4. Epidemiology: vector-host and vector-parasite interactions

[See also 27: nos. 12677, 12706, 12713, 12759, 12807.]

A microsimulation model of the spread of Gambian sleeping sickness is described. The model focuses on the randomness of epidemic trajectories brought about merely by the random nature of the tsetse fly bites on humans. There is a high level of variability in the trajectories, primarily due to the small sizes of the populations involved. There is an inverse relationship between the probability of initial extinction and the size of an epidemic flare-up when the disease takes hold. When a stream of one infected fly enters the focus every three days, a low-level epidemic can be sustained with less variability. Implications and further subjects of study are briefly discussed.

Four thousand nine hundred and seventy-one trypanosomosis-surveillance records from an open population of Orma Boran cattle raised under natural trypanosomosis challenge in Galana Ranch, Kenya between the years 1990 and 2000 were analysed. The objective of the analysis was to identify epidemiological factors that influenced time-to-treatment of trypanosomosis cases. Under the surveillance programme, blood was being examined fortnightly for trypanosomosis using the buffy coat technique. Infected animals were treated when their packed cell volumes (PCV) fell to 25 percent or lower. The number of days between the first diagnosis and treatment of trypanosomosis cases was obtained from the difference between diagnosis and treatment dates. Days-to-treatment clustered around the screening periods; therefore, time-to-treatment was represented by a series of time points 0-8 at 14-day intervals. Factors postulated to affect the outcome (time-to-treatment) were: age of an animal at time of diagnosis, sex, number of trypanosome infections, trypanosome species and season of the year. The majority of the cases (89.5 percent) were treated on the same day of diagnosis. Trypanosome infections were more likely to be treated after time 0 in dry than in wet season. Similarly, the rate of treatment was lower in the dry than the wet season. An increase in number of previous trypanosome infections reduced the odds of an animal being treated after time 0. Animals that had been exposed to many infections before had higher rates of treatment than those that had minimal experiences. We offer possible reasons for these observations and conclude that selection of animals for breeding purposes in programmes geared towards improving trypanotolerance should take into consideration the environmental factors that affect classification of an animal as being resistant or susceptible.

The pathogenic trypanosomes Trypanosoma vivax and Trypanosoma congolense are transmitted by tsetse and cause trypanosomosis, a debilitating disease affecting livestock in Africa and South America. Here, we discuss the need, relevance and advantage of having the Trypanosoma genomes sequenced, annotated and analysed in a comparative genomics context. We also propose the increased involvement of scientists from countries in which the diseases are endemic, in the initiative to aid the development and maintenance of capacity in bioinformatics research in these countries.

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 zebu × Baoulé), 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 at 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 percent 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.

The trypanosomes pathogenic to livestock in Africa (Trypanosoma congolense, T. vivax, and T. brucei) are mainly cyclically transmitted by tsetse. However, T. vivax can also be mechanically transmitted by other haematophagous insects. Laboratory studies have demonstrated the mechanical transmission of T. congolense, but confirmation of this under natural conditions is 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 percent 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.

An experiment was carried out in Burkina Faso to evaluate the potential for mechanical transmission of Trypanosoma vivax by the African tabanid Atylotus fuscipes. The experiment was carried out in a corral (10 m × 10 m) completely covered by a mosquito net (12 m × 12 m and 2.5 m high). Eight heifers (cross-bred Zebu × Baoulé), free of trypanosome infection, were kept together with two heifers experimentally infected with a local stock of T. vivax. An average of 539 A. fuscipes per day, freshly captured with two Nzi traps, were introduced into the mosquito net from Day 1 to 20, to allow mechanical transmission of the parasites among cattle. Daily parasitological examinations (BCM) of cattle blood samples indicated that six of the eight receiver heifers were positive from days 9, 10, 15, 16, 19 and 29. Mechanical transmission of T. vivax by A. fuscipes was demonstrated unequivocally in close to natural conditions, at a high rate (75 percent incidence over a 20-day period) under a mean challenge of 54 insects per heifer per day. These results, in addition to previous demonstration of mechanical transmission of T. vivax by Atylotus agrestis, confirm that mechanical transmission can be a significant route of infection.

Interaction experiments between hematophagous insects and monoxenous trypanosomatids have become relevant, once cases of human infection involving these protozoa have been reported. Moreover, investigations related to the interaction of insects with trypanosomatids that harbour an endosymbiotic bacterium and thereby lack the paraflagellar rod structure are important to elucidate the role of this structure in the adhesion process. In this work, we compared the interaction of endosymbiont-bearing trypanosomatids and their aposymbiotic counterpart strains (without endosymbionts) with cell lines of Anopheles gambiae, Aedes albopictus and Lutzomyia longipalpis, and with explanted guts of the respective insects. Endosymbiont-bearing strains interacted better with insect cells and guts when compared with aposymbiotic strains. In vitro binding assays revealed that the trypanosomatids interacted with the gut epithelial cells via flagellum and cell body. Flagella attached to the insect gut were enlarged, containing electrondense filaments between the axoneme and flagellar membrane at the point of adhesion. Interactions involving the flagellum lacking paraflagellar rod structure were mainly observed close to tight junctions, between epithelial cells. Endosymbiont-bearing trypanosomatids were able to colonise Aedes aegypti guts after protozoa feeding.

The existence of multiple groups of HIV-1 and HIV-2 suggests that zoonotic transmissions of SIV have occurred at a low rate for centuries. Hence, an increase in the rate of human-to-human transmission may be necessary and sufficient to explain the emergence of HIV as an epidemic in the twentieth century. Three common hypotheses to explain accelerated transmission are (1) social changes accelerated sexual transmission, (2) health care changes accelerated parenteral transmission, and (3) serial passaging adapted HIV for persistent infection and sexual transmission. These hypotheses can be compared against a range of evidence. Temporal and geographic discontinuities in HIV epidemic growth are not easily explained by supposed increases in sexual transmission over time. Large historic changes in sexual transmission are hard to explain based on weak evidence associating HIV prevalence in African communities with differences in sexual behaviour. On the other hand, documented iatrogenic outbreaks show high rates of parenteral transmission. The distribution of hepatitis C virus infections and the history of multi-injection treatment for trypanosomiasis in Central Africa suggest widespread parenteral transmission of blood-borne viruses during 1920-40, coinciding in time and place with the early HIV epidemic. This suggests an important role for parenteral transmission in the early spread of HIV. Further research could improve our understanding of the early HIV epidemic.

Domestic dogs were screened for Trypanosoma brucei infection using the haematocrit centrifugation technique as part of routine active surveillance exercises in the Busia and Teso districts of Kenya. The purpose was to assess the role of dogs as sentinels for the occurrence of human sleeping sickness. Out of 200 dogs screened, five were found to be infected at the various test sites. These five succumbed to the disease within four weeks, and exhibited a distinct and pronounced corneal opacity before death. Blood from two naturally infected dogs were tested for the presence of the serum resistance associated (SRA) gene and one tested positive, confirming it as human infective (T. brucei rhodesiense) prevalence (0.5 percent). It is considered that the occurrence of this clinical sign could be used as an early warning prediction of future outbreaks. This type of prediction could form an integral part of an indigenous technical knowledge set in areas lying at the edges of the tsetse belts where T. brucei is the main trypanosome species that affects dogs. The occurrence of corneal opacity in dogs could indicate a rise in the levels of T. brucei a proportion of which could be human infective T. b. rhodesiense circulating in the population early enough before disease outbreak occurs. It is thought that during sleeping sickness epidemics the domestic dog will be the first casualty rapidly succumbing to disease long before it is noticed in man. Prompt prediction of disease outbreaks would thus enable early interventions that would reduce the morbidity, mortality and the general economic losses associated with sleeping sickness to be instituted.

An agent-based model (AMB) used to simulate the spread of human African trypanosomiasis is presented together with the results of simulations of a focus of the disease. This model is a completely spatialized approach taking into account a series of often overlooked parameters such as human behaviour (activity-related movements), the density and mobility of the disease vectors (tsetse flies) and the influence of other tsetse feeding hosts (livestock and wild animal populations). The agents that represent humans and tsetse flies move in a spatially structured environment managed by specialized location agents. Existing compartmental mathematical models governed by differential equations fail to incorporate the spatial dimension of the disease transmission. Furthermore, on a small scale, transmission is unrealistically represented by entities less than one. This ABM was tested with data from one village of the Bipindi sleeping sickness focus (southern Cameroon) and gave realistic simulations of stable transmission involving an animal reservoir. By varying different spatial configurations, we note that the stability of spread is linked to the spatial complexity (number of heterogeneous locations). The prevalence is very sensitive to human densities and to the number of tsetse flies initially infected in a given location. A relatively low and durable prevalence is obtained with shortening the duration of the first phase. In addition, we discuss some upgrading possibilities, in particular the linkage to a Geographical Information System (GIS). The agent-based approach offers new ways to understanding the spread of the disease and is a tool for evaluating risk and test control strategies.

Camel trypanosomiasis was confirmed in selected northern Kenya herds using the enzyme linked immunosorbent assay (ELISA), Mouse Inoculation (MI) and Blood Smear (BS) techniques. The ELISA results indicated current or past trypanosome prevalence rates of 72-95 percent. MI and BS techniques revealed current infection rates up to 19.2 and 11.5 percent, respectively. Trypanosome infection rates were significantly elevated during the wet season based on the MI diagnostic technique. The mean infection rates were 13.7 ± 5 and 4.55 ± 2.2 for the wet and dry season, respectively. There were no seasonal differences in infection rates in camels based on the ELISA technique. Active transmission of camel trypanosomiasis was ascertained by regular monitoring of 10 sentinel camels. Three of these sentinel camels became infected 8-9 months after introduction, which in the absence of Glossina spp., supported the concept of mechanical transmission. The trypanosome species involved was confirmed to be Trypanosoma evansi from the results of a preliminary survey.

Three major human diseases, malaria, sleeping sickness and leishmaniasis, are caused by protozoan parasites that are transmitted by blood-sucking insects. These insects are not mere ‘flying syringes’ that mechanically transfer parasites from one mammal to the next. Instead, they provide a specific environment - albeit not a particularly hospitable one - in which the parasites differentiate, proliferate and migrate to the correct tissues to ensure transmission to the next mammalian host. Recent studies on the role of parasite surface molecules in insect vectors have delivered some surprises and could provide insights on ways to interrupt transmission.

Laboratory experiments and field observations clearly show that tsetse flies can be carriers of mixed trypanosome infections. The 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 (unmixed) infection 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.

A study was conducted to determine whether Varanus niloticus can serve as host for Glossina fuscipes fuscipes, the disease vector of African trypanosomiasis. Tsetse flies were collected from three assigned zones in southeastern Uganda. A form of ELISA was used to detect V. niloticus blood in the blood meals of the tsetse flies. Glossina fuscipes fuscipes was the only species recorded in zones 1 and 2. In zone 3, 0.28 percent of the tsetse flies were classified as G. pallidipes, while 99.72 percent were G. fuscipes fuscipes. 27.12 percent of the blood meals were found to have V. niloticus blood. It is concluded that V. niloticus is a possible host for Trypanosoma spp. and a cause for the persistence of African trypanosomiasis in Uganda.

Many insect species rely on intracellular bacterial symbionts for their viability and fecundity. Large-scale DNA-sequence analyses are revealing the forces that shape the evolution of these bacterial associates and the genetic basis of their specialization to an intracellular lifestyle. The full genome sequences of two obligate mutualists, Buchnera aphidicola of aphids and Wigglesworthia glossinidia of tsetse flies, reveal substantial gene loss and an integration of host and symbiont metabolic functions. Further genomic comparisons should reveal the generality of these features among bacterial mutualists and the extent to which they are shared with other intracellular bacteria, including obligate pathogens.