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Implications of Heterogeneous Biting Exposure and Animal Hosts on Trypanosomiasis brucei gambiense Transmission and Control.

Stone CM, Chitnis N - PLoS Comput. Biol. (2015)

Bottom Line: However, the parasite persists in human populations at levels of considerable rarity and as such the existence of animal reservoirs has been posited.We developed a mathematical model allowing for heterogeneous exposure of humans to tsetse, with animal populations that differed in their ability to transmit infections, to investigate the effectiveness of two established techniques, screening and treatment of at-risk populations, and vector control.If they did not serve as reservoirs, sensitivity analyses suggested their attractiveness may instead function as a sink for tsetse bites.

View Article: PubMed Central - PubMed

Affiliation: Department of Epidemiology and Public Health, Swiss Tropical and Public Health Institute, Basel, Switzerland; University of Basel, Basel, Switzerland.

ABSTRACT
The gambiense form of sleeping sickness is a neglected tropical disease, which is presumed to be anthroponotic. However, the parasite persists in human populations at levels of considerable rarity and as such the existence of animal reservoirs has been posited. Clarifying the impact of animal host reservoirs on the feasibility of interrupting sleeping sickness transmission through interventions is a matter of urgency. We developed a mathematical model allowing for heterogeneous exposure of humans to tsetse, with animal populations that differed in their ability to transmit infections, to investigate the effectiveness of two established techniques, screening and treatment of at-risk populations, and vector control. Importantly, under both assumptions, an integrated approach of human screening and vector control was supported in high transmission areas. However, increasing the intensity of vector control was more likely to eliminate transmission, while increasing the intensity of human screening reduced the time to elimination. Non-human animal hosts played important, but different roles in HAT transmission, depending on whether or not they contributed as reservoirs. If they did not serve as reservoirs, sensitivity analyses suggested their attractiveness may instead function as a sink for tsetse bites. These outcomes highlight the importance of understanding the ecological and environmental context of sleeping sickness in optimizing integrated interventions, particularly for moderate and low transmission intensity settings.

No MeSH data available.


Related in: MedlinePlus

Zero-growth isoclines (R0 = 1) of T.b. gambiense under perturbation of specific parameters.The parameter values used were the median values obtained for the high transmission setting, except for those varied in the analysis. In the left plot, isoclines at different levels of vector mortality are shown, depending on the daily removal rate of infected humans (r) and the proportion of time commuters spend in the high exposure area (ξ). The areas above the isoclines represent values of R0 greater than 1, and below and to the right of the isoclines values smaller than 1. In the middle plot the impact of screening humans in the low risk setting (ra) in combination with screening commuting humans (rb) is shown for different levels of animal to human ratios (A/H1). In the right plot isoclines are depicted along removal rates (r) and tsetse density (V/H) in both areas when animals either do not contribute to transmission (ca = 0) or they can infect tsetse (ca = 0.003).
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pcbi.1004514.g003: Zero-growth isoclines (R0 = 1) of T.b. gambiense under perturbation of specific parameters.The parameter values used were the median values obtained for the high transmission setting, except for those varied in the analysis. In the left plot, isoclines at different levels of vector mortality are shown, depending on the daily removal rate of infected humans (r) and the proportion of time commuters spend in the high exposure area (ξ). The areas above the isoclines represent values of R0 greater than 1, and below and to the right of the isoclines values smaller than 1. In the middle plot the impact of screening humans in the low risk setting (ra) in combination with screening commuting humans (rb) is shown for different levels of animal to human ratios (A/H1). In the right plot isoclines are depicted along removal rates (r) and tsetse density (V/H) in both areas when animals either do not contribute to transmission (ca = 0) or they can infect tsetse (ca = 0.003).

Mentions: To gain further insight into how the efficacy of different parameters and potential control approaches (screen & treat and vector control) depend on variation in other parameters, we investigate the impact on the basic reproduction number, R0, by looking at zero-growth isoclines, i.e., the parameter space where R0 = 1 (Fig 3). As the human populations are more separated (with increasing ξ) a greater removal rate is required to interrupt transmission, and removal rates leading to R0 < 1 become smaller as vector mortality increases (left plot). When we compare the impact of screening humans in the low risk setting (ra) and screening commuting humans (rb), it is clear that screening the humans commuting to high exposure areas is critical, as transmission can be sustained by this group even at very high rates of screening the non-commuting population (ra). Depending on the density of animals in the low risk area (which here do not contribute to transmission, but can lead to “wasted bites” among tsetse), it can be sufficient to target only the commuting population. When animal densities are low, resulting in higher biting rates on humans in the village (Nh1), then transmission could also be sustained among this population and all populations will need to be screened (middle plot). When animals can contribute to transmission (ca = 0.003), treatment of infected humans will not lead to interruption of transmission above a certain vector-human density threshold, and tsetse control will have to be relied on (right plot).


Implications of Heterogeneous Biting Exposure and Animal Hosts on Trypanosomiasis brucei gambiense Transmission and Control.

Stone CM, Chitnis N - PLoS Comput. Biol. (2015)

Zero-growth isoclines (R0 = 1) of T.b. gambiense under perturbation of specific parameters.The parameter values used were the median values obtained for the high transmission setting, except for those varied in the analysis. In the left plot, isoclines at different levels of vector mortality are shown, depending on the daily removal rate of infected humans (r) and the proportion of time commuters spend in the high exposure area (ξ). The areas above the isoclines represent values of R0 greater than 1, and below and to the right of the isoclines values smaller than 1. In the middle plot the impact of screening humans in the low risk setting (ra) in combination with screening commuting humans (rb) is shown for different levels of animal to human ratios (A/H1). In the right plot isoclines are depicted along removal rates (r) and tsetse density (V/H) in both areas when animals either do not contribute to transmission (ca = 0) or they can infect tsetse (ca = 0.003).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4591123&req=5

pcbi.1004514.g003: Zero-growth isoclines (R0 = 1) of T.b. gambiense under perturbation of specific parameters.The parameter values used were the median values obtained for the high transmission setting, except for those varied in the analysis. In the left plot, isoclines at different levels of vector mortality are shown, depending on the daily removal rate of infected humans (r) and the proportion of time commuters spend in the high exposure area (ξ). The areas above the isoclines represent values of R0 greater than 1, and below and to the right of the isoclines values smaller than 1. In the middle plot the impact of screening humans in the low risk setting (ra) in combination with screening commuting humans (rb) is shown for different levels of animal to human ratios (A/H1). In the right plot isoclines are depicted along removal rates (r) and tsetse density (V/H) in both areas when animals either do not contribute to transmission (ca = 0) or they can infect tsetse (ca = 0.003).
Mentions: To gain further insight into how the efficacy of different parameters and potential control approaches (screen & treat and vector control) depend on variation in other parameters, we investigate the impact on the basic reproduction number, R0, by looking at zero-growth isoclines, i.e., the parameter space where R0 = 1 (Fig 3). As the human populations are more separated (with increasing ξ) a greater removal rate is required to interrupt transmission, and removal rates leading to R0 < 1 become smaller as vector mortality increases (left plot). When we compare the impact of screening humans in the low risk setting (ra) and screening commuting humans (rb), it is clear that screening the humans commuting to high exposure areas is critical, as transmission can be sustained by this group even at very high rates of screening the non-commuting population (ra). Depending on the density of animals in the low risk area (which here do not contribute to transmission, but can lead to “wasted bites” among tsetse), it can be sufficient to target only the commuting population. When animal densities are low, resulting in higher biting rates on humans in the village (Nh1), then transmission could also be sustained among this population and all populations will need to be screened (middle plot). When animals can contribute to transmission (ca = 0.003), treatment of infected humans will not lead to interruption of transmission above a certain vector-human density threshold, and tsetse control will have to be relied on (right plot).

Bottom Line: However, the parasite persists in human populations at levels of considerable rarity and as such the existence of animal reservoirs has been posited.We developed a mathematical model allowing for heterogeneous exposure of humans to tsetse, with animal populations that differed in their ability to transmit infections, to investigate the effectiveness of two established techniques, screening and treatment of at-risk populations, and vector control.If they did not serve as reservoirs, sensitivity analyses suggested their attractiveness may instead function as a sink for tsetse bites.

View Article: PubMed Central - PubMed

Affiliation: Department of Epidemiology and Public Health, Swiss Tropical and Public Health Institute, Basel, Switzerland; University of Basel, Basel, Switzerland.

ABSTRACT
The gambiense form of sleeping sickness is a neglected tropical disease, which is presumed to be anthroponotic. However, the parasite persists in human populations at levels of considerable rarity and as such the existence of animal reservoirs has been posited. Clarifying the impact of animal host reservoirs on the feasibility of interrupting sleeping sickness transmission through interventions is a matter of urgency. We developed a mathematical model allowing for heterogeneous exposure of humans to tsetse, with animal populations that differed in their ability to transmit infections, to investigate the effectiveness of two established techniques, screening and treatment of at-risk populations, and vector control. Importantly, under both assumptions, an integrated approach of human screening and vector control was supported in high transmission areas. However, increasing the intensity of vector control was more likely to eliminate transmission, while increasing the intensity of human screening reduced the time to elimination. Non-human animal hosts played important, but different roles in HAT transmission, depending on whether or not they contributed as reservoirs. If they did not serve as reservoirs, sensitivity analyses suggested their attractiveness may instead function as a sink for tsetse bites. These outcomes highlight the importance of understanding the ecological and environmental context of sleeping sickness in optimizing integrated interventions, particularly for moderate and low transmission intensity settings.

No MeSH data available.


Related in: MedlinePlus