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A Stochastic Model to Study Rift Valley Fever Persistence with Different Seasonal Patterns of Vector Abundance: New Insights on the Endemicity in the Tropical Island of Mayotte.

Cavalerie L, Charron MV, Ezanno P, Dommergues L, Zumbo B, Cardinale E - PLoS ONE (2015)

Bottom Line: Transmission rates had to be divided by more than five to best fit observed data.Five years after introduction, RVF persisted in more than 10% of the simulations, even under this scenario of low transmission.Hence, active surveillance must be maintained to better understand the risk related to RVF persistence and to prevent new introductions.

View Article: PubMed Central - PubMed

Affiliation: CRVOI, Centre de Recherche et de Veille sur les maladies émergentes dans l'Océan Indien, F-97490 Sainte Clotilde, La Réunion, France; CIRAD, UMR CMAEE, F-97490, Sainte Clotilde, France; INRA, UMR 1309 CMAEE, F-34398, Montpellier, France; AgroParisTech, F-75005, Paris, France; Université de la Réunion, F-97715 Saint Denis, La Réunion, France.

ABSTRACT
Rift Valley fever (RVF) is a zoonotic vector-borne disease causing abortion storms in cattle and human epidemics in Africa. Our aim was to evaluate RVF persistence in a seasonal and isolated population and to apply it to Mayotte Island (Indian Ocean), where the virus was still silently circulating four years after its last known introduction in 2007. We proposed a stochastic model to estimate RVF persistence over several years and under four seasonal patterns of vector abundance. Firstly, the model predicted a wide range of virus spread patterns, from obligate persistence in a constant or tropical environment (without needing vertical transmission or reintroduction) to frequent extinctions in a drier climate. We then identified for each scenario of seasonality the parameters that most influenced prediction variations. Persistence was sensitive to vector lifespan and biting rate in a tropical climate, and to host viraemia duration and vector lifespan in a drier climate. The first epizootic peak was primarily sensitive to viraemia duration and thus likely to be controlled by vaccination, whereas subsequent peaks were sensitive to vector lifespan and biting rate in a tropical climate, and to host birth rate and viraemia duration in arid climates. Finally, we parameterized the model according to Mayotte known environment. Mosquito captures estimated the abundance of eight potential RVF vectors. Review of RVF competence studies on these species allowed adjusting transmission probabilities per bite. Ruminant serological data since 2004 and three new cross-sectional seroprevalence studies are presented. Transmission rates had to be divided by more than five to best fit observed data. Five years after introduction, RVF persisted in more than 10% of the simulations, even under this scenario of low transmission. Hence, active surveillance must be maintained to better understand the risk related to RVF persistence and to prevent new introductions.

No MeSH data available.


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Sampled farms and zones in Mayotte.
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pone.0130838.g004: Sampled farms and zones in Mayotte.

Mentions: The RVF dynamics observed in the simulations are represented in Fig 4. After the introduction of an infectious animal, epizootics could occur in repetitions for all scenarios with a peak of around 40% of the ruminant population being infectious (maxIH1). At the end of the 1st year, in repetitions in which the virus persisted, almost 100% of the host population had become infectious and had recovered. From the second year on, behaviours differed according to vector emergence scenarios. In scenario a, new infections occurred constantly with the renewal of the host population so that the infectious hosts remained steady at 0.4% of the population. In the three other scenarios, when the virus persisted, small epizootics occurred early in the favourable season and reached about the same level every year. The maximal proportion of infectious hosts in the 2nd year (maxIH2) attained 1.4, 3.4, and 4% for scenarios b, c, and d, respectively. Peaks always occurred several days (5–15 days on average) before the maximal vector abundance was reached. Yearly maximal proportion of infectious hosts was roughly stable from the 2nd year onwards (maxIH2 and maxIH3). In one repetition (over 1500), in scenario c, we observed an increase in year 4 of infectious hosts above the value of previous years, reaching 8% of the host population (Fig 4 section cI). Meanwhile, the mean proportion of recovered animals in case of persistence (meanRH) from the 2nd year until the end of the simulation was of 98, 98, 95, and 94% for scenarios a, b, c, and d, respectively; the mean number of infected vectors in aquatic stage in case of persistence was very low (1.10−3, 2.10−3, 5,1.10−3, and 8,0.10−3% of the vectors in the aquatic stage for scenarios a, b, c, and d, respectively). Direct transmission was responsible for very few new cases (5.4 x 10−2%, 5.2 x 10−2%, 6.1 x 10−2%, and 5.7 x 10−2% of the total cases for scenario a, b, c, and d, respectively).


A Stochastic Model to Study Rift Valley Fever Persistence with Different Seasonal Patterns of Vector Abundance: New Insights on the Endemicity in the Tropical Island of Mayotte.

Cavalerie L, Charron MV, Ezanno P, Dommergues L, Zumbo B, Cardinale E - PLoS ONE (2015)

Sampled farms and zones in Mayotte.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4493030&req=5

pone.0130838.g004: Sampled farms and zones in Mayotte.
Mentions: The RVF dynamics observed in the simulations are represented in Fig 4. After the introduction of an infectious animal, epizootics could occur in repetitions for all scenarios with a peak of around 40% of the ruminant population being infectious (maxIH1). At the end of the 1st year, in repetitions in which the virus persisted, almost 100% of the host population had become infectious and had recovered. From the second year on, behaviours differed according to vector emergence scenarios. In scenario a, new infections occurred constantly with the renewal of the host population so that the infectious hosts remained steady at 0.4% of the population. In the three other scenarios, when the virus persisted, small epizootics occurred early in the favourable season and reached about the same level every year. The maximal proportion of infectious hosts in the 2nd year (maxIH2) attained 1.4, 3.4, and 4% for scenarios b, c, and d, respectively. Peaks always occurred several days (5–15 days on average) before the maximal vector abundance was reached. Yearly maximal proportion of infectious hosts was roughly stable from the 2nd year onwards (maxIH2 and maxIH3). In one repetition (over 1500), in scenario c, we observed an increase in year 4 of infectious hosts above the value of previous years, reaching 8% of the host population (Fig 4 section cI). Meanwhile, the mean proportion of recovered animals in case of persistence (meanRH) from the 2nd year until the end of the simulation was of 98, 98, 95, and 94% for scenarios a, b, c, and d, respectively; the mean number of infected vectors in aquatic stage in case of persistence was very low (1.10−3, 2.10−3, 5,1.10−3, and 8,0.10−3% of the vectors in the aquatic stage for scenarios a, b, c, and d, respectively). Direct transmission was responsible for very few new cases (5.4 x 10−2%, 5.2 x 10−2%, 6.1 x 10−2%, and 5.7 x 10−2% of the total cases for scenario a, b, c, and d, respectively).

Bottom Line: Transmission rates had to be divided by more than five to best fit observed data.Five years after introduction, RVF persisted in more than 10% of the simulations, even under this scenario of low transmission.Hence, active surveillance must be maintained to better understand the risk related to RVF persistence and to prevent new introductions.

View Article: PubMed Central - PubMed

Affiliation: CRVOI, Centre de Recherche et de Veille sur les maladies émergentes dans l'Océan Indien, F-97490 Sainte Clotilde, La Réunion, France; CIRAD, UMR CMAEE, F-97490, Sainte Clotilde, France; INRA, UMR 1309 CMAEE, F-34398, Montpellier, France; AgroParisTech, F-75005, Paris, France; Université de la Réunion, F-97715 Saint Denis, La Réunion, France.

ABSTRACT
Rift Valley fever (RVF) is a zoonotic vector-borne disease causing abortion storms in cattle and human epidemics in Africa. Our aim was to evaluate RVF persistence in a seasonal and isolated population and to apply it to Mayotte Island (Indian Ocean), where the virus was still silently circulating four years after its last known introduction in 2007. We proposed a stochastic model to estimate RVF persistence over several years and under four seasonal patterns of vector abundance. Firstly, the model predicted a wide range of virus spread patterns, from obligate persistence in a constant or tropical environment (without needing vertical transmission or reintroduction) to frequent extinctions in a drier climate. We then identified for each scenario of seasonality the parameters that most influenced prediction variations. Persistence was sensitive to vector lifespan and biting rate in a tropical climate, and to host viraemia duration and vector lifespan in a drier climate. The first epizootic peak was primarily sensitive to viraemia duration and thus likely to be controlled by vaccination, whereas subsequent peaks were sensitive to vector lifespan and biting rate in a tropical climate, and to host birth rate and viraemia duration in arid climates. Finally, we parameterized the model according to Mayotte known environment. Mosquito captures estimated the abundance of eight potential RVF vectors. Review of RVF competence studies on these species allowed adjusting transmission probabilities per bite. Ruminant serological data since 2004 and three new cross-sectional seroprevalence studies are presented. Transmission rates had to be divided by more than five to best fit observed data. Five years after introduction, RVF persisted in more than 10% of the simulations, even under this scenario of low transmission. Hence, active surveillance must be maintained to better understand the risk related to RVF persistence and to prevent new introductions.

No MeSH data available.


Related in: MedlinePlus