Tsetse-infested Area Research Articles

Human-associated scarcity of hosts for tsetse flies (Diptera: Glossinidae) is related to an increase in prevalence of trypanosome infection in flies in north-eastern Zambia.

Occurrence of nutritional stress (due to depletion of fat reserves) in tsetse flies, associated with inadequate levels of access to blood meals, enhances susceptibility of the flies to trypanosome infection. Thus, in a tsetse-infested area, a spatial gradient of reducing tsetse habitat quality is potentially a gradient of increasing prospects for occurrence of stress in tsetse flies. This study investigated prevalence of trypanosome infection in Glossina morsitans morsitans and G. pallidipes along a transect line hypothesised to represent such a gradient, in relation to the edge of the tsetse belt and distribution of human settlements. This was undertaken in three sites located in Lundazi, Mpika and Rufunsa districts, respectively, in north-eastern Zambia. Human settlement was concentrated at the edge of the tsetse belt in the Mpika and Rufunsa sites and evenly distributed along transect line in the Lundazi site. Tsetse fly samples were collected using black-screen fly rounds and Epsilon traps. Detection of trypanosome infection was by dissection and microscopy in Lundazi and Mpika sites and loop-mediated isothermal amplification (LAMP) test in Rufunsa site. Multiple logistic regression models were applied to determine whether the following factors, 'change in distance from edge of tsetse belt', 'tsetse sampling method' and 'sex of tsetse fly', had effect on 'prevalence of trypanosome infection' in the tsetse flies. Only 'increase in distance from the edge of tsetse belt' for G. m. morsitans was significantly associated with 'prevalence of trypanosome infection' in the flies, in the Mpika and Rufunsa sites. Distance was associated with reduced likelihood of infection with 'one or more subgenera of trypanosomes' and with 'Nannomonas trypanosomes', in the case of 'all sites collectively', 'Lundazi and Mpika sites collectively', Mpika site alone, and Rufunsa site alone. Per site, increase in distance entailed reduced prospects for Trypanozoon infection but only in the Mpika and Rufunsa sites. We conclude that in the Mpika and Rufunsa sites, increase in distance from human settlements entailed reduced likelihood of trypanosome infection, likely due to reducing tsetse habitat degradation, increasing availability of hosts, and hence increasing levels of nutrition and fat reserves, thus enhancing tsetse immunity against trypanosome infection.

Molecular identification of diminazene aceturate resistant trypanosomes in tsetse flies from Yoko in the Centre region of Cameroon and its epidemiological implications

African animal trypanosomiases are caused by trypanosomes cyclically or mechanically transmitted by tsetse and other biting flies. Although molecular tools have been developed to identify drug-resistant trypanosomes in mammals, little or no investigation on drug-resistance has been undertaken on trypanosomes harbored by tsetse flies. Moreover, no data on mechanical vectors of African trypanosomes is available in most endemic areas of Cameroon. This study was designed to update our knowledge on the cyclical and mechanical vectors of African trypanosomes, and using molecular tools to identify different trypanosome species as well as diminazene aceturate resistant trypanosomes in tsetse flies trapped at Yoko in the Centre region of Cameroon.For this study, traps were used to catch tsetse and mechanical vectors of African trypanosomes. The flies trapped were counted and identified by sex and species. DNA was extracted from tsetse and species-specific primers were used to identify different trypanosome species. PCR-RFLP was used to detect diminazene aceturate resistant strains of Trypanosoma congolense.In all, 454 flies comprising 168 (37%) Tabanus spp., 71 (15.6%) Stomoxys spp. and 215 (47.4%) tsetse fly (i.e. 107 (49.8%) Glossina fusca congolensis, 71 (33%) Glossina fusca fusca and 37 (17.2%) Glossina palpalis palpalis) were trapped. Trypanosome infections were identified in 12.6% (27/215) of tsetse flies: 13 in G. f. congolensis, 6 in G. p. palpalis and 5 in G. f. fusca. From 24 T. congolense positive samples, PCR-RFLP was successful on 37.5% of the samples. Four samples (16.2%) harbored T. congolense strains that were resistant to diminazene aceturate while the remaining samples had drug-sensitive strains.These results show for the first time the applicability of molecular tools for the identification of drug-resistant trypanosomes in tsetse. They revealed the existence of diminazene aceturate resistant strains of T. congolense in the tsetse-infested area of Yoko in the Centre region of Cameroon. Detection of drug-resistant trypanosomes in tsetse may enable scientists to map with accuracy specific areas where these parasites are transmitted. With such mapping, control strategies against African trypanosomiases could be improved by adapting control measures according to drug resistance distribution.

Effect of a Natural Trypanosome Infection in Trypanotolerant Cattle Reared in a Tsetse-Infested Area in Southern Gabon and Monitored Under Field Conditions

Effect of a Natural Trypanosome Infection in Trypanotolerant Cattle Reared in a Tsetse-Infested Area in Southern Gabon and Monitored Under Field Conditions

THE IMMUNE TRYPANOLYSIS TEST: AN ACCURATE SEROLOGICAL MARKER TO MANAGE ELIMINATION OF T.B. GAMBIENSE HUMAN AFRICAN TRYPANOSOMIASIS

BackgroundContinued post-elimination monitoring is required to ensure sustainability of zero transmission of human African trypanosomosis (HAT) and to avoid re-emergence caused by potential remaining Trypanosoma brucei gambiense reservoirs (animal and/or...

Spatial distribution and trypanosome infection of tsetse flies in the sleeping sickness focus of Zimbabwe in Hurungwe District.

BackgroundIn Zimbabwe, cases of human African trypanosomiasis (HAT) are caused by the unicellular protozoan Trypanosoma brucei, sub-species T. b. rhodesiense. They are reported from the tsetse-infested area in the northern part of the country, broadly corresponding to the valley of the Zambezi River. Tsetse-transmitted trypanosomes, in particular T. congolense and T. vivax, also cause morbidity and mortality in livestock, thus generating poverty and food insecurity. Two species of tsetse fly, Glossina morsistans morsitans and G. pallidipes, are known to be present in the Zambezi Valley, although their distributional patterns and densities have not been investigated in detail. The present study tries to address this gap by providing some insight into the dynamics of trypanosomiasis in humans and livestock.MethodsTsetse distribution and trypanosome infections were studied using traps and fixed fly rounds located at 10 km intervals along a 110 km long transect straddling the southern escarpment of the Zambezi Valley. Three km long fly rounds were conducted on 12 sites, and were repeated 11 times over a 7-month period. Additional traps were deployed and monitored in selected sites. Microscopic examination of 2092 flies for trypanosome infections was conducted.ResultsSurveys confirmed the presence of G. morsitans morsitans and G. pallidipes in the Zambezi Valley floor. Moving south, the apparent density of tsetse flies appears to peak in the vicinity of the escarpment, then drops on the highlands. Only one fly was caught south of the old game fence separating protected and settled areas. A trypanosome infection rate of 6.31% was recorded in tsetse flies dissected. Only one infection of the T. brucei-type was detected.ConclusionsTsetse fly distribution in the study area appears to be driven by ecological factors such as variation in land use and altitude-mediated climatic patterns. Although targeted control of tsetse flies have played a role in determining distribution, no major control operations have been implemented in the area for 15 years. Trypanosome infections in tsetse flies are consistent with HAT epidemiological data, which considers the situation to be generally ‘low risk’. Nonetheless, underreporting is likely to conceal the true epidemiological picture, and efforts are needed to strengthen the diagnostic capacities of health facilities.

Ribosomal DNA analysis of tsetse and non-tsetse transmitted Ethiopian Trypanosoma vivax strains in view of improved molecular diagnosis

Animal trypanosomosis caused by Trypanosoma vivax (T. vivax) is a devastating disease causing serious economic losses. Most molecular diagnostics for T. vivax infection target the ribosomal DNA locus (rDNA) but are challenged by the heterogeneity among T. vivax strains. In this study, we investigated the rDNA heterogeneity of Ethiopian T. vivax strains in relation to their presence in tsetse-infested and tsetse-free areas and its effect on molecular diagnosis.We sequenced the rDNA loci of six Ethiopian (three from tsetse-infested and three from tsetse-free areas) and one Nigerian T. vivax strain. We analysed the obtained sequences in silico for primer-mismatches of some commonly used diagnostic PCR assays and for GC content. With these data, we selected some rDNA diagnostic PCR assays for evaluation of their diagnostic accuracy. Furthermore we constructed two phylogenetic networks based on sequences within the smaller subunit (SSU) of 18S and within the 5.8S and internal transcribed spacer 2 (ITS2) to assess the relatedness of Ethiopian T. vivax strains to strains from other African countries and from South America.In silico analysis of the rDNA sequence showed important mismatches of some published diagnostic PCR primers and high GC content of T. vivax rDNA. The evaluation of selected diagnostic PCR assays with specimens from cattle under natural T. vivax challenge showed that this high GC content interferes with the diagnostic accuracy of PCR, especially in cases of mixed infections with T. congolense. Adding betain to the PCR reaction mixture can enhance the amplification of T. vivax rDNA but decreases the sensitivity for T. congolense and Trypanozoon. The networks illustrated that Ethiopian T. vivax strains are considerably heterogeneous and two strains (one from tsetse-infested and one from tsetse-free area) are more related to the West African and South American strains than to the East African strains.The rDNA locus sequence of six Ethiopian T. vivax strains showed important differences and higher GC content compared to other animal trypanosomes but could not be related to their origin from tsetse-infested or tsetse-free area. The high GC content of T. vivax DNA renders accurate diagnosis of all pathogenic animal trypanosomes with one single PCR problematic.

Mapping the economic benefits to livestock keepers from intervening against bovine trypanosomosis in Eastern Africa

Endemic animal diseases such as tsetse-transmitted trypanosomosis are a constant drain on the financial resources of African livestock keepers and on the productivity of their livestock. Knowing where the potential benefits of removing animal trypanosomosis are distributed geographically would provide crucial evidence for prioritising and targeting cost-effective interventions as well as a powerful tool for advocacy. To this end, a study was conducted on six tsetse-infested countries in Eastern Africa: Ethiopia, Kenya, Somalia, South Sudan, Sudan and Uganda. First, a map of cattle production systems was generated, with particular attention to the presence of draught and dairy animals. Second, herd models for each production system were developed for two scenarios: with or without trypanosomosis. The herd models were based on publications and reports on cattle productivity (fertility, mortality, yields, sales), from which the income from, and growth of cattle populations were estimated over a twenty-year period. Third, a step-wise spatial expansion model was used to estimate how cattle populations might migrate to new areas when maximum stocking rates are exceeded. Last, differences in income between the two scenarios were mapped, thus providing a measure of the maximum benefits that could be obtained from intervening against tsetse and trypanosomosis. For this information to be readily mappable, benefits were calculated per bovine and converted to US$ per square kilometre. Results indicate that the potential benefits from dealing with trypanosomosis in Eastern Africa are both very high and geographically highly variable. The estimated total maximum benefit to livestock keepers for the whole of the study area amounts to nearly US$ 2.5 billion, discounted at 10% over twenty years – an average of approximately US$ 3300 per square kilometre of tsetse-infested area – but with great regional variation from less than US$ 500 per square kilometre to well over US$ 10,000. The greatest potential benefits accrue to Ethiopia, because of its very high livestock densities and the importance of animal traction, but also to parts of Kenya and Uganda. In general, the highest benefit levels occur on the fringes of the tsetse infestations. The implications of the models’ assumptions and generalisations are discussed.

Distribution and Density of Tsetse Flies (Glossinidae: Diptera) at the Game/People/Livestock Interface of the Nkhotakota Game Reserve Human Sleeping Sickness Focus in Malawi

In large parts sub-Saharan Africa, tsetse flies, the vectors of African human or animal trypanosomiasis, are, or will in the foreseeable future, be confined to protected areas such as game or national parks. Challenge of people and livestock is likely to occur at the game/livestock/people interface of such infested areas. Since tsetse control in protected areas is difficult, management of trypanosomiasis in people and/or livestock requires a good understanding of tsetse population dynamics along such interfaces. The Nkhotakota Game Reserve, an important focus of human trypanosomiasis in Malawi, is a tsetse-infested protected area surrounded by a virtually tsetse-free zone. The abundance of tsetse (Glossina morsitans morsitans) along the interface, within and outside the game reserve, was monitored over 15 months using epsilon traps. A land cover map described the vegetation surrounding the traps. Few flies were captured outside the reserve. Inside, the abundance of tsetse at the interface was low but increased away from the boundary. This uneven distribution of tsetse inside the reserve is attributed to the uneven distribution of wildlife, the main host of tsetse, being concentrated deeper inside the reserve. Challenge of people and livestock at the interface is thus expected to be low, and cases of trypanosomiasis are likely due to people and/or livestock entering the reserve. Effective control of trypanosomiasis in people and livestock could be achieved by increasing the awareness among people of dangers associated with entering the reserve.

Seasonal prevalence of bovine trypanosomosis in a tsetse-infested zone and a tsetse-free zone of the Amhara Region, north-west Ethiopia

During a period of four consecutive years, trypanosomosis surveys were conducted in a tsetse-infested and tsetse-free area of the Amhara Region of north-west Ethiopia. In each study area randomly selected communal cattle were sampled and their blood was investigated using parasitological diagnostic methods. At the same time the population of biting flies was sampled. The monthly average prevalence of trypanosome infections in cattle did not differ significantly between study areas. In both study areas, the prevalence of trypanosome infections was highest during the long rainy season. Trypanosome infections were mainly due to Trypanosoma vivax and they significantly reduced the average packed cell volume and the body condition of the animals. The monthly prevalence of infection was correlated with the density of biting flies, such as Tabanidae and Stomoxys spp., in the preceding month suggesting an important role of mechanical transmission in the epidemiology of trypanosomosis in both areas.

Characterisation of shops selling veterinary medicines in a tsetse-infested area of Kenya

In a tsetse-infested area of Kenya, we characterised veterinary-drug outlets based on personnel and business characteristics to assess their capacities to provide clinical veterinary services. Structured questionnaires were administered to the retailers and sought information on the characteristics of the owners, salespersons and businesses. A total of 41 retail outlets (20 agro-veterinary, 11 pharmacy, and 10 general shops) were identified. There was poor response to questions on owner characteristics. Proprietors, who had no more than secondary education owned 15 out of 28 shops. Few shops (4/29) were owned by proprietors, who had professional qualifications (in animal health). Most salespersons had only secondary education but no qualifications. Animal-health assistants (AHAs), veterinarians and manufacturer’s package inserts (drug leaflets) were the preferred information sources for the retailers. We concluded that drug retailers were poorly equipped with the technical knowledge necessary for drug dispensation and advice.

An analysis of trypanocidal drug use in the Eastern Province of Zambia

As part of the development of a strategy for the control of bovine trypanosomosis in Zambia, a survey was conducted to quantify and qualify the current use of trypanocidal drugs (diminazene aceturate and isometamidium chloride) in a tsetse-controlled and a tsetse-infested area of the Eastern Province. A total of 207 trypanocide users were interviewed. Questions were posed on herd structure, trypanocidal drug preference, treatment strategy, reason for treatment, method of treatment and treatment frequency. The majority of the cattle owners preferred to use diminazene aceturate rather than isometamidium chloride. Both trypanocides were mainly used to treat clinically sick animals (not necessarily infected with trypanosomes) and preference was given to the treatment of oxen and cows. The proportion of animals treated and the frequency of drug application did not differ between the two areas. Hence, in the tsetse-controlled area, a high proportion of the trypanocide treatments was inappropriate. In the tsetse-infested area, on the other hand, the treatment of clinically sick animals significantly reduced the trypanosomosis-related mortality but was insufficient to boost reproduction in cows. Despite the fact that the cattle owners administered most trypanocides themselves, evidence from the survey suggests that most of the farmers did not under-dose with either diminazene aceturate or isometamidium chloride. Moreover, other factors enhancing the development of resistance to trypanocides in trypanosomes were not present in the areas surveyed. Conclusions are drawn on the usefulness of this type of survey in determining appropriate methods to control bovine trypanosomosis.

Determination of serum isometamidium levels in sheep and goats under field conditions using isometamidium-ELISA

Twenty eight sheep and 28 goats were treated with isometamidium chloride (Samorin®, hone Merieux, Lyon, France) at a dose of 1 mg/kg body weight (bw) by intramuscular injection. All the animals were grazed in a tsetse-infested area. They were monitored for anaemia, body weight, anti-trypanosome antibodies and serum isometamidium concentration using the isometamidium enzyme-linked immunosorbent assay (ELISA) for more than 80 days after treatment. Serum isometamidium levels were higher in goats than in sheep throughout the experimental period. Isometamidium was still detectable in sheep and goats for up to 77 and 98 days, respectively, alter treatment (detection limit=0.1 ng/ml). The isometamidium elimination half-lives in sheep and goats were approximately 13.8 and 17.4 days, respectively. No trypanosomes were detected in either the isometamidium-treated or untreated control animals. The present study demonstrated that the isometamidium-ELISA, originally developed for use in cattle, may be equally useful in monitoring the drug in sheep and goats. The elimination half-lives and serum isometamidium levels were markedly higher in goats than in sheep. This could have important implications for chemoprophylaxis in small ruminants under field conditions.

Isometamidium concentrations in the sera of cattle maintained under a chemoprophylactic regime in a tsetse-infested area of Zimbabwe.

An experiment was carried out to determine the concentrations of the trypanocidal drug isometamidium chloride in the sera of cattle maintained under a chemoprophylactic regimen at Rekomitjie, Zimbabwe, an area of high tsetse challenge in the Zimbabwe valley. In February 1993, 24 cattle at this site were treated intramuscularly with isometamidium chloride at a dose of 1.0 mg/kg body weight. Thereafter all animals were monitored regularly for 6 months for the presence of trypanosomes and sera were collected to determine the concentrations of isometamidium using an ELISA. Isometamidium treated cattle appeared to be protected against trypanosome infections for at least 18 weeks following treatment. Thereafter, three trypanosome infections were detected, between 20 and 22 weeks following treatment. In contrast, in 18 untreated control cattle at the same site, 9 trypanosome infections were detected over the first 18 weeks of the experiment. Quantification of the isometamidium concentration in sera from the drug treated cattle indicated that the apparent half-life of isometamidium in these animals was 23 days. This was similar to the half-life observed previously in cattle treated under laboratory conditions. The isometamidium ELISA was shown to be capable of quantifying drug levels in 20 out of 23 cattle for at least 70 days after treatment. There was no evidence of drug resistant trypanosomes at this site.

The economics of village cattle production in a tsetse-infested area of southwest Ethiopia

Cattle raised in the Ghibe valley of southwest Ethiopia are exposed to medium to high levels of trypanosomiasis risk and are often given trypanocidal drugs. A benefit-cost analysis was undertaken to evaluate the financial and economic returns generated by cattle raised in the area under a systematic regime of drug therapy. The results show that, despite the high level of trypanosomosis risk and the prevalence of drug-resistant trypanosomes, cattle production can generate attractive economic returns for herd owners and the overall Ethiopian economy. Sensitivity analyses show that most herd owners would continue to obtain good returns on their investments even if they paid higher prices for trypanocidal drugs and the full costs of veterinary services.