Improved understanding of Rift Valley Fever (RVF) etiology and outbreaks in relation to intra-

seasonal rainfall variability

J-A Ndione1&4, M. Diop2, J-P Lacaux3, A.T. GAYE4

1- Centre de Suivi Ecologique, BP 15 532 Dakar Fann, Tel : (+221) 825 80 66/67 Fax (+221) 825 81 68 SENEGAL

Email : jacques-andre.ndione@cse.sn, www.cse.sn 2- Laboratoire d'Enseignement et de Recherche en Géomatique (LERG), Campus universitaire de l'ESP BP

25275 Dakar-Fann SENEGAL 3- MEDIAS-France, CNES, 18, avenue Edouard Belin BPi 2102, 31401 Toulouse Cedex 9, France

4- Laboratoire de Physique de l’Atmosphère Siméon Fongang/ESP-UCAD, BP 5085

OutlineOutline

Introduction

Some considerations on RVF

Study area: data and methods

Results and discussions

Conclusion and Perspectives

RVF : Rift Valley Fever is acute arthropod-borne virus infection with a wide range of vertebrate disease hosts (FAO, 2003)

The RVF virus belongs to Phlebovirus genus of the Bunyaviridae family (FAO, 2003)

The RVF virus is transmitted by mosquitoes of at least 6 genera and probably over 30 different species. It can be also transmitted by aerosols of viremic blood during hemorrhage (FAO, 2003)

Disease notifiable to OIE (OIE: World Organisation for Animal Health)

Signs of RVF (FAO, 2003): * ruminants: mass abortion and high mortality in newborn kids and lambs

are observed in naive ruminant populations ;

* humans: a flu-like syndrome is often the only clinical manifestation, but haemorrhagic fever, encephalitis or ocular disease may occur in rare cases.

What is Rift Valley Fever (RVF) ?

© De la Roque (2005)

RVF : a disease at the crossraods of 3 systems

RVF in the world…RVF has been for the 1st time in 1931 au Kenya by Montgomery and Stordy (Christophe et al., 1997)

November 2007: RVF epidemics in Soudan 161 human death

June 2007: 1062 of RVF cases (315 human death in Kenya, Tanzania and Somalia)

Since 2000, RVF went out from Africa to Asia (Yemen and Saoudi Arabia RVF events; OIE, 2000; WHO, 2000)

2008: RVF in Mayotte (Sissoko, 2009)

Some historical reference marks about RVF in Senegal…

October 1987: Rosso RVF epidemics (Sall, 2001)

October 1993: RVF virus was isolated from the floodwater mosquitoes Aedes vexans and Ae. Ochraceus (Fontenille et al, 1998) and from one of the sheep

October-december 1994: outbreak of RVF in Ross-Béthio (Sall, 2001)

November 1998: outbreak of RVF in Diawara ; isolation of RVF virus for the first time in Cu. poicilipes (Sall, 2001 ; Diallo et al, 2000)

October-november 1999: outbreak of RVF in Ranérou (Sall, 2001)

October-november 2002: outbreak of RVF in Barkédji (Ba et al, 2005), in Galoya and Dabia Olédji (OIE, 2002)

November 2003: outbreak of RVF in Saint-Louis, Dagana, Podor, Matam and Bakel (OIE, 2003)

November 2004: outbreak of RVF in Dagana (OIE, 2004)

RVF in summary: what we know today?

RVF vectors in Senegal : Aedes vexans, Aedes ochraceus, Aedes dalzieli, Culex poicilipes (Fontenille et al, 1998 ; Diallo et al, 2000)

RVF and rainfall: in Senegal (Ndione et al, 2003; Ndione and al, 2008; Ba et al, 2005; Diallo et al., 2005), RVF epidemics do not seem to follow the same relationships as over East Africa (Linthicum et al, 1999) at inter-annual time scale.

We decide to focus on rainfall intra-seasonal variability

Feedind behaviour : RVF vectors use the same type of breeding sites and also feed on cattle and sheep as the main vectors in East and South Africa

RVF epidemiological cycle in Ferlo (Diallo, 1995)

3 new vectors since 2003 (Ba et al, 2009)

Study area : Senegal river basin (area at risk)

Climatic regions in Senegal (Sagna, 2000)

The analysis consisted in determining the onset and end of the rainy season using strictly climatological criteria

Onset corresponds to a succession of two rainy episodes, either by having recorded at least 10 mm during one day from July 1st

The threshold of 0.1 mm is used to characterize rainy day.

Methods

The analysis of dry spells frequency during the season was based on a classic method: F = (nb_seqi /nb_seqtot)*100 With: • nb_seqi: number of dry spells during the rainy season; • nb_seqtot: total number of dry spells observed in the same period. We only

highlight dry spells superior or equal to 13 days.

Data: daily rainfall from July to October Met. stations and years: Barkedji (1993 et 2002),

Ross-Bethio (1994), Ranerou (1999)

Fig. 1 : Répartition des pluies à Barkedji en 1993.

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23/0

7/19

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Barkedji

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01/0

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(jours)

(mm

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18/0

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Ranérou

Ross-Béthio

1 remark: a similar profile at the end of the rainy season

Results

All these rainy seasons ended by heavy rainfall peak:

23 mm (Ross-Bethio 1994); 27 mm (Ranerou 1999); and 40 mm (Barkedji 1993).

Results (2)

We also observed long dry spells before theses rainfall peaks :19 days at Ross-Bethio (1994); 15 days at Ranerou (1999);and 13 days at Barkedji (1993).

Recently (Barkedji 2002) : after 19 days of dry spell (19 sept to 8 oct.), a rainfall peak of 24 mm occured…

2 major questions:

1- Is it any link between RVF emergence at the end of rainy season and the length of last dry spell?

2- What is the incidence of this last dry spell on the vector dynamics?

Aedes populations? Culex populations?

Discussions

16June

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Pond levels (cm)

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2003

Rainfall, ponds levels and Aedes mosquitoes in Furdu

Ponds levels (cm) & Aedes females in Furdu (Mondet et al, 2005)

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pluvio LimniNgao pond 2002

Niaka pond 2002

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mm

pluvio Limni

Furdu pond 2002

Rainfall distribution and periodicity is a key factor controlling mosquitoes dynamics

r

h

Culex eggs

Aedes eggs

t0Juin Dec

© adapted PIN-DIOP, 2006

t20

h

Aedes eggs

r

Culex eggs

t0

Juin Dec© adapted PIN-DIOP, 2006

Discussions (3)

These heavy rainfall at the end of the rainy season have a double impact on vector dynamics (Ndione et al, 2004; Mondet et al, 2005; Ba et al, 2005; Ndione et al, 2008):

1) the maintenance to a high level of Culex population density;

2) be at the origin of bursting Aedes eggs; in fact, they allow Aedes eggs to meet suitable environment.

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Aedes vexans

Culex poicilipesSecond peak of Aedes collected in October 1993 at Barkédji

(Données entomologiques, Fontenille et al, 1998)

Discussions (4)

Identification of rainy season profile involved in RVF emergence in Senegal river basin

RVF event Bimodal population dynamics curve for Ae. Vexans (Ba et al, 2005)

Whatever the RVF event « virus circulation with no visible manifestations » (Barkedji 1993 & 2002) and «  local RVF outbreak »  (Ross-Bethio & Ranerou 1999), the rainy season profile is the same…

Dispite of the environment difference between Ferlo (Barkedji & Ranerou) et Senegal river (Ross-Béthio), we have the same rainy season profile involved in RVF emergence…

How to improve seasonal forecast using the news findings?

Discussions (5)

Our main concern :

Document more ponds in differents aspects (hydrology, vegetation, water quality…) in relation with:

- moonson dyanmics (climate, rainfall…);

- RVF vectors aquatic stages populations dynamics in relation with water quality monitoring;

- Collecting informations on livestock survey, mapping villages surrounding ponds…

Seasonal forecast and RVF ???

Integration of risk maps through:- GIS;- Devoloping an Early Warning System for forecasting RVF

epidemics.

Conclusion and Perspectives

DIREL