Description of Events Where African Buffaloes (Syncerus caffer) Strayed from the Endemic Foot-and-Mouth Disease Zone in South Africa, 1998-2008.

African buffaloes (Syncerus caffer) are reservoir hosts of Southern African Territories (SAT) foot-and-mouth disease (FMD) virus strains. In South Africa, infected buffaloes are found in the FMD-infected zone comprising the Kruger National Park (KNP) and its adjoining reserves. When these buffaloes stray into livestock areas, they pose a risk of FMD transmission to livestock. We assessed 645 records of stray buffalo events (3124 animals) from the FMD infected zone during 1998-2008 for (i) their temporal distribution, (ii) group size, (iii) age and gender composition, (iv) distance from the infected zone fence and (v) outcome reported for each event. A maximum entropy model was developed to evaluate spatial predictors of stray buffalo events and assess current disease control zones. Out of all buffaloes recorded straying, 38.5% escaped from the FMD infected zone during 2000/2001, following floods that caused extensive damage to wildlife fences. Escape patterns were not apparently influenced by season. The median size of stray groups was a single animal (IQR [1-2]). Adult animals predominated, comprising 90.4% (620/686) of the animals for which age was recorded. Of the 315 events with accurate spatial information, 204 (64.8%) were recorded within 1 km from the FMD infected zone. During late winter/spring (June-October), stray buffaloes were found significantly closer to the FMD infected zone (median = 0.3 km, IQR [0.1-0.6]). Less than 13% (40/315) of stray groups reached the FMD protection zone without vaccination, posing a higher risk of spreading FMD to these more susceptible livestock. Model outputs suggest that distance from the FMD infected zone, urban areas and permanent water sources contributed almost 85% to the spatial probability of stray buffalo events. Areas with a high probability for stray buffalo events were well covered by current disease control zones, although FMD risk mitigation could be improved by expanding the vaccination zone in certain areas.

Understanding the effects of time on collaborative learning processes in problem based learning: a mixed methods study.

Little is known how time influences collaborative learning groups in medical education. Therefore a thorough exploration of the development of learning processes over time was undertaken in an undergraduate PBL curriculum over 18 months. A mixed-methods triangulation design was used. First, the quantitative study measured how various learning processes developed within and over three periods in the first 1,5 study years of an undergraduate curriculum. Next, a qualitative study using semi-structured individual interviews focused on detailed development of group processes driving collaborative learning during one period in seven tutorial groups. The hierarchic multilevel analyses of the quantitative data showed that a varying combination of group processes developed within and over the three observed periods. The qualitative study illustrated development in psychological safety, interdependence, potency, group learning behaviour, social and task cohesion. Two new processes emerged: 'transactive memory' and 'convergence in mental models'. The results indicate that groups are dynamic social systems with numerous contextual influences. Future research should thus include time as an important influence on collaborative learning. Practical implications are discussed.

Ghibe river basin in Ethiopia: present situation of trypanocidal drug resistance in Trypanosoma congolense using tests in mice and PCR-RFLP.

A cross-sectional study was carried out in the Ghibe valley from August to October 2010. 411 head of cattle were sampled in eight villages for buffy coat examination (BCE) and blood spots were collected from each animal for trypanosomose diagnosis by 18S-PCR-RFLP and diminazene aceturate (DA) resistance by Ade2-PCR-RFLP. Three villages were selected in a zone where trypanosomosis control operations are currently on-going whereas the other 5 villages were located outside these control operations. Twenty-four samples (5.84%) were diagnosed positive for Trypanosoma congolense by BCE and injected in mice for further characterization. Twelve of those isolates successfully multiplied in mice and were tested by an in vivo mouse test for diminazene (DA) (10 and 20mg/kg B.W.) and isometamidium (ISM) (1mg/kg B.W.) resistance. All were shown to be resistant to both drugs at all doses. The use of the Ade2-PCR-RFLP on these isolates confirmed their DA-resistance profile. Seventy-three of the collected blood spots (17.8%) were diagnosed positive for T. congolense by 18S-PCR-RFLP of which 37 (50.7%) gave amplification products with the Ade2-PCR-RFLP. Here, 35 (94.6%) showed a resistant profile, 1 (2.7%) a sensitive profile and 1 (2.7%) a mixed profile. The data were analysed by logistic regression model and the relapsing time in mice tests was assessed using the Cox regression model. There was no significant intervention effect (P=0.83) with odds ratio equal to 1.21 when using the BCE data. 18S-PCR-RFLP test also showed no significant intervention effect (P=0.60) with odds ratio equal to 1.43. The hazard ratio of getting parasitaemic after treatment with DA at 20mg/kg B.W. compared to the control group was 0.38 which differs significantly from one (P<0.001). Relapsing time after treatment with DA 10mg/kg B.W. or ISM 1mg/kg B.W. was also significantly longer than the prepatent period of the control group. The situation of drug resistance in the Ghibe valley is further discussed.

Field detection of resistance to isometamidium chloride and diminazene aceturate in Trypanosoma vivax from the region of the Boucle du Mouhoun in Burkina Faso.

A longitudinal study assessed the chemoresistance to isometamidium chloride (ISM) and diminazene aceturate (DA) in the region of the Boucle du Mouhoun in Burkina Faso. A preliminary cross-sectional survey allowed the identification of the 10 villages with the highest parasitological prevalences (from 2.1% to 16.1%). In each of these 10 villages, two herds of approximately 50 bovines were selected, one being treated with ISM (1mg/kg b.w.) and the other remaining untreated as control group. All animals (treated and untreated herds) becoming infected were treated with DA (3.5mg/kg b.w.). In total, 978 head of cattle were followed up. Fortnightly controls of the parasitaemia and PCV were carried out during 8 weeks. The main trypanosome species was Trypanosoma vivax (83.6%) followed by Trypanosoma congolense (16.4%). In two villages, less than 25% of the control untreated cattle became positive indicating no need to use prophylactic treatment. These two villages were not further studied. Resistance to ISM was observed in 5 of the remaining 8 villages (Débé, Bendougou, Kangotenga, Mou and Laro) where the relative risk (control/treated hazard ratios) of becoming infected was lower than 2 i.e. between 0.89 (95% CI: 0.43-2.74) and 1.75 (95% CI: 0.57-5.37). In contrast, this study did not show evidence of resistance to DA in the surveyed villages with only 8.6% (n=93) of the cattle relapsing after treatment. Our results suggest that because of the low prevalence of multiple resistances in the area a meticulous use of the sanative pair system would constitute the best option to delay as much as possible the spread of chemoresistance till complete eradication of the disease by vector control operations.

Bovine trypanosomosis in the Upper West Region of Ghana: entomological, parasitological and serological cross-sectional surveys.

Baseline surveys were conducted in the Upper West Region of Ghana to assess the distribution and densities of tsetse species, as well as the prevalence of bovine trypanosomosis. The entomological survey was designed to cover the suitable tsetse habitats along the three main rivers in the study area (i.e. Black Volta, Kulpawn and Sissili). Results indicated the presence of Glossina tachinoides in all three river basins, whilst Glossina palpalis gambiensis was only found close to the southern limit of the study area. A random sampling of 1800 cattle of the West African Short Horn, Sanga and Zebu breeds from 36 randomly selected grid cells covering the study area showed substantial differences between parasitological and serological prevalences. The average parasitological prevalence was estimated at 2.5% (95% CI: 1.06-5.77) with the majority of the infections due to Trypanosoma vivax. Most of the infected cattle were found close to the major river systems. The serological prevalence, measured using Enzyme Linked Immunosorbent Assay (ELISA), test was 19% (95% CI: 14.03-25.35). Cattle with anti-trypanosomal antibodies were also found throughout the study area.

Options for the control of tsetse-transmitted livestock trypanosomosis. An epidemiological perspective.

Tsetse-transmitted livestock trypanosomosis affects livestock in large parts of sub-Saharan Africa. In southern Africa two epidemiological situations can be distinguished. The disease can have an endemic nature with high morbidity and low mortality in the livestock population. Endemic livestock trypanosomosis is found mainly in areas where cattle constitute the main host of tsetse and reservoirs of trypanosomes. Epidemic trypanosomosis, with high morbidity and high mortality is found in areas where wildlife persist as main reservoir and where livestock come into contact with tsetse flies transmitting trypanosomes from the sylvatic reservoir. Based on the differences in impact of the disease on livestock health in these two epidemiological settings, the appropriateness of the available trypanosomosis control tools differs. In trypanosomosis endemic areas, trypanocidal drug use could be the most suitable approach. Possible problems associated with the development of resistance in trypanosomes to the drugs need to be investigated further. In epidemic situations, vector control seems the most appropriate long-term solution.

Virulence in Trypanosoma congolense Savannah subgroup. A comparison between strains and transmission cycles.

Trypanosoma congolense strains have been shown to differ in their virulence both between subgroups and within the Savannah subgroup between strains. This review revisits these findings and complements them with information on the virulence of T. congolense Savannah subgroup strains isolated from cattle (domestic transmission cycle) in different geographical areas and of strains isolated in protected areas where trypanotolerant wildlife species are the reservoir of the trypanosomes (sylvatic transmission cycle). The virulence of a total of 62 T. congolense Savannah subgroup strains (50 domestic and 12 sylvatic), determined using a standard protocol in mice, was compared. Virulence varied substantially between strains with, depending on the strain, the median survival time of infected mice varying from five to more than sixty days. The proportion of highly virulent strains (median survival time <10 days) was significantly (P = 0·005) higher in strains from the sylvatic transmission cycle. The analysis highlights repercussions of the domestication of the trypanosomiasis transmission cycle that may have to be taken in consideration in the development of trypanosomiasis control strategies.

Bovine trypanosomiasis risk in an endemic area on the eastern plateau of Zambia.

The control of bovine trypanosomiasis could be improved by using the available control tools during periods when the incidence of the disease is highest. The present study assessed the monthly risk of bovine trypanosomiasis in 85 sentinel cattle kept on the tsetse-infested eastern plateau of Zambia during a period of 19 consecutive months. To avoid problems associated with persistence of infections because of trypanocidal drug resistance and/or the time lag between sampling and molecular analysis, a survival analysis and the subsequent calculation of risk was used as an indicator of challenge. Results showed that the average monthly risk of infection (92.3% due to Trypanosoma congolense) was 6%. It was significantly higher (7.7%) during the beginning of the rainy season (December-February). According to the outcome of the study, bovine trypanosomiasis control in the study area can be improved through increasing control efforts during this period of highest challenge.

Occurrence of Theileria parva and other haemoprotozoa in cattle at the edge of Hluhluwe-iMfolozi Park, KwaZulu-Natal, South Africa.

Theileria parva, the most important bovine theilerial species in sub-Saharan Africa, causes widespread mortality and morbidity in endemic areas. A survey was conducted using buffy-coat specimens from 60 apparently healthy adult communally herded Nguni-type cattle at the northeastern edge of the Hluhluwe-iMfolozi Park to determine, by means of PCR and Reverse Line Blot (RLB) hybridisation, the occurrence of Theileria and Babesia species. The presence of Trypanosoma species was determined using PCR-RFLP. Results showed that 6.7 % of the specimens were positive for Theileria parva. This significant finding suggests that cattle in South Africa, and not only African buffaloes (Syncerus caffer), may be subclinical carriers of T. parva. Other species identified were T. mutans (83.3%), T. velifera (70.0%), Theileria sp. (sable) (46.8%) and T taurotragi (1.7%). Two specimens (3.3%) were positive for Babesia bovis and single specimens (1.7%) positive for B. bigemina and B. rossi, respectively. Mixed infections, of up to 4 species, were common (65.0%). Only 1 specimen was found to be positive for Trypanosoma vivax, and 2 for T theileri, of which only the first species is pathogenic.

Transmissibility, by Glossina morsitans morsitans, of Trypanosoma congolense strains during the acute and chronic phases of infection.

In order to verify whether chronic trypanosomal infections can affect the transmissibility of Trypanosoma congolense by tsetse flies, batches of Glossina morsitans morsitans were fed on mice infected with the same level of parasitemia (10(8.1)trypanosomes/ml of blood) of two cloned low virulent T. congolense strains during the acute and the chronic phases of infection. Results showed that the proportions of procyclic infections in flies that were fed during the acute phase (32.6% and 45.4% for isolates 1 and 2, respectively) were significantly higher (chi(2)=4.7, P<0.05 and chi(2)=23.7, P<0.0001, respectively) compared to the proportions of procyclic infections of flies fed during the chronic phase of infection (18.8% and 14.9% for isolates 1 and 2, respectively). Similarly the proportions of metacyclic infections in flies fed during the acute phase (32.6% and 45.4% for isolates 1 and 2, respectively) were significantly higher (chi(2)=6.3, P<0.05 and chi(2)=23.7, P<0.0001, respectively) compared to the proportions of metacyclic infections in flies fed during the chronic phase of infection (16.8% and 14.9% for isolates 1 and 2, respectively). No significant difference was found in the maturation rate of both strains during the acute phase compared to the chronic phase of infection (P>0.05). The results of this study suggest that T. congolense loses part of its transmissiblity by tsetse flies during the chronic phase of infection.

Maturation of a Trypanosoma brucei infection to the infectious metacyclic stage is enhanced in nutritionally stressed tsetse flies.

We report on the effect of tsetse fly starvation on the maturation of an established Trypanosoma brucei brucei midgut infection, i.e., the development of procyclic infection into the infectious metacyclic parasites in the tsetse fly salivary glands. Glossina morsitans morsitans flies were nutritionally stressed 10 d after the uptake of a T. b. brucei-infected bloodmeal by depriving these flies from feeding for seven consecutive days, whereas the control fly group (nonstarved group) continued to be fed three times a week. After this period, both fly groups were again fed three times per week on uninfected rabbit. Thirty days after the infected bloodmeal, all surviving flies were dissected and examined for the presence of an immature midgut and a mature salivary gland trypanosome infections. Results showed a significantly increased proportion of flies with salivary gland infection in the nutritionally stressed fly group suggesting an enhanced maturation of the trypanosome infection. These data suggest that environmental factors that cause nutritional stress in a tsetse population do not only make tsetse flies significantly more susceptible to establish a midgut infection as was shown previously but also boost the maturation of these midgut infections.

Nutritional stress affects the tsetse fly's immune gene expression.

Tsetse-transmitted trypanosomiasis poses a serious threat to human and animal health in sub-Saharan Africa. The majority of tsetse flies (Glossina spp.) in a natural population will not develop a mature infection of either Trypanosoma congolense or Trypanosoma brucei sp. because of refractoriness, a phenomenon that is affected by different factors, including the tsetse fly's immune defence. Starvation of tsetse flies significantly increases their susceptibility to the establishment of a trypanosome infection. This paper reports the effects of nutritional stress (starvation) on (a) uninduced baseline levels of gene expression of the antimicrobial peptides attacin, defensin and cecropin in the tsetse fly, and (b) levels of expression induced in response to bacterial (Escherichia coli) or trypanosomal challenge. In newly emerged, unfed tsetse flies, starvation significantly lowers baseline levels of antimicrobial peptide gene expression, especially for attacin and cecropin. In response to trypanosome challenge, only non-starved older flies showed a significant increase in antimicrobial peptide gene expression within 5 days of ingestion of a trypanosome-containing bloodmeal, especially with T. brucei bloodstream forms. These data suggest that a decreased expression of immune genes in newly hatched flies or a lack of immune responsiveness to trypanosomes in older flies, both occurring as a result of fly starvation, may be among the factors contributing to the increased susceptibility of nutritionally stressed tsetse flies to trypanosome infection.

Zoonotic tuberculosis and brucellosis in Africa: neglected zoonoses or minor public-health issues? The outcomes of a multi-disciplinary workshop.

Late in 2007, veterinary, medical and anthropological professionals from Europe and Africa met in a 2-day workshop in Pretoria, South Africa, to evaluate the burden, surveillance and control of zoonotic tuberculosis and brucellosis in sub-Saharan Africa. Keynote presentations reviewed the burden of these diseases on human and livestock health, the existing diagnostic tools, and the available control methods. These presentations were followed by group discussions and the formulation of recommendations. The presence of Mycobacterium bovis and Brucella spp. in livestock was considered to be a serious threat to public health, since livestock and animal products are the only source of such infections in human beings. The impact of these pathogens on human health appears to be relatively marginal, however, when compared with Mycobacterium tuberculosis infections and drug resistance, HIV and malaria. Appropriate diagnostic tools are needed to improve the detection of M. bovis and Brucella spp. in humans. In livestock, the 'test-and-slaughter' approach and the pasteurization of milk, which have been used successfully in industrialized countries, might not be the optimal control tools in Africa. Control strategies should fit the needs and perceptions of local communities. Improved intersectoral and international collaboration in surveillance, diagnosis and control, and in the education of medical and veterinary personnel, are advocated.

The impact of habitat fragmentation on tsetse abundance on the plateau of eastern Zambia.

Tsetse-transmitted human or livestock trypanosomiasis is one of the major constraints to rural development in sub-Saharan Africa. The epidemiology of the disease is determined largely by tsetse fly density. A major factor, contributing to tsetse population density is the availability of suitable habitat. In large parts of Africa, encroachment of people and their livestock resulted in a destruction and fragmentation of such suitable habitat. To determine the effect of habitat change on tsetse density a study was initiated in a tsetse-infested zone of eastern Zambia. The study area represents a gradient of habitat change, starting from a zone with high levels of habitat destruction and ending in an area where livestock and people are almost absent. To determine the distribution and density of the fly, tsetse surveys were conducted throughout the study area in the dry and in the rainy season. Landsat ETM+ imagery covering the study area were classified into four land cover classes (munga, miombo, agriculture and settlements) and two auxiliary spectral classes (clouds and shadow) using a Gaussian Maximum Likelihood Classifier. The classes were regrouped into natural vegetation and agricultural zone. The binary images were overlaid with hexagons to obtain the spatial spectrum of spatial pattern. Hexagonal coverage was selected because of its compact and regular form. To identify scale-specific spatial patterns and associated entomological phenomena, the size of the hexagonal coverage was varied (250 and 500 m). Per coverage, total class area, mean patch size, number of patches and patch size standard deviation were used as fragmentation indices. Based on the fragmentation index values, the study zone was classified using a Partitioning Around Mediods (PAM) method. The number of classes was determined using the Wilks' lambda coefficient. To determine the impact of habitat fragmentation on tsetse abundance, the correlation between the fragmentation indices and the index of apparent density of the flies was determined and habitat changes most affecting tsetse abundance was identified. From this it followed that there is a clear relationship between habitat fragmentation and the abundance of tsetse flies. Heavily fragmented areas have lower numbers of tsetse flies, but when the fragmentation of natural vegetation decreases, the number of tsetse flies increases following a sigmoidal-like curve.

Cross-protection between Trypanosoma congolense strains of low and high virulence.

The aim of this study was to assess the existence of possible cross-protection between Trypanosoma congolense strains of low and extreme virulence circulating in the same trypanosomiasis focus. Groups of six mice were infected using one of three strains of low virulence and challenged with one of three strains of extreme virulence. A group of six mice was used as control for each strain of low and extreme virulence. The results showed that mice infected with one of the strains of extreme virulence developed high parasitaemia and a significant drop of the PCV compared to mice infected with a strain of low virulence and challenged with one of the strains of extreme virulence. With an exception of one strain of extreme virulence (strain F), the survival time of mice infected with the strains of extreme virulence was shorter compared to mice infected with strains of low virulence and subsequently challenged with a strain of extreme virulence. These results suggest that in an area where trypanosomes of various virulence profiles circulate, livestock infected with T. congolense strains of low virulence can be protected against the adverse effects of extremely virulent T. congolense strains.

Nutritional stress of adult female tsetse flies (Diptera: Glossinidae) affects the susceptibility of their offspring to trypanosomal infections.

The epidemiology of tsetse-transmitted trypanosomiasis depends, among other factors, on the proportion of infected flies in a tsetse population. A wide range of intrinsic and extrinsic factors seem to determine the ability of a tsetse fly to become infected and to transmit the parasite. In this paper, we investigated the effect of nutritional stress of reproducing female Glossina morsitans morsitans on the susceptibility of their offspring to trypanosomal infections. Adult female flies that were nutritionally stressed by feeding only once a week, produced pupae with a significant lower weight and offspring with a significant lower fat content as well as a lower baseline immune peptide gene expression. Moreover, infection experiments showed that the emerging teneral flies were significantly more susceptible to a Trypanosoma congolense or Trypanosoma brucei brucei infection than flies emerging from non-starved adult females. These findings suggest that in the field, substantial nutritional stress of adult tsetse flies, as is often experienced during the hot dry season, can increase significantly the vectorial capacity of the emerging teneral flies and thus result in an increased infection rate of the tsetse population.

Endemic type of animal trypanosomiasis is not associated with lower genotype variability of Trypanosoma congolense isolates circulating in livestock.

In order to verify whether the low impact on livestock production in endemic areas is related to a low number of trypanosome strains circulating in livestock, 37 Trypanosoma congolense isolates collected from cattle in 11 sites in an endemic trypanosomiasis area in Eastern Zambia were characterised for genotype variability using a modified amplified fragment length polymorphism technique (AFLP). Isolates were further cloned to evaluate the occurrence of mixed infections in individuals. The results obtained revealed a high genotype diversity (94.6%) among these isolates. Apart from one site, all isolates gave different AFLP profiles in each of the sites. When clones were compared, three (8%) of the 37 isolates had mixed infections. These results indicate the circulation of a high number of strains in this trypanosomiasis endemic area despite the low impact the disease has on livestock production.

Climate change and animal health in Africa.

Climate change is expected to have direct and indirect impacts on African livestock. Direct impacts include increased ambient temperature, floods and droughts. Indirect impacts are the result of reduced availability of water and forage and changes in the environment that promote the spread of contagious diseases through increased contact between animals, or increased survival or availability of the agent or its intermediate host. The distribution and prevalence of vector-borne diseases may be the most significant effect of climate change. The potential vulnerability of the livestock industry will depend on its ability to adapt to such changes. Enhancing this adaptive capacity presents a practical way of coping with climate change. Adaptive capacity could be increased by enabling the African livestock owner to cope better with animal health problems through appropriate policy measures and institutional support. Developing an effective and sustainable animal health service, associated surveillance and emergency preparedness systems and sustainable disease control and prevention programmes is perhaps the most important strategy for dealing with climate change in many African countries.

Evaluating the use of packed cell volume as an indicator of trypanosomal infections in cattle in eastern Zambia.

In this study, packed cell volume-values (PCV) are evaluated as indicator of trypanosomiasis infections in cattle. A total of 734 blood samples were collected in 11 different sampling sites in eastern Zambia: 84 calves (< 1 year), 52 young females and 40 young males (between 1 and 3 years), 228 cows, 317 oxen and 13 bulls (> 3 years). All samples were subjected to three diagnostic tests: parasitological examination using the buffy coat method, PCR/RFLP and PCV determination. The results were compared and analysed in a Bayesian model, which allowed the estimation of the infection prevalence and the respective test sensitivities and specificities. The presence of a trypanosomal infection significantly reduced the PCV, independently of the age and sex of the infected animal. The estimated prevalence of trypanosomal infections in the study area was 34% (95% credibility interval: 30-38%). While the specificity of both the parasitological and the PCR/RFLP tests were set to 1, the parasitological diagnosis had a low sensitivity (37%) compared to the PCR/RFLP (96%). When using a cut-off value of 24, the PCV had a high specificity (98%) but a rather low sensitivity (53%) for identifying trypanosomiasis infections. Using 26% as a cut-off increased the sensitivity to 76% without much affecting the specificity (94%). A parallel combination of the parasitological diagnosis and the PCV improved the diagnostic sensitivity (74% and 89% for PCV cut-off values of 24% or 26%, respectively) while specificity remained high (98% and 94% for PCV cut-off values of 24% or 26%, respectively). These results suggest that such a combination could advantageously be used for the diagnosis of cattle trypanosomiasis in the field: it is much more sensitive than parasitological examination alone and it is much cheaper than molecular tests. However, the value of this approach depends largely on the determination of an appropriate cut-off value to consider a sample positive, depending on the required test sensitivities and specificities.

Investigations on the transmissibility of Trypanosoma congolense by the tsetse fly Glossina morsitans morsitans during its development in a mammalian host.

Experiments were conducted to investigate the effect of the developmental stage of a monomorphic T. congolense IL1180 strain, in a vertebrate host, on its transmissibility by the tsetse fly Glossina morsitans morsitans Westwood (Diptera: Glossinidae). Batches of 160 male teneral tsetse flies were given a single bloodmeal on mice infected with this T. congolense strain 4, 5, 6, 7 or 10 days post-infection. The proportion of infected flies in each of those batches showed that the stage of development of the trypanosome does affect the proportion of flies that develop a mature or immature infection with immature and mature infection rates of flies infected on days 5 or 10 significantly higher. The proportion of infected flies was not affected by the parasitaemia at the moment of infection. Results show that tsetse flies can become infected at any phase of the development of the T. congolense IL 1180 strain but the ease with which trypanosomes develop in the fly depends on the phase in the parasite's development in the host. Those observations suggest that in analogy with the pleomorphic T. brucei s.l. adaptation of the monomorphic T. congolense to development in the fly may also determine the parasite's transmissibility. Moreover, the findings stress the importance of standardising experiments in which the vectorial capacity of tsetse flies is determined and compared.