Mini-review on CRISPR-Cas9 and its potential applications to help controlling neglected tropical diseases caused by Trypanosomatidae

The CRISPR-Cas system, which was originally identified as a prokaryotic defense mechanism, is increasingly being used for the functional study of genes. This technology, which is simple, inexpensive and efficient, has aroused a lot of enthusiasm in the scientific community since its discovery, and every month many publications emanate from very different communities reporting on the use of CRISPR-Cas9. Currently, there are no vaccines to control neglected tropical diseases (NTDs) caused by Trypanosomatidae, particularly Human African Trypanosomiasis (HAT) and Animal African Trypanosomoses (AAT), and treatments are cumbersome and sometimes not effective enough. CRISPR-Cas9 has the potential to functionally analyze new target molecules that could be used for therapeutic and vaccine purposes. In this review, after briefly describing CRIPSR-Cas9 history and how it works, different applications on diseases, especially on parasitic diseases, are reviewed. We then focus the review on the use of CRISPR-Cas9 editing on Trypanosomatidae parasites, the causative agents of NTDs, which are still a terrible burden for human populations in tropical regions, and their vectors.

A Spatio-temporal Model of African Animal Trypanosomosis Risk

BACKGROUND

African animal trypanosomosis (AAT) is a major constraint to sustainable development of cattle farming in sub-Saharan Africa. The habitat of the tsetse fly vector is increasingly fragmented owing to demographic pressure and shifts in climate, which leads to heterogeneous risk of cyclical transmission both in space and time. In Burkina Faso and Ghana, the most important vectors are riverine species, namely Glossina palpalis gambiensis and G. tachinoides, which are more resilient to human-induced changes than the savannah and forest species. Although many authors studied the distribution of AAT risk both in space and time, spatio-temporal models allowing predictions of it are lacking.

METHODOLOGY/PRINCIPAL FINDINGS

We used datasets generated by various projects, including two baseline surveys conducted in Burkina Faso and Ghana within PATTEC (Pan African Tsetse and Trypanosomosis Eradication Campaign) national initiatives. We computed the entomological inoculation rate (EIR) or tsetse challenge using a range of environmental data. The tsetse apparent density and their infection rate were separately estimated and subsequently combined to derive the EIR using a "one layer-one model" approach. The estimated EIR was then projected into suitable habitat. This risk index was finally validated against data on bovine trypanosomosis. It allowed a good prediction of the parasitological status (r2 = 67%), showed a positive correlation but less predictive power with serological status (r2 = 22%) aggregated at the village level but was not related to the illness status (r2 = 2%).

CONCLUSIONS/SIGNIFICANCE

The presented spatio-temporal model provides a fine-scale picture of the dynamics of AAT risk in sub-humid areas of West Africa. The estimated EIR was high in the proximity of rivers during the dry season and more widespread during the rainy season. The present analysis is a first step in a broader framework for an efficient risk management of climate-sensitive vector-borne diseases.

Importance of vector-borne infections in different production systems: bovine trypanosomosis and the innovation dynamics of livestock producers in Senegal

In Senegal, a project has been undertaken to eradicate a population of tsetse flies (Glossina palpalis gambiensis) from a prime area for intensifying livestock production--the coastal region of Niayes. The project is intended to remove the constraint of trypanosomosis and allow the ecological intensification of cattle production. A cross-sectional analysis of ten case studies was the inductive phase of an assessment to gauge the impact of removing trypanosomosis on livestock production strategies. The methodology used was comprehensive analysis, with participatory epidemiology tools to understand farmers' rationales. The authors analysed the strategies of three main types of livestock producer (agro-pastoralists, mixed crop/livestock farmers and intensive dairy farmers). The strategies were in line with the farmers' goals and their ability to mobilise the socio-technical network. The risk management of trypanosomosis has been incorporated into livestock management practices through the use of trypanotolerant breeds, medical prophylaxis or placing livestock in low-risk areas. Removing the risk of disease would therefore have a major impact on decisions about the composition and strategic direction of herds. This change in the animal health environment would steer livestock production along different routes of intensification in a highly competitive environment. The indicators of innovation capacity revealed by this study will be used to quantitatively monitor various change scenarios, taking livestock producers' reasoning into account, in order to assess the socio-economic impact of eradicating the tsetse fly population in this area. The methodology presented in the study can be used to understand the impact of controlling other vector-borne infections on the innovation dynamics of livestock producers.

Spatio-temporal distribution of tsetse and other biting flies in the Mouhoun River basin, Burkina Faso

In the Mouhoun River basin, Burkina Faso, the main vectors of African animal trypanosomoses are Glossina palpalis gambiensis Vanderplank and Glossina tachinoides Westwood (Diptera: Glossinidae), both of which are riverine tsetse species. The aim of our study was to understand the impact of landscape anthropogenic changes on the seasonal dynamics of vectors and associated trypanosomosis risk. Three sites were selected on the basis of the level of disturbance of tsetse habitats and predominant tsetse species: disturbed (Boromo, for G. tachinoides) and half-disturbed (Douroula for G. tachinoides and Kadomba for G. p. gambiensis). At each of these sites, seasonal variations in the apparent densities of tsetse and mechanical vectors and tsetse infection rates were monitored over 17 months. Tsetse densities differed significantly between sites and seasons. Of 5613 captured tsetse, 1897 were dissected; 34 of these were found to be infected with trypanosomes. The most frequent infection was Trypanosoma vivax (1.4%), followed by Trypanosoma congolense (0.3%) and Trypanosoma brucei (0.05%). The mean physiological age of 703 tsetse females was investigated to better characterize the transmission risk. Despite the environmental changes, it appeared that tsetse lived long enough to transmit trypanosomes, especially in half-disturbed landscapes. A total of 3021 other biting flies from 15 species (mainly Tabanidae and Stomoxyinae) were also caught: their densities also differed significantly among sites and seasons. Their relative importance regarding trypanosome transmission is discussed; the trypanosomosis risk in cattle was similar at all sites despite very low tsetse densities (but high mechanical vector densities) in one of them.

Survey on bovine trypanosomosis and its vector in Metekel and Awi zones of Northwest Ethiopia

A cross-sectional study was conducted between November 2009 and December 2009 in the riverbank of Abay river tributaries, located in three districts of Awi and Metekel zones, Northwest Ethiopia. The prevalence of bovine trypanosomosis, associated risk factors and distribution as well as vector identification in the study area were considered. Blood samples were collected from 540 randomly selected local (zebu) breed of cattle in nine peasant associations of three districts and the assumed risk factors were recorded. The collected samples were examined using hematological and parasitological techniques. In this study, sixty-seven animals (12.42%) were infected with different species of trypanosomes. Most of the infections were due to T. congolense (77.6%) followed by T. vivax (14.9%), T. brucei (6.0%) and mixed infection of T. congolense and T. vivax (1.5%). There was no statistical significance (p>0.05) between sex, age and coat color of skin, but significant differences were observed in body condition, altitude and districts (p<0.05). Mean PCV value of infected (19.42%) and non-infected (24.13%) group of animals had significant variation; and mean PCV value of poor body condition was significantly different (p<0.001) from good body condition. A total of 3072 tsetse flies of riverine species or palpalis group (Glossina tachinoides) and biting flies were caught, of these 2792 (90.9%) were tsetse flies and the remaining were Stomoxys and Tabanus. The overall apparent densities of tsetse and biting flies were 6.49 and 0.65flies/trap/day, respectively and the difference was significant (p<0.05). The study revealed that bovine trypanosomosis is more prevalent in low land and in poor body condition animals in the study area. Tsetse distribution also coincides with altitude, where there was high tsetse catch in low land, but none in mid land. Therefore, prompt control strategy has to be designed and implemented in the area to minimize the distribution of tsetse as well as trypanosomosis prevalence.

Tsetse fly control and trypanosomiasis in Africa, quo vadis?

National and international efforts to eradicate tsetse fly-borne human and animal trypanosomiasis are critically evaluated, and possible reasons for their failure in many cases are discussed. Some formerly performed campaigns in specific areas with positive results cannot be taken as examples to solve the main problems. In future, a significant reduction of trypanosomiasis cases will be possible to achieve only if a concerted long-term Pan-African approach, based on financial security, the continuity of expert staff, and a well-planned, ecologically sound land use, is generally accepted.

Immunodiagnosis of bovine trypanosomiasis in Anambra and Imo states, Nigeria, using enzyme-linked immunosorbent assay: zoonotic implications to human health

BACKGROUND & OBJECTIVES

The prevalence of trypanosomiasis was studied in cattle, being a major source of animal protein in Nigeria, thus, a very likely means of spread of Human African Trypanosomosis (HAT).

METHODS

Enzyme-linked immunosorbent assay (ELISA) was used to diagnose bovine trypanosomiasis in 264 samples collected from adult cattle of mixed breeds, age and sex, in Anambra and Imo states, Nigeria.

RESULTS

Out of 264 samples analysed, 21 (7.96%) were seropositive for Trypanosoma congolense while 20 (7.58%) were seropositive for T. vivax and 8 (3.03%) were seropositive for T. brucei infections in both the states.

INTERPRETATION & CONCLUSION

The predominant species was found to be T. congolense. Mixed infection of three species, T. vivax, T. congolense and T. brucei was found to dominate other mixed infections in both the states. ELISA detected the infection of the three species of trypanosomes in the same group of animals. The usefulness of antigen capture ELISA in the diagnosis of human or animal trypanosomiasis was established, and the possibility of the spread of HAT caused by T. brucei gambiense and T.b. rhodesiense through cattle was expressed.

Epizootiological importance of Glossina morsitans submorsitans (Diptera: Glossinidae) (Newstead) in the Ghibe River Valley, Southwest Ethiopia

The epizootiological importance of Glossina morsitans submorsitans in Ghibe River Valley was undertaken from October 2000 to September 2001. The flies were collected using baited monoconical traps. G. m. submorsitans occurred with a mean apparent density of 4.26+/-0.49 flies/trap/day and the apparent density was characterized by an increase during the wet season and a decrease during the dry season. Among 450 G. m. submorsitans, approximately 5% were found to be infected with trypanosome. Of these infected flies, 76% were female. Nanomonas, Duttonella and Trypanozoon were the three trypanosome subgenera detected and occurred in the proportions of 57.1%, 38.1% and 4.8%, respectively. Among 139 blood meals of G. m. submorsitans collected, 54.68% were identified to group or species levels. Accordingly, 36.84%, 25%, 11.84% and 10.53% accounted for cattle, kudu, suidae (warthog and/or wild pig) and human, respectively and others such as goats (6.58%), bovidae (5.26%), baboon (2.63%) and water buck (1.32%). While 21.05% of the blood meals were found to be out of detection range.

A comparative longitudinal study of bovine trypanosomiasis in tsetse-free and tsetse-infested zones of the Amhara Region, northwest Ethiopia

A study was conducted to determine the incidence of trypanosome infections in cattle in tsetse-free and tsetse-infested zones of the Amhara Region of northwest Ethiopia. A total of six sentinel herds were established and the cattle observed during a period of 8 consecutive months. The prevalence of seropositive cattle was high in both the tsetse-free and tsetse-infested zones. The average monthly incidence of trypanosome infection, determined using molecular diagnostic tools, was 20.9% and 25.7% in the tsetse-free and the tsetse-infested zones, respectively. In the tsetse-free, Trypanosoma vivax was responsible for 90.9% of the cattle trypanosome infections. In the tsetse-infested zone, Trypanosoma congolense and T. vivax contributed almost equally to the trypanosome infections in cattle. Trypanosome infection, regardless of species, resulted in anaemia as evidenced by a significant decrease in the packed cell volume of the infected animal. The outcome of this longitudinal study suggests that control of trypanosomiasis in the Amhara Region cannot be achieved by tsetse control alone. Supplemental measures to include drug therapy and biting fly control are discussed.

Comparison of the transmissibility ofTrypanosoma congolensestrains, isolated in a trypanosomiasis endemic area of eastern Zambia, byGlossina morsitans morsitans

Transmission experiments were conducted to compare the transmissibility of genetically differentTrypanosoma congolense(Savannah subgroup) strains isolated from cattle in a trypanosomiasis endemic area of eastern Zambia. A total of 17 strains were compared. Three strains were extremely virulent with a short pre-patent period, high parasitaemia and a short median survival time (between 5 and 9 days) in mice. The remainder of the strains belonged to the moderate (6 strains) or low (8 strains) virulence categories with median survival times between 10 and 30 days and >30 days, respectively. Batches of 40 teneralGlossina morsitans morsitans(Diptera: Glossinidae) were offered a single bloodmeal on mice infected with one of those strains. Flies were dissected to determine their infection status 21 days later. The proportion of flies with procyclic and metacyclic infections differed significantly between trypanosome strains and were significantly higher in flies infected with extremely virulent strains (P=0·033 andP=0·016 for the differences in the procyclic infection rate of strains with moderate and low virulence, respectively andP=0·005 andP=0·019 for the differences in the metacyclic infection rate of strains with moderate and low virulence, respectively). On the other hand, moderately virulent strains had, in general, higher procyclic and metacyclic infection rates compared to low virulent strains. But the differences were not significant (P>0·05). The outcome of those experiments shows clear differences in transmissibility of trypanosome strains associated with their virulence. This observation confirms the theory for the evolution and maintenance of virulence in a parasite population and may explain the persistence of virulent trypanosome strains in a susceptible host population.

The transmissibility of Trypanosoma congolense seems to be associated with its level of resistance to isometamidium chloride

In large parts of Africa the control of livestock trypanosomiasis relies on the use of trypanocidal drugs. Resistance against the available compounds is developing rapidly in the trypanosome population. The effect of the development of drug resistance on the fitness of the trypanosome is not well known. To determine the effect of the development of resistance to isometamidium chloride on the trypanosome's transmissibility, transmission experiments were conducted. Use was made of three isogenic clones of Trypanosoma congolense with different susceptibility to the drug. The infection rate in Glossina morsitans morsitans differed significantly between clones and was significantly higher in tsetse flies infected with the T. congolense clone with the highest level of drug resistance.

Observations on the trypanosomosis problem outside the tsetse belts of Sudan

Entomological and trypanosomosis surveys were conducted in the Blue Nile area between Admazien and Khartoum. The surveys showed the area to be tsetse free. The only species of trypanosome found to infect cattle in this study was Trypanosoma vivax, which infected some of the local cattle that had no history of entering tsetse belts. The prevalence of disease varied with the season. High disease prevalence coincided with the periods when tabanid and stomoxy flies were abundant. The study showed that the months when biting flies were most numerous coincided with trypanosomosis outbreaks, but even minimal numbers of these flies may cause the cycle of mechanical transmission to continue in stable enzootic conditions.

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.

Geographical Information Systems in parasitology: a review of potential applications using the example of animal trypanosomosis in West Africa

The epidemiology of vector-borne diseases is complex due to the variability in the ecology of the different actors involved, i.e. hosts, parasites and vectors. The transmission of African animal trypanosomosis in the West-African savannah region is an excellent example of this complexity: riverine tsetse flies have an heterogeneous distribution along the rivers, depending of suitable habitats, and transmit pathogenic trypanosomes were they use domestic animal as feeding hosts. Contrasting epidemiological situations may thus occur at the local scale, and a broad view of the overall environment is necessary to quantify the interfaces in time and space between hosts and vectors. Geographical Information Systems (GIS) can provide new insight into the study of such complex epidemiological processes. GIS is a powerful technology that has been used mainly in map-making, and an enormous amount of knowledge can be gained simply by geographical data projection. GIS also allows juxtaposition of different types of information, creation of new variables, testing of theories and correlation, and generating of predictive models. The purpose of the present paper is to exemplify the potential application of GIS using a recent study carried out on animal trypanosomosis in a cattle-raising area of Burkina Faso.

A longitudinal epidemiological survey of bovine trypanosomosis and its vectors in the White Volta river basin of Northern Ghana

A longitudinal epidemiological survey of bovine trypanosomosis and its vectors was carried out in the Volta river basin of Northern Ghana to determine the relationship between cattle management and the incidence of bovine trypanosomosis. Two groups of sentinel cattle under different systems of management, classified as "fully-sedentary" and "partially-sedentary" (depending on the type of management) were followed over a 1-year period starting from March 2003 onwards. Cattle were screened at intervals of 3 months using the buffy coat technique (BCT). Buffy coat specimen from animals that were positive for the BCT and those that were negative, but with a packed cell volume (PCV) of less than 21% were further tested using the polymerase chain reaction (PCR). Plasma from all animals were tested for antibody using the indirect antibody enzyme-linked immunosorbent assay (ELISA). Trypanosomosis challenge was determined in tandem with the epidemiological survey with watering sites of sentinel cattle being the foci of interest. The parasitological prevalence at the start of the survey was higher in the fully-sedentary group (9%) than in the partially-sedentary group (3%). In subsequent visits, however, the parasitological incidence was consistently higher in the partially-sedentary group than in the fully-sedentary group. The mean seroprevalence (ELISA) of both groups increased from 3% in March to 54% in December. Statistical analysis of the serological results using a random effect logistic regression, showed a significant difference in incidence of bovine trypanosomosis between the two groups. There was also a significant effect of time. The influence of cattle herding on host-vector-parasite interface and its consequence on the incidence of trypanosomosis are discussed.

Implications of the re-invasion of Southeast Uganda by Glossina pallidipes on the epidemiology of bovine trypanosomosis

A study to assess the influence of re-invasion of Glossina pallidipes on the epidemiology of bovine trypanosomosis was conducted in Southeast Uganda. A total of 1,992 cattle were screened in villages, with (949) and without G. pallidipes (1043) for trypanosomosis using a combination of the BCT and HCT methods. The prevalence of trypanosomosis (15.5%), Trypanosoma brucei infection (1.4%), T. congolense infection (7.2%), T. vivax infection (5.3%) and mixed infection (1.6%) in cattle in villages with was significantly higher than in those without G. pallidipes: trypanosomosis (7.1%), T. brucei infection (0.6%), T. congolense infection (2.0%), T. vivax infection (3.3%) and mixed infection (1.2%) (overall trypanosome infection, chi2=35.5, d.f.=1, P<0.05; T. brucei infection, chi2=8.06, d.f.=1, P<0.05; T. congolense infection, chi2=22.8, d.f.=1, P<0.05 and T. vivax infection, chi2=6.4, d.f.=1, P<0.05). Infections of Trypanosoma congolense were predominant in cattle in villages with G. pallidipes, while T. vivax infections were predominant in cattle in villages without. In all villages, T. brucei infections were fewer than either T. congolense or T. vivax infections. The risk of transmission of T. brucei, T. congolense and T. vivax infections was 3, 2.7 and 1.6 times, respectively, higher in villages with G. pallidipes than in those without, despite the presence of G. f. fuscipes in either set of villages. The mean PCV (28.27+/-0.41, 95% CI) and mean herd size (3+/-0.46) of cattle in villages with G. pallidipes were significantly (P<0.05) lower than in those in villages without (mean PCV, 29.48+/-0.34; mean herd size, 4+/-0.72). It is evident that presence of G. pallidipes brings about an increase in the prevalence of T. congolense, which causes a more severe disease in cattle than other species of trypanosomes. This is a rare case of a re-invasion of a tsetse species whose disease transmission capability calls for refocusing of the traditional national tsetse and trypanosomosis control strategies to contain it.

High infection rates of the tick Hyalomma anatolicum anatolicum with Trypanosoma theileri

A crossbred calf (3 months old) obtained from a farm where regular control of ticks was practised and found to be free of blood parasites was inoculated with 20 ml pooled blood collected from four field cattle which had very low Trypanosoma theileri parasitaemias (one parasite per 70 microl blood as determined by the haematocrit centrifugation technique). Trypanosoma theileri was present in the blood 6 days after injection and a peak parasitaemia of 42 parasites per 70 microl blood was recorded by day 12. Hyalomma anatolicum anatolicum nymphs were applied on the ears of the calf on day 8 and they dropped engorged by days 13 and 14. The resulting adult ticks were examined for the presence of T. theileri by severing a leg and making a smear of the clear haemolymph which exuded from the wound. The smear was fixed in methanol and stained with Giemsa stain. The infection rate with T. theileri in the ticks was 43.3% (26 out of 60). The intensity of infection was very high and various developmental stages of the flagellates were observed (epimastigotes, sphaeromastigotes, trypomastigotes and other intermediate stages). The haemolymph from 12 ticks was also collected in tissue culture medium and the trypanosomes survived for 25 weeks before eventually dying. The results demonstrated unequivocally the high vectorial capacity of the tick H. a. anatolicum for T. theileri.

Admixture and diversity in West African cattle populations

We present a population genetic analysis of microsatellite variation in 16 West African cattle populations. West Africa represents a unique juxtaposition of different climatic and ecological zones in a relatively small geographical area. While more humid coastal regions are inhabited by the tsetse fly, a vector which spreads trypanosomiasis among cattle, the disease is not transmitted in the drier areas outside this zone. This is the most thorough study of genetic diversity in cattle within this area, which contains genetically important trypanotolerant Bos taurus breeds. Genetic relationships among the many breeds are examined and levels of diversity are assessed. Admixture levels were determined using a variety of methods. Ancestry informative or population-associated alleles (PAAs) were selected using populations from India, the Near East and Europe. Multivariate analysis, the admix program and model-based Bayesian admixture analysis approaches were also employed. These analyses reveal the direct impact of ecological factors and the profound effect of admixture on the cattle of this region. They also highlight the importance of efforts to prevent further dilution of African taurine breeds by B. indicus cattle.

The transmission of mixed Trypanosoma brucei brucei/T. congolense infections by tsetse (Glossina morsitans morsitans)

Laboratory experiments and field observations clearly show that tsetse flies can be carriers of mixed trypanosome infections. Question remains how easy it is for the tsetse fly to acquire such a mixed infection during the first bloodmeal. This is of particular importance in the epidemiology of Trypanosoma brucei s.l., often a cryptic infection and difficult to transmit to non-teneral tsetse flies. To determine the transmission rate of T. brucei as part of a mixed infection, teneral Glossina morsitans morsitans were fed once on cattle with a mixed (Trypanosoma brucei brucei/Trypanosoma congolense) or single (T. brucei) infection. Of the 140 flies fed on animals with a mixed infection and examined 30 days later, 4 had a metacylic T. brucei infection, 29 a T. congolense infection and 13 a mixed T. brucei/T. congolense infection. There was no significant difference between the transmission rate of T. brucei as a single or as part of a mixed infection. The high proportion of mixed T.b. brucei/T. congolense infections was explained best by a model implying that if a fly is refractory to T. congolense, it is also refractory to T.b. brucei and vice versa. Hence, results suggest that the transmission of T.b. brucei is affected mainly by the vectorial capacity of flies and not by concurrent trypanosome infections in the host.

Mechanical transmission of Trypanosoma congolense in cattle by the African tabanid Atylotus agrestis

The trypanosomes pathogenic to livestock in Africa (Trypanosoma congolense, Trypanosoma vivax, and Trypanosoma brucei) are mainly cyclically transmitted by tsetse (Glossina). However, T. vivax, can also be mechanically transmitted by haematophagous insects. Laboratory studies have demonstrated the mechanical transmission of T. congolense, but confirmation of this under natural conditions was necessary. An experiment was therefore carried out in Lahirasso, Burkina Faso, in a corral completely covered by mosquito net, to avoid exposure to tsetse. Eight receiver heifers, free of trypanosome infection, were kept together with two donor heifers, experimentally infected with local stocks of T. congolense. On average, 291 Atylotus agrestis, freshly captured in Nzi traps, were introduced into the mosquito net daily for a period of 20 days to initiate mechanical transmission among cattle. Daily microscopical observation of their blood indicated that two of the eight receiver heifers became infected with T. congolense from days 42 and 53. Mechanical transmission of T. congolense by A. agrestis was demonstrated unequivocally with a 25% incidence over a 20-day period of exposure under a mean challenge of 29 insects/animal/day. These results, in addition to previous reports, demonstrate the ability of A. agrestis to transmit T. vivax and T. congolense to cattle in Africa by mechanical means. Efforts to eliminate cattle trypanosomosis should therefore consider the eventual persistence of disease as a result of mechanical transmission of trypanosomes by tabanids. Index descriptor and abbreviations: Trypanosoma congolense (Trypanosomatidae) is a pathogenic trypanosome found in wild and domestic herbivores, principally in cattle (Bos taurus, Bos indicus, and cross-breds), in Africa. It is cyclically transmitted by tsetse (Glossina, Diptera); however, mechanical transmission by biting insects may also occur. The present study demonstrates unequivocally the mechanical transmission of T. congolense to cattle by one of the most common African tabanids, A. agrestis. The main conclusion is that tabanids are able to transmit T. congolense; however, the incidence of transmission was lower than in studies carried out under the same conditions with T. vivax. Better models of mechanical transmission are required to understand why, on the one hand, epidemiological studies support the mechanical transmission of T. vivax but not T. congolense, and, on the other hand, experimental studies confirm that both species can be mechanically transmitted. Our studies suggest that the epidemiology of trypanosomosis in cattle involves tabanids, and hence, the eradication of tsetse-flies in Africa will not necessarily lead to the eradication of trypanosomosis in domestic livestock. ADT, apparent density of insects per trap per day (mean number of insects caught in one type of trap per 24h of trapping); D, day; NS, not statistically significant

Trypanosoma vivax: mechanical transmission in cattle by one of the most common African tabanids, Atylotus agrestis

The role of mechanical vectors in the transmission of African livestock trypanosomes has always been controversial relative to tsetse flies, their cyclical vectors. An experiment was carried out in Burkina Faso to demonstrate mechanical transmission of Trypanosoma vivax by one of the most common tabanids in Africa: Atylotus agrestis. Eight heifers (crossbred zebuxBaoulé), free of trypanosome infection, were kept in a corral covered by a mosquito net, together with two heifers infected experimentally with a local stock of T. vivax. On average, 324 A. agrestis, freshly captured with Nzi traps, were introduced daily over 20 days. Parasitological, PCR and serological examinations were carried out regularly to assess infections and levels of parasitaemia. Microscopic examination of buffy-coats indicated that five of the eight receiver-heifers were infected on days 8, 13, 32, 41, and 48. PCR results indicated that these five heifers were already infected by day 13. Mechanical transmission of T. vivax by A. agrestis was demonstrated unequivocally, at a high rate (63% in 13-20 days). Conditions of transmission in this experiment are discussed in terms of natural rates of challenge. The importance of tabanids as mechanical vectors of T. vivax should be re-considered, in light of these results. Creation of tsetse free zones in Africa will generally lead to the disappearance of T. congolense, T. brucei, and most often T. vivax as well; however, in areas where T. vivax can be mechanically transmitted, clearance of tsetse may not be sufficient to eradicate livestock trypanosomosis.

Seasonal variations in the distribution and abundance of the tsetse fly, Glossina morsitans morsitans in eastern Zambia

The seasonal changes in the distribution of Glossina morsitans morsitans Westwood (Diptera: Glossinidae) and its main host, cattle, were examined in a cultivated area of the plateau of eastern Zambia. During four consecutive years, the tsetse and cattle populations were monitored along a fly-round transect traversing the two main vegetation types in the study area. These were miombo, a one-storied open woodland with the genera Brachystegia and Julbernardia dominant, and munga, a one- or two-storied woodland where the principal tree genera were Acacia, Combretum and Terminalia. Concurrently, a capture/mark/release/recapture (CMRR) exercise was conducted along two other transects also traversing both vegetation types. The index of apparent abundance of tsetse (IAA) in miombo increased at the beginning of the rainy season (November), reached its peak at the end of the rainy season (April) and was low during the cold season (May to late August), but especially the hot dry season (September to late October). The IAA of tsetse in munga showed a pattern that was the reverse of that in miombo. The seasonal changes in the IAA of tsetse in both vegetation types were in accordance with changes in the movement patterns of tsetse between the two vegetation type as observed using CMRR. The distribution and abundance of cattle along the transect also showed a seasonal trend. This was especially so in munga, during the first three years of observations, where cattle abundance increased gradually from June onwards, reached a maximum at the end of the hot dry season (October-November) and declined steeply at the start of the rainy season (November-December). In both vegetation types, the monthly mean IAA of tsetse was positively correlated with the abundance of cattle in the previous month. It is concluded that the distribution of tsetse in cultivated area of the eastern plateau of Zambia undergoes substantial seasonal changes, which can partly be attributed to changes in the distribution of cattle. The implications of these observations for the control of tsetse are discussed.

Comparing apples and oranges--model-based assessment of different tsetse-transmitted trypanosomosis control strategies

The current control strategies for tsetse-transmitted trypanosomosis in cattle (trypanocidal drugs, tsetse control and trypanotolerant cattle) are briefly reviewed and their adoption rates in different geographic regions of sub-Saharan Africa are presented. The impact of these control strategies and the potential use of vaccines, should they be developed, on trypanosomosis transmission were compared using a mathematical model. The relative trypanosomosis prevalence compared with no control was estimated across a range of control coverages (from none to complete control coverage) by varying the change in specific model parameters influenced by individual control measures. Based on this comparison, the relative rankings of the effect of control strategies on reducing disease prevalence were: vector control, vaccination, and drug use, in that order. In this model, trypanotolerance was assumed to decrease disease prevalence, but not to influence transmission. Differences in the predicted impact of control measures on the transmission of human sleeping sickness are discussed. Finally, the role of transmission model outputs as inputs for economic models to guide investment decisions for trypanosomosis control is emphasised.

The distribution of tsetse (Diptera: Glossinidae) and bovine trypanosomosis in the Matutuine District, Maputo Province, Mozambique

A tsetse and bovine trypanosomosis survey was conducted during 1998 and 1999 in the Matutuine District of Maputo Province (Mozambique). A total of 59 Glossina brevipalpis and 17 Glossina austeni were captured throughout the district. Survey results suggest that Glossina brevipalpis is mainly concentrated in dense vegetation along the Maputo River and in the wetlands east of the river. G. austeni, on the other hand, was captured mainly in dense thickets in drier areas. Both tsetse species are suspected to be vectors of bovine trypanosomosis. Bovine trypanosomosis (75,5% Trypanosoma congolense) was diagnosed in 53 animals (13,9%) from seven sampling sites distributed throughout the district. The prevalence of cattle with anti-trypanosomal antibodies was high (29,9%). The incidence of trypanosomal infections in sentinel cattle was also high. The widespread distribution of bovine trypanosomosis and the high prevalence of infection are likely to have a significant impact on cattle production and, hence, the cattle restocking exercise in the district.

Study on the sequential tsetse-transmitted Trypanosoma congolense, T. brucei brucei and T. vivax infections to African buffalo, eland, waterbuck, N'Dama and Boran cattle

Susceptibility of African buffalo, eland, waterbuck, N'Dama and Boran cattle to sequential Glossina morsitans centralis-transmitted infections of Trypanosoma congolense, T. brucei brucei and T. vivax was compared, and their possible role as reservoirs of these parasites for G. moristans centralis, G. pallidipes, G. austeni, G. brevipalpis and G. longipennis determined. The buffalo, eland, waterbuck and N'Dama controlled T. congolense parasitaemias and were able to prevent anaemia. By contrast, one Boran became severely anaemic whilst the other controlled parasitaemia and anaemia. When the above five species of Bovidae were rechallenged with T. brucei brucei they showed persistent parasitaemias but did not develop anaemia. The buffalo died of other causes. When the remaining four bovids were rechallenged with T. vivax they became infected with mixed T. vivax/T. b. brucei parasites. Eland, waterbuck and N'Dama controlled parasitaemias and anaemia whereas the Boran became anaemic. Cyclical development of T. congolense occurred in G. moristans centralis when fed on the bovid hosts, with buffalo being infective for tsetse flies for a much longer period. There was no relationship between the levels of T. congolense parasitaemia in the bovid hosts and the resultant infection rates in tsetse flies. Glossina m. centralis was more susceptible than G. pallidipes to T. brucei brucei whilst G. austeni the least; G. brevipalpis and G. longipennis were refractory to the mature infection. Again there was no relationship between T. brucei brucei parasitaemia levels in the hosts and infection rates in the flies. Glossina m. centralis and G. pallidipes showed mixed T. brucei brucei/T. vivax infections whilst G. austeni, G. brevipalpis and G. longipennis became infected with T. vivax alone. Tsetse flies showed higher T. vivax infection rates when fed on the hosts with high parasitaemias than thosewith low parasitaemias. Thus trypanotolerant African buffalo, eland, waterbuck, N'Dama as well as some trypanosusceptible Boran cattle can serve as reservoirs of single or mixed trypanosome infections for tsetse flies. This study has also shown that the Ag-ELISA on the sera from the five bovid hosts had low sensitivity and species-specificity. Examinations of thin wet blood films and buffy coats with a phase-contrast microscope were not sensitive enough to detect the parasites on all occasions. Xenodiagnosis using mice for T. brucei brucei and T. congolense infections, and tsetse flies for all the three trypanosome species were most sensitive for the detection of trypanosome infections in the bovid hosts.

Intra-species differentiation of Trypanosoma evansi by DNA fingerprinting with arbitrary primered polymerase chain reaction

A method was developed to obtain reproducible DNA fingerprints from isolates of Trypanosoma evansi by PCR-based amplification using arbitrary primers (AP-PCR). Only one out of 10 randomly designed 12-mer primers generated DNA fingerprint profiles that revealed intra-species differences in T. evansi. The technique was applied in association with parasitological and serological examinations to investigate animal-to-animal transmission during an outbreak of surra in Thailand. The AP-PCR method has the advantage of being simple, fast and sensitive to diagnosis and characterization of the parasites since it does not require prior DNA sequence information. The technique should prove useful for the proper understanding of epidemiology and for designing rational control programs for trypanosomosis.

Effect of diminazene aceturate on the infectivity and transmissibility of drug-resistant Trypanosoma congolense in Glossina morsitans centralis

To determine the duration after treatment of cattle with diminazene aceturate that the drug influences the tsetse infectivity and transmissibility of a drug-resistant Trypanosoma congolense, six Boran cattle were infected with T. congolense IL 3338 via the bites of Glossina morsitans centralis. At the first peak of parasitaemia, different groups of 120 teneral G. m. centralis were fed on one occasion on each animal, 1 h before treatment with diminazene aceturate at a dose of 3.5 mg kg-1 body weight. Thereafter, on Days 1, 2, 3, 7, 14 and 21 after treatment, six different groups of 120 teneral G. m. centralis were similarly fed on each animal. After 28 days maintenance on uninfected goats, all the flies were probed onto slides at 37 degrees C to identify those extruding metacyclic trypanosomes. Flies with mature infections from each group were then fed on one occasion on individual mice to determine the transmissibility index. After dissection of flies on Day 30 after their feed on the cattle, the mean mature (+/-SE) infection rates in the seven groups of flies were 32.1 +/- 2.2, 1.0 +/- 0.7, 0.4 +/- 0.4, 0.5 +/- 0.3, 20.0 +/- 1.7, 33.3 +/- 2.2 and 23.4 +/- 2.0% for flies fed on Days 0, 1, 2, 3, 7, 14 and 21 after treatment with diminazene, respectively. The transmissibility rates for the seven groups ranged from 94 to 100%. Thus, when cattle were infected with a diminazene-resistant T. congolense, treatment with diminazene aceturate caused a substantial reduction in the ability of the trypanosomes to establish mature infections in tsetse for at least the first 7 days after treatment. In contrast, no significant effect on the transmissibility of the parasites to mice was observed at different intervals after treatment.

The importance of cattle as a food source for Glossina morsitans morsitans Katete district, Eastern Province, Zambia

The feeding habits of Glossina morsitans morsitans in the Eastern Province of Zambia were studied. A total of 687 meals were identified. Results show that 75.1% of the meals were taken on cattle. These results are discussed in relation to the published data on feeding patterns of Glossina morsitans morsitans and the control of tsetse or tsetse-transmitted trypanosomosis in the study area.

Trypanosomes, tsetse and trypanotolerance: coevolution in tropical Africa

Trypanotolerance reaches varying degrees of stability in wild and domestic animals according to their co-evolutionary contacts with tsetse flies and trypanosomes. In this context, various mechanisms developed by wild and domestic Bovidae to cope with tsetse and trypanosome challenge are discussed.

Effect of tsetse control on trypanosome prevalence in livestock: problems of experimental design and statistical interpretation--a case study in northern Côte d'Ivoire

As part of a study on livestock productivity under trypanosomosis risk in the region of Boundiali, northern Côte d'Ivoire, 21 herds of cattle (N'Dama, Baoulé and Zebu crosses) and 20 flocks of Djallonké and Djallonké x Sahel sheep were monitored monthly for body weight, packed red cell volume and trypanosomal parasitaemia over various periods between January 1984 and December 1992. A tsetse control campaign using biconical traps impregnated with alpha-cypermethrin started in December 1987. Tsetse control reduced the relative tsetse density by over 95% between 1988 and 1992, and this was associated with reductions in the prevalence of Trypanosoma congolense over the same period of over 90% both in sheep and cattle. Average reductions in the prevalence of T. vivax were lower, on average 68% in adults and 85% in young animals. Attempts were made in the design of the study to allow comparisons between controlled and uncontrolled areas; however, there were too many confounding and uncontrollable factors to allow such comparisons to be made. It was necessary, therefore, to compare data collected from all herds and flocks before and after the intervention, with the consequential difficulties in accounting for uncontrollable year-to-year variations in factors affecting trypanosome prevalence in livestock.

The impact of nagana

The disease in cattle, called nagana in Zululand, was linked with trypanosomal parasitaemia and tsetse flies. Nagana occurs in livestock throughout the tsetse belts of Africa. Wild animals are tolerant of trypanosomal infections. Nagana affects individual animals, herds and socio-economic development. In susceptible animals nagana may be acute, but chronic infections are more common. The host-parasite interaction produces extensive pathology and severe anaemia. Clinically affected animals lose condition and become weak and unproductive. Nagana is often fatal and, at herd level, its impact is wide ranging. All aspects of production are depressed: fertility is impaired; milk yields, growth and work output are reduced; and the mortality rate may reduce herd size. Africa has to feed its rapidly growing human population, and animal products are a vital dietary component. However, in most tsetse areas, there is not enough meat and milk. Furthermore, animal draft power is often not available, which limits cultivation and local transport. These factors lower household incomes and retard socio-economic development. Sustainable rural development requires that nagana be controlled. This in turn needs considerable resources, whichever control strategy is adopted.

Tsetse-trypanosomiasis challenge to village N'Dama cattle in The Gambia: field assessments of spatial and temporal patterns of tsetse-cattle contact and the risk of trypanosomiasis infection

The severity of the trypanosomiasis problem in a particular location is traditionally assessed in terms of a challenge index-the product of some measure of tsetse abundance and infection-rate-which is assumed to be proportional to the force of infection. However, this index masks variation in the force of infection between herds and among individuals within herds. It is also not comparable between sites since the relative abundance of tsetse to hosts may vary. We have studied spatial distribution of herds of cattle in relation to tsetse in The Gambia and calculated an index of challenge based on the ratio of vectors to hosts over the livestock ranging area. This index is strongly correlated with estimates of the force of infection calculated from the incidence of infection in susceptible zebu; and it provides information on heterogeneity in exposure of different herds to tsetse.

Trypanosoma vivax in Colombia: epidemiology and production losses

The present paper reports information obtained between 1982 and 1989 in the tropical lowlands of the Atlantic coast of Colombia. The studies confirmed that Trypanosoma vivax was widely but unevenly distributed within the study area and that incidence ranged from sporadic transmission to localised epidemics in which virtually all susceptible animals become infected within a period of 3 months. Quantitative epidemiological methods substantiated impressions of previous workers that T. vivax infections were associated with low-lying swampy areas and tabanid activity. Overt clinical trypanosomosis was rare in herds in which T. vivax was endemic. However, in these herds primary T. vivax infections were shown to cause subclinical alterations in calves. The infection consistently caused a temporary depression of packed cell volume (PCV) as well as a reduction in growth rate. There was no evidence of compensatory growth at a later stage.

Perinatal infection with Trypanosoma vivax in a calf in Venezuela

A case of perinatal infection by Trypanosoma vivax is reported in a 5-hr-old Carora local dairy breed calf. The levels of parasitemia were followed during 15 mo in the infected animal. Similarly, its immune response was monitored during 4 mo by the indirect fluorescent assay. Trypanosoma vivax was observed in Giemsa-stained blood films collected during the first 4 mo of life from the infected calf. Positivity was detected on days 0, 35, 80, and 125 after first sampling. Antibodies against T. vivax were detected each time the indirect fluorescent assay was performed, showing low titers (1:80-1:160). The importance of a congenital route of transmission for this Trypanosoma species in cattle herds, on a continent where Glossina spp. flies are absent, is discussed.

Suppression of T-Cell Responsiveness during Tsetse-Transmitted Trypanosomiasis in Cattle

In the present study, we demonstrate that lymph node cells from cattle infected with T. congolense through tsetse fly challenge were unable to proliferate in vitro following activation with the T-cell mitogen Concanavalin A. This was associated with a simultaneous suppression of interleukin 2 (IL-2) production and interleukin 2 receptor (IL-2R) expression. However, the capacity of the cells to secrete interferon gamma following the mitogenic activation was not affected by the infection.

Pathogenicity of tsetse-transmitted Trypanosoma congolense for waterbuck (Kobus defassa) and Boran cattle (Bos indicus)

Five waterbuck (Kobus defassa) and four Boran cattle (Bos indicus) were infected with Trypanosoma congolense IL2895 using Glossina morsitans morsitans. At the same time, two waterbuck and two cattle were inoculated intravenously with bloodstream forms. With both methods of challenge, cattle had short prepatent periods followed by a continuous high parasitaemia. All cattle became severely anaemic and had to be treated with trypanocidal drugs to prevent death. In contrast, tsetse and intravenous challenge of waterbuck resulted in a long prepatent period, followed by brief, intermittent levels of low parasitaemia, and eventual selfcure. Waterbuck did not become anaemic, even during short bouts of parasitaemia which in general were very low. Both cattle and waterbuck developed parasite-specific antibodies, but some waterbuck failed to develop neutralizing antibodies. These results suggest that the ability of the waterbuck to resist trypanosome infection may not be mediated entirely by antibody-dependent immune processes.

Infection rates in Glossina morsitans morsitans fed on waterbuck and Boran cattle infected with Trypanosoma congolense

Teneral Glossina morsitans morsitans were fed on waterbuck (Kobus defassa) and Boran cattle (Bos indicus) infected experimentally with Trypanosoma congolense clone IL2895. Infection rates in tsetse varied from 9 to 31% when fed on cattle, and from 2 to 59% when fed on waterbuck. In waterbuck, infections were often detected through the development of parasites in tsetse at times when parasitaemia could not be detected through microscopic examination of blood. Male and female, and 1- and 2-day-old flies were equally susceptible to infection on both hosts. Infection in tsetse was associated with a 14% absolute reduction in survival during the month following the infective feed.

An analysis of survey measurements of tsetse challenge to trypanotolerant cattle in relation to aspects of analytical models of trypanosomiasis

The recent development of analytical models of trypanosomiasis has increased the general applicability of models to the strategic control of the disease. An analysis of detailed data on tsetse abundance and infection rates and trypanosome prevalence in village-based trypanotolerant cattle over 4 years in The Gambia showed that seasonal patterns of abundance in Glossina morsitans-infested areas were consistent, and that the rates of trypanosome infection remained relatively unchanging. However, there were two distinct seasonal trypanosome prevalence patterns in cattle, with peaks occurring either in May/June/July or November/December. The peaks of trypanosome prevalence therefore occurred either 4 months before or after the times of peak challenge from G. morsitans, not 1 or 2 months after as predicted by analytical models. In G. palpalis-infested areas there was little seasonal variation in abundance or trypanosome infections, but peak trypanosome prevalence still occurred mostly in June/July. Despite the incongruity between the months of peak prevalence and challenge, the overall annual prevalence rates and tsetse challenge indices showed a significant linear relationship. It is concluded that existing analytical models need to be refined to take into account trypanotolerance and the various influences on the expression of this trait.

Susceptibility of African buffalo and Boran cattle to Trypanosoma congolense transmitted by Glossina morsitans centralis

Four African buffalo (Syncerus caffer) and four Boran cattle (Bos indicus) were each exposed to the bites of 10 tsetse flies infected with Trypanosoma congolense. Although both groups of animals became infected, the buffalo showed no clinical signs of trypanosomiasis while the cattle suffered from the disease characterized by pronounced skin reactions, high parasitaemia and severe anaemia. The prepatent periods in the buffalo varied from 18 to 27 days in comparison with 11 to 14 days in the cattle. In the buffalo, skin reactions were only detectable by histological examination of skin biopsies, the peak of parasitaemia was at least a hundredfold below that in cattle and after 54 days parasites were no longer detected. In contrast, the cattle had a continuous high parasitaemia until they were treated with a trypanocidal drug 60 days after infection. Neutralizing antibody to metacyclic trypanosomes appeared in the buffalo during the prepatent period, 15-20 days after infection, whereas in cattle neutralizing antibody was not detected until 10 days after the first peak of the parasitaemia, 25-30 days after infection.

Experimental infection of N'Dama cattle with trypanosomes using Glossina palpalis gambiensis caught in the wild

The transmissibility of trypanosome infection to N'Dama cattle by tsetse flies caught in the field was examined. Wild-caught Glossina palpalis gambiensis were transferred singly into small numbered cages and allowed to feed on 14 uninfected N'Dama cattle. Following a completed feed the tsetse were dissected and infection in the proboscis, the salivary glands and the gut was recorded. Each animal was bitten by a number of tsetse ranging from five up to 64 flies. Following dissection of the tsetse flies, seven of the cattle were found to have been bitten by a single infected tsetse, five by two, while the remaining two were each fed upon by three infected tsetse. The tsetse were harbouring either Trypanosoma vivax or a trypanosome species belonging to the Nannomonas subgenus or both species. The experimental animals were monitored daily over a period of three months for the appearance of trypanosomes in the blood and for antibodies in their sera. Other parameters such as body temperature, local skin reactions, packed red cell volume and weight changes were also measured. Trypanosomes were first detected eight days after the infective bite. Only five of the 14 cattle became infected, of which three had been exposed to a single infected tsetse fly. Trypanosoma vivax was detected in one animal, Trypanosoma congolense in two cattle and mixed infections of both species in the remaining two animals. These findings show that N'Dama cattle can become infected with trypanosomes through the bite of a single infected tsetse fly under field conditions. However, only five (possibly eight) of the 23 infected tsetse were able to transmit the parasites successfully.

[Tabanus importunus, experimental mechanical vector of Trypanosoma vivax in French Guiana]

The pathogen of american bovine trypanosomiasis Trypanosoma vivax was transmitted between zebu bull calves by blood meal interruption and immediate resumption of female Tabanus importunus horse flies. Transmission was achieved by 44 transfers from the donor animal to the recipient animal carried out from 8 to 10 days after experimental inoculation of the donor animal. Parasitemia was detected in the recipient animal 10 days after transfer beginning.

Mechanical transmission of Trypanosoma brucei rhodesiense by Glossina morsitans morsitans (Diptera:Glossinidae)

Interrupted feedings of teneral, laboratory-reared Glossina morsitans morsitans were used to study mechanical transmission of Trypanosoma brucei rhodesiense. Intervals between exposure of individual flies on parasitaemic rats and refeeding on clean rats were varied from five minutes to 24 hours. Direct transmissions were demonstrated at each interval up to 160 minutes after exposure. Proboscis dissections showed that active trypanosomes were present up to 320 minutes after exposure. No mechanical transmissions from bovine to bovine occurred in 39 attempts, when groups of 20-120 flies exposed on parasitaemic bovines were transferred immediately to uninfected cattle, but two of 40 individual flies exposed on parasitaemic bovines mechanically transmitted trypanosomes to clean rats. Proboscis dissections made immediately after flies were exposed to a bovine with a parasitaemia of 4.8 x 10(-4) trypanosomes/microliters of blood showed that 11 of 20 (55%) had active trypanosomes in the food canal. The mean number of trypanosomes per proboscis was 29.4 (+/- 20.5). Of 20 flies exposed on a bovine with a low parasitaemia, however, only one trypanosome was seen in proboscis dissections. The parasitaemia of the infected donor was an important factor in mechanical transmission. The mechanical transmission of trypanosomes from one host to another may largely depend on the infectivity threshold of the recipient host, and individual mechanically-infected tsetse flies may not transmit an infective dose.

A model of tsetse-transmitted animal trypanosomiasis

The data needed to develop analytical models of trypanosomiasis transmission have become available only recently. By making some simplifying assumptions, models of the dynamics of the disease in vector, cattle and wild mammal populations can be constructed in order to determine criteria for successful disease control by mass and targetted chemotherapy, and by vector control. The heterogeneity in transmission due to tsetse fly feeding preferences and the variability of immunological characteristics among the vertebrate hosts account for differences in prevalence of Trypanosoma vivax and T. congolense, and also lead to an increase in the basic reproductive rates of the parasites and a corresponding decrease in the vector population density threshold for disease eradication or persistence. The long life-span of the vectors relative to the duration of the parasites' developmental period lead to high infection rates in the vector and high values of R0. The efficacy of chemotherapeutic regimes depends on the relationship between treatment rate and the duration of prophylaxis conferred by the drugs used. The model's predictions of the effects of vector control are shown to be in broad agreement with published field data for Mkwaja Ranch, Tanzania. Vector control programmes are frequently blighted by reinvasion, and the implications of this are discussed in terms of a model for fly immigration. With immigration of vectors, the disease is always endemic, though the infection rate in the fly population is modified by the effect of differential mortalities inside and outside the controlled area on cohorts of incubating flies. Sensitivity analysis of the model, using Monte-Carlo methods, enables an assessment of the relative importance of the parameters to be made. The results emphasize the need for studies of the wild animal reservoir to be carried out alongside entomological surveys. The relative accuracy with which field measurements need to be made in order to minimize the uncertainty in predictions of trypanosomiasis prevalence is discussed.

Therapeutic and prophylactic activity of isometamidium chloride in Boran cattle against Trypanosoma vivax transmitted by Glossina morsitans centralis

Ten Boran steers were infected with Trypanosoma vivax, transmitted by Glossina morsitans centralis; five steers with a T vivax clone from Nigeria and five with a T vivax clone from Kenya. Eleven days after infection all 10 animals were treated with 0.5 mg kg-1 isometamidium chloride. Four steers infected with the Nigerian T vivax and all five infected with the Kenyan T vivax were completely cured. When different steers received a single prophylactic dose of 0.5 mg kg-1 isometamidium chloride and subjected to monthly tsetse-transmitted challenge with the same T vivax clones, complete protection was afforded for at least two months against challenge with the Nigerian T vivax, but for less than one month against the Kenyan T vivax. The findings indicate that the level of sensitivity of a T vivax population to the prophylactic activity of isometamidium chloride cannot be concluded from sensitivity studies based on the therapeutic action of the drug.

Trypanosoma congolense: host responses following tsetse transmitted infection of Kilifi isolates in goats

East African x Galla goats, when infected with Trypanosoma congolense isolates from the Kilifi area of Kenya by Glossina morsitans centralis, did not develop the characteristic chancre reaction at the bite sites, whereas bites of tsetse infected with the cloned T. congolense IL.1180 from Serengeti, Tanzania, resulted in chancres in the same goats. Histological changes could not be observed in skin biopsies collected 8 or 9 days after infection with Kilifi isolates. However, all goats became parasitemic about 10 days after challenge. It is concluded that the absence of chancre development is a characteristic feature of T. congolense parasites from Kilifi. The isoenzyme analysis of clones of two T. congolense Kilifi isolates and the T. congolense clone IL.1180 indicated that they belong to different zymodemes. Neutralizing antibodies to homologous metacyclic variable antigen types were detected in six out of seven (86%) of the sera from goats infected with a clone or stock of a T. congolense Kilifi isolate, 20 days after infection. Goats primed by tsetse transmitted infection with a stock or clone of T. congolense from Kilifi and treated with Berenil were, in three out of eight cases (37%), not immune to homologous challenge. It is suggested that the reduced immune response to metacyclic variable antigen types could be a result of the absence of cellular infiltration, i.e., chancre development in the skin at the tsetse bite site. It is concluded that the use of the chancre reaction as a marker for serodeme analysis of recently isolated stocks of T. congolense from Kilifi was not feasible.

Transmission of Trypanosoma theileri to cattle by Tabanidae

Tabanids were collected in an area in northern Germany, where pastured cattle were abundant. Trypanosomatidae were identified in 14% of tabanids examined. Twelve cattle became infected with Trypanosoma theileri after applying usually 2-5 infected tabanids, to the intact oral mucosa. Haematopota pluvialis, Haematopota italica, Hybomitra micans and Tabanus bromius were identified as vectors. Infective stages of Trypanosoma theileri were identified in the gut and in the faeces of tabanids by transmission experiments. The minimum prepatent period was less than 4 days. No apparent signs of disease were observed in the infected cattle.

Apparent exhaustion of the variable antigen repertoires of Trypanosoma vivax in infected cattle

Three groups of cattle, each group comprising six animals, were inoculated intravenously with populations of bloodstream forms of Trypanosoma vivax. The first group received T. vivax clone ILDat 1.3 derived from an isolate from Nigeria, while the other two received T. vivax stocks IL 1875 or IL 2133 isolated from Coast Province, Kenya. One animal from the group that was infected with IL 1875 died 8 weeks postinfection. The remaining 17 animals became aparasitemic in 8 to 12 weeks without intervention by drug therapy. The recovered animals developed serodeme-specific immunity against Glossina morsitans subsp. centralis-transmitted challenge. There was complete cross-protection between the two East African T. vivax stocks, although they were isolated from areas 80 to 90 km apart, indicating that they belong to the same serodeme. Antibodies to the homologous metacyclic variable antigen types (VATs) were not detected in sera from recovered animals, suggesting that the immunity displayed by the recovered animals was directed at the bloodstream and not the metacyclic VATs. It is thus suggested that recovery in these animals is due to exhaustion of the repertoire of bloodstream VATs expressed in the animals by the infecting T. vivax clone or stocks.