.2% (31/63). Trypanosoma congolense accounted for 77.4% (24/31) of failed ISMM treatments. The 62 cattle treated with 3.5 mg/kg bw. DIM resulted in 30.6% (19/62) failed treatments. Although 42.2% (19/45) of T. congolense positive cattle did not respond to DIM treatment, all T. vivax positive cattle responded positively to DIM treatment.

Conclusions

The overreliance on trypanocides in the control of trypanosomosis will ultimately lead to multiple drug-resistant trypanosome populations as detected in villages in south-east Mali rendering the use of drugs doubtful. Effective alternative methods for trypanosomosis control ought to substitute chemotherapy to ensure sustainable cattle production in these villages. Since there is no single strategy for containing trypanocidal drug resistance, promotion of an integrated approach combining proven trypanosomosis control approaches in high trypanosomosis risk areas is most desirous. The best-bet strategy this study recommended for areas with multiple drug resistance included area-wide community tsetse control, control of co-infections to exploit self-cure against resistant trypanosome populations and the rational use of trypanocidal drugs which should be urgently promoted at all levels as a way of containing or reversing resistance.

An evaluation of food intake, digestive physiology and live-weight changes in N'dama and Gobra Zebu bulls following experimental Trypanosoma congolense infection

The effects of experimental Trypanosoma congolense infection on the digestive physiology and nutrient utilization in Gobra zebu and N'Dama cattle were examined in a 16-week trial. A pair-feeding procedure permitted examination of the effects of both food intake and trypanosomosis infection. Twenty Gobra and 16 N'dama bulls aged between 1 and 2 years were paired on a live-weight basis within each breed. One of each pair was chosen at random to serve as an uninfected control while the other was inoculated intradermally with 104 T. congolense in mice blood in the 6th week of the experiment. Packed cell volume and parasitaemia data were collected weekly throughout the trial. Total dry-matter intake (TDMI) and live-weight changes were measured weekly in all animals during the 16-week trial. Rate of passage (RoP) and dry matter digestibility were evaluated before and after infection.

While infection significantly depressed TDMI in both breeds (P < 0·05), neither infection nor breed affected the RoP and the apparent digestibility of the dry matter. Significant changes in live weight attributable solely to the infection were observed in both breeds. Loss of body weight was more severe (P < 0·05) in infected Gobra bulls compared with N'dama bulls possibly implying a superior efficiency of nutrient utilization by the N'dama during infection. In the course of the trial, one N'dama and three Gobra bulls that presented severe clinical symptoms of trypanosomosis were treated and withdrawn from the experiment along with their pair-fed controls.

In conclusion, the RoP and the digestive efficiency were not affected by infection and breed differences. Also, the trypanotolerance mechanism does not seem to be affected by anorexia but rather by the ability to conserve body reserves during infection.

Characterization of trypanosome isolates from naturally infected horses on a farm in Kenya

Following an outbreak of trypanosomosis in horses on a farm in Kenya, 18 trypanosome isolates were collected from the infected animals over a period of one and a half years and cryopreserved for characterization. The characterization was done on the basis of morphology using Giemsa-stained blood and buffy coat smears, infectivity to mice, recombinant DNA hybridization, and chromosome separation by orthogonal field alternation gel electrophoresis (OFAGE). Morphologically, all the trypanosome isolates were identified as belonging to the subgenus Nannomonas, and a total of 16 out of the 18 isolates grew in mice. Using the recombinant DNA hybridization technique, the isolates were further classified as the 'savannah' type of Trypanosoma congolense. Furthermore, chromosome separation by OFAGE, carried out on six clones derived from different isolates, exhibited a profile characteristic of T. congolense, 'savannah' type. However, there were differences in the number and positions of the medium-sized and minichromosomes indicating a diversity of serodemes within the isolates. Hence the infecting trypanosomes in this disease outbreak were T. congolense, 'savannah' type, and comprised several serodemes or strains.

Haematological changes in N'Dama and Gobra Zebu bulls during Trypanosoma congolense infection maintained under a controlled feeding regimen

Haematological changes were monitored in Gobra-Zebu and N'Dama bulls following infection with Trypanosoma congolense. The cattle were offered a diet which provided levels of protein and energy above maintenance requirement and a pair feeding regimen was used in order to eliminate the confounding anorexic effects of trypanosomosis on the traits studied. Packed red cell volume (PCV), red blood cells (RBC) and haemoglobin (Hb) were monitored weekly. Mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), and mean corpuscular volume (MCV) were derived by computation. Significant breed differences (P < 0.001) were observed in the baseline data collected with N'Dama bulls having higher (P < 0.001) values for RBC and Hb. PCV levels were similar in both breeds pre-infection. Post-infection, there was a significant (P < 0.001) infection effect on the RBC, Hb and PCV in both breeds. The pathogenic effects were more severe in the Gobra-Zebu bulls where three out of ten bulls compared with only one out of eight infected N'Dama bulls attained the low PCV threshold, treated and withdrawn from the study along with their pair mates. Throughout the infection in N'Dama cattle and during the first 6 weeks of infection in the Gobra-Zebu bulls, the infection presented a normochromic normocytic anaemia. However, in the chronic phase, the Gobra-Zebu bulls became macrocytic. The infection reduced total dry matter intake in both breeds although this persisted longer in the Gobra-Zebus. However, their pair-fed controls showed no haematological changes indicating that the anorexia was not compounding the effects of the infection. The severity and type anaemia in N'Dama correlates with their innate ability to resist the effects of trypanosome infection compared to the Gobra-Zebu bulls.

Frequency of diminazene-resistant trypanosomes in populations of Trypanosoma congolense arising in infected animals following treatment with diminazene aceturate

The frequency of trypanosomes resistant to diminazene aceturate at a dose of 25 mg/kg of body weight was investigated for populations of Trypanosoma congolense IL 3274 which reappeared in infected mice after intraperitoneal treatment with diminazene aceturate at the same dosage. At inoculum sizes of 10(2), 10(3), 10(4), 10(5), and 10(6) trypanosomes per mouse, the relapse populations were used to initiate infections in five groups of 100 mice each by the intravenous route. Immediately after infection, 50 mice in each group were treated intraperitoneally with diminazene aceturate at the aforementioned dosage; the other 50 mice functioned as untreated controls. Thereafter, all animals were monitored for 100 days for the presence of trypanosomes. In each group, trypanosomes were detected in 50 of 50 control mice, indicating 100% infectivity for all five inoculum sizes. In contrast, in the groups of 50 mice infected with 10(2), 10(3), 10(4), 10(5) and 10(6) trypanosomes and treated with diminazene aceturate, trypanosomes were detected in 4, 11, 13, 28, and 39 of 50 mice, respectively. By logistic regression, a good fit was found between the number of mice identified as parasitemic and the inoculum sizes. Maximum likelihood estimates for the proportions of trypanosomes resistant to diminazene aceturate at 25 mg/kg of body weight for the inoculum of 10(2), 10(3), 10(4), 10(5), and 10(6) organisms were 8.335 x 10(-4), 2.485 x 10(-4), 3.02 x 10(-5), 8.3 x 10(-6), and 1.6 x 10(-6), respectively. These finding indicate that the majority of the relapse trypanosomes were susceptible the the drug dosage used for selecting the population and that, surprisingly, the calculated proportion of organisms which survived drug exposure varied inversely with the inoculum size. Further experiments with mice indicated that the inverse relationship did not result from alterations in the pharmacokinetics of the drug with different inoculum sizes. The data therefore suggest that parasite inoculum size and drug dosage are important factors in estimating the apparent frequency of diminazene-resistant trypanosomes in populations of T. congolense occurring in vivo.

Apparent rarity of diminazene-resistant trypanosomes in goats infected with a diminazene-resistant population of Trypanosoma congolense

Experiments were carried out in goats to determine the frequency with which diminazene-resistant trypanosomes occur in parasite populations before and after the intramuscular treatment of the goats with diminazene aceturate. Trypanosoma congolense IL 3274, a diminazene-resistant clone, was used to initiate infections in three groups of five goats. The goats in the first group were treated with diminazene aceturate at a dose of 7.0 mg kg-1 bodyweight within 10 seconds of infection; one of the goats was cured. All of the second group, which received no treatment, became parasitaemic. The third group of goats received the same dose of drug as the first group but three days after all of them were first detected parasitaemic; trypanosomes reappeared in all the five goats. When this third group was treated, the frequency of trypanosomes resistant to the drug dosage was estimated to be less than 1 in 10(3). The parasites which reappeared after the treatment of these animals were used to infect two additional groups of five goats intravenously. The goats in one group were treated with the same dose of drug as before, within 10 seconds of infection and were all cured. In contrast, the five goats in the second, untreated, group became parasitaemic. Finally, when the goats in which the infections had relapsed were retreated with diminazene aceturate at the same dose rate, the level of parasitaemia temporarily decreased by at least 10(3) trypanosomes ml-1. These findings suggest that diminazene-resistant T congolense occur at low levels in trypanosome populations despite attempts to select for a population resistant to the dose of drug used.

Variation in sensitivity of Trypanosoma congolense to diminazene during the early phase of tsetse-transmitted infection in goats

Twenty-five goats were randomly allocated to five groups of five animals each and infected with Trypanosoma congolense IL 3274 via the bites of infected Glossina morsitans centralis. At intervals of 1, 4, 8, 12 or 19 days following infection, each group of five animals was treated intramuscularly with diminazene aceturate at a dose of 7.0 mg kg-1 body weight (b.w.). While treatment on Day 1 eliminated infections in all five goats, treatment on Day 19 did not cure any of the animals; in groups treated 4, 8 or 12 days following infection, two of five goats in each group were cured. Since the alteration in apparent resistance of T. congolense IL 3274 between Day 1 and Day 19 could have been due to alteration in expression of drug resistance by trypanosomes as the population expanded, the experiment was repeated using trypanosomes that reappeared in the animals that had been treated with diminazene aceturate on Day 19. On Day 36, when all five animals were parasitaemic, five groups of teneral G. m. centralis, each containing 160 flies, were fed on one occasion on each of the five goats (one group of testse flies per goat). Thereafter, each group of tsetse flies was maintained on clean rabbits. When infective, five flies from each group were allowed to feed on two naive goats each (i.e. two goats per group of tsetse flies). One animal in each pair was treated 24 h after infection with diminazene aceturate at a dose of 7.0 mg kg-1 b.w., the other was treated on Day 19, when parasitaemic, with the same drug dosage. As before, treatment 24 h following infection eliminated infections in all animals, but when treatment was delayed until Day 19, trypanosomes in all animals were refractory to treatment. Thus, although tsetse flies were infected with trypanosomes that had arisen in infected goats following treatment with diminazene aceturate at a dose of 7.0 mg kg-1 b.w., when the same flies were allowed to feed on clean goats, the resultant infections were sensitive to treatment with the same drug dosage when administered 24 h following infection. These data therefore indicate that there is a significant alteration in diminazene sensitivity of IL 3274 between Day 1 and Day 19 and that this is associated with an alteration in the resistance phenotype of the trypanosomes.

An in vitro assay for drug sensitivity of Trypanosoma congolense using in vitro-derived metacyclic trypanosomes

The sensitivity of seven populations of T. congolense to the salts of three trypanocides, diminazene, isometamidium and homidium, were determined in vitro using in vitro-derived metacyclic trypanosomes. The trypanosomes were incubated at 35 degrees C for 48 h with various drug concentrations (0.5 ng-50 micrograms/ml) and then transferred to cultures containing bovine endothelial-cell monolayers, to assess their viability over the following 5 days as compared to control trypanosomes that had been incubated without drug. The sensitivity to each drug was expressed as the minimum effective drug concentration which killed 100% of the trypanosomes in a given population within the 5 days. Using this assay, population IL 1180, characterised as being highly sensitive to all three drugs in vivo, required 10 ng/ml isometamidium chloride, 50 ng/ml homidium bromide or chloride and 5000 ng/ml diminazene aceturate to kill the entire population in vitro. In contrast, two derivatives of IL 1180 in which resistance to isometamidium had been induced in mice, IL 3343 and IL 3344, required isometamidium chloride at a concentration of 1000 ng/ml and 2000 ng/ml, respectively, to eliminate the populations. The in vitro results showed that the increase in level of resistance to isometamidium in these populations was associated with at least a 200-fold increase in resistance in both populations to homidium, but no increase in resistance to diminazene. KE 2887 and CP 81, two isolates expressing high levels of resistance to both isometamidium and homidium in mice and cattle, were both resistant in vitro to isometamidium chloride and homidium salts at 100 ng/ml. Furthermore, while the former population was resistant to 10,000 ng/ml diminazene aceturate, the latter was sensitive to 5000 ng/ml. IL 3274 and IL 3330, characterised as expressing high levels of resistance to all three drugs in vivo, were shown to be resistant to isometamidium chloride and homidium salts at 1000 ng/ml, and to diminazene aceturate at 10,000 ng/ml. Finally, the in vitro IC100 (concentration of drug required to eliminate 100% of the population) results were consistent with the maximum amounts of each drug detected in vivo.