**4. Discussion**

162 Current Topics in Tropical Medicine

season

Parity rate 40.4 (673) 36.7-44.1 31.1 (1,157) 28.4-33.8 16.28 <0.001

Parity rate 61.9 (63) 49.8-74.2 36.7 (139) 28.6-44.8 11.16 <0.001

Parity rate 52.9 (194) 45.2-60.6 46.2 (199) 40.6-52.0 1.71 0.191

Parity rate 0.0 (0) 0.0 0.0 (0) 0.0 0 0

Parity rate 0.0 (9) 0.0-0.2 58.3 (12) 25.6-91.1 7.88 0.005

4.8 (1,120) 3.6-6.1 4.2 (978) 3.0-5.5 0.33 0.564

0.0 (65) 0.0-0.2 1.7 (176) 0.0-3.6 1.12 0.290

2.3 (127) 0.2-5.8 3.4 (136) 1.7-5.2 0.37 0.542

0.0 (0) 0.0 0.0 (0) 0.0 0 0

0.0 (18) 0.0-0.2 2.3 (44) 0.0-6.8 0.42 0.519

8.3 (8) 0.0-28.4 0.0 (3) 0.0 0.87 0.824

χ² or LRT

P value

Dry season Rainy

2002a Biting rate 38.7 36.3-41.1 59.9 56.9-68.2 3.79 <0.001

2003b Biting rate 7.1 5.4-9.3 8.3 5.0-11.6 0.13 0.900

2005c Biting rate 18.3 11.8-24.7 58.6 23.8-93.4 20.50 <0.001

2002a Biting rate 0.0 0.0-0.2 0.0 0.0 0 0

2003b Biting rate 1.4 0.7-2.4 1.2 0.6-2.1 0.17 0.879

2005c Biting rate 2.7 1.1-4.4 0.6 0.0-1.5 0.61 0.435

 Parity rate 70.9 24.3-84.3 60.0 0.0-100.0 0.58 0.216 Total EIR 41 0 0

Table 2. Monthly average biting rate, infection rate, parity rate and entomological

inoculation rate (EIR) o**f** *An. gambiae* and *An. funestus* during the dry season and the rainy

In brackets are the number of malaria vectors analyzed; LRT (likelihood ratio test)

Total EIR 0 - 26 -

Mean (n) 95% CI Mean (n) 95% CI

Total EIR 338 - 458 -

Total EIR 77 365

Total EIR 0 0

Total EIR 0 - 5 -

aIrrigated rice farming performed in a synchronized manner

cIrrigated rice farming performed in a synchronized manner

season in 2002, 2003 and 2005 in Zatta, central Côte d'Ivoire

Malaria vector

*An. gambiae* 

 Infection rate

 Infection rate

 Infection rate

 Infection rate

 Infection rate

 Infection rate

bInterruption of rice cultivation

*An. funestus*

Entomological parameter

> The interruption of irrigated rice farming due to a farmers' dispute over land property rights, coupled with an unstable socio-political situation in the face of the 2002-2004 armed conflict (Betsi et al., 2006; Fürst et al., 2009) offered a unique opportunity to study the dynamics of malaria transmission. Our analyses complement previous publications

Effects of Irrigated Rice Fields and Seasonality on

increase in the EIR (Bonnet *et al.*, 2002).

play a key role (Mabaso et al., 2007).

paradox' (Ijumba & Lindsay, 2001).

interventions and their monitoring.

**6. Acknowledgements** 

**5. Conclusion** 

*Plasmodium* Transmission in West Africa, Particularly in Central Côte d'Ivoire 165

*An. gambiae* population at the beginning of the short rainy season was followed by an

With regard to *An. funestus*, the highest EIRs were usually observed during the dry season. Indeed, *An. funestus* is often abundant and has high EIR during dry season compared to the rainy season (Fontenille et al., 1997; Manga et al., 1997). *An. funestus* was identified as the main malaria vector in the Guinean climatic region, in East Africa and Madagascar (Robert et al., 1985; Severini et al., 1990). As shown in our study, despite the presence of irrigated rice field, there is a great variability in the annual EIR values and seasonality would seem to

Finally, an important finding of our study is that in Zatta, where irrigated rice farming was interrupted in 2003/2004, *Plasmodium* prevalence rates and the number of presumptive malaria cases decreased. This observation is corroborated by a significant decrease in the EIR from 2002 to 2003 (Koudou et al., 2005) and a significant increase from 2003 to 2005 (Koudou et al., 2007). This study demonstrated also that irrigated rice cultivation is associated with elevated malaria prevalence rates, as well as high numbers of presumptive malaria cases, as seen in Burundi (Coosemans, 1985), Kenya (Githeko et al*.*, 1993) and Madagascar (Marrama et al*.*, 1995). However, research carried out in Tanzania showed that irrigated rice farming was not associated with a higher risk of malaria. One important reason for this observation is that farmers engaged in irrigated rice have the opportunity to gain some extra money, part of which is spent for protective measures against malaria. A reduced risk of malaria despite enhanced rice production has been termed 'paddies

In conclusion, analyses of our entomological data revealed that malaria transmission in two different agro-ecological settings of central Côte d'Ivoire is very high, but there are clear seasonal patterns. Whilst the interruption of irrigated rice farming in one of the two study villages resulted in a highly significant reduction in the EIR, seasonal patterns of transmission remained. Hence, even in intensive agriculture areas, the effect of season on malaria transmission must be taken into consideration for the design of integrated

Additionally, in Zatta, from 2002 to 2003, the highly significant reduction in the annual EIR was paralleled by a significant reduction in the *Plasmodium* prevalence rate, and the proportions of presumptive and clinically-confirmed malaria cases. Once irrigated rice farming was resumed, there was an increase in entomological and parasitological parameters of malaria. In Tiémélékro, despite the significant increase in the EIR from the year 2002 to 2005 (Koudou et al., 2005, 2007), malaria prevalence rates, and the presumptive and clinical malaria cases decreased. Hence, the reduction of malaria transmission in endemic areas does not necessary reduce the incidence of clinical malaria episodes (Charlwood et al., 1998), highlighting the complex relationship between these parameters.

Our thanks are addressed to the local authorities and villagers of Tiémélékro and Zatta, and the district health officers of Dimbokro and Yamoussoukro, for their commitment in the present study. This investigation received financial support from the 'Fonds Ivoiro Suisse de

(Girardin et al., 2004; Koudou et al., 2005, 2007, 2009), now with an explicit focus on the effect of seasonality on malaria transmission under changing agro-ecological conditions. The following points are offered for discussion.

Firstly, biting rates of *An. gambiae* in both villages were usually significantly higher in the rainy season than in the dry season. When irrigated rice farming was interrupted in Zatta in 2003, much lower biting rates were observed than in the preceding year and in 2005, but there were no seasonal differences. Hence, the interruption of irrigated rice farming appeared to have hidden the effect of season on *An. gambiae* biting rate. These findings are in agreement with previous investigations in the humid savannah of Côte d'Ivoire: in an area characterised by intensive agriculture, the biting rate of *An. gambiae* increased significantly a few weeks after the beginning of the rainy season, whereas it decreased and became lowest towards the end of the dry season (Doannio et al., 2006). Moreover, the blunting of seasonal differences in biting rates due to changing patterns in irrigated rice farming has been documented previously for the savannahs of Senegal (Faye et al., 1993) and Mali (Dolo et al., 2004). In contrast to *An. gambiae* with the highest biting rates usually observed in the rainy season, the highest biting rates of *An. funestus* were consistently recorded in the dry season regardless of the prevailing agricultural activity. Moreover, interruption of irrigated rice farming in Zatta showed no effect.

Secondly, with the only exception of a significantly higher infection rate of *An. funesuts* in Tiémélékro in the rainy season compared to the dry season of 2005, infection rates of both *An. gambiae* and *An. funestus* showed no clear seasonal patterns. Different results were reported from Dielmo, a holoendemic area in Senegal, where the infection rate of malaria vectors showed considerable seasonal variation (Fontenille et al., 1997). The observations made in Senegal corroborated previous findings obtained in the savannah area in the north of Côte d'Ivoire (Dossou-Yovo et al., 1995), and other findings documenting a high infection rate of *An. funestus* at the beginning of the dry season in an irrigated rice area compared to a non-irrigated rice farming area (Dossou-Yovo, 2000). It should be noted, however that the mean annual infection rate of *An. gambiae* in Zatta was significantly higher when irrigated rice farming was in place (in 2002 and 2005) compared to a year with interrupted irrigated rice farming (Koudou et al., 2005).

Thirdly, the influence of changing patterns of irrigated rice farming on the *An. gambiae*specific EIR in Zatta has been discussed elsewhere (Koudou et al., 2005, 2007). In brief, interruption of irrigated rice farming resulted in several-fold lower EIRs compared to normal years. Here, we now document that seasonal patterns of transmission remained. Indeed, considerably higher EIRs were observed for *An. gambiae* in the rainy season compared to the dry season. Of note, the EIR of *An. gambia* in the dry season of 2003 in Zatta dropped to zero. In Tiémélékro, high EIRs were recorded throughout the study period for *An. gambiae* and, in general, EIRs were higher in the rainy season compared to the dry season. *An. funestus* seemed to play an important role in the transmission of malaria, particularly in the dry season. Our results therefore confirm previous observations made elsewhere in the northern savannah of Côte d'Ivoire (Dossou-Yovo, 2000) and in southern Cameroon (Bonnet et al., 2002). Whilst *An. gambiae* was the key *P. falciparum* transmitter mainly during the rainy season, *An. funestus* was the main vector species during the dry season. It is interesting to note that a previous study focusing on climatic models for suitable malaria transmission in Africa, based on monthly rainfall and temperature data, concluded that an average of 80 mm rainfall per month, for at least 3-5 months, is a minimum to ascertain stable malaria transmission (Craig et al., 1999). Usually, a rapid rise in the *An. gambiae* population at the beginning of the short rainy season was followed by an increase in the EIR (Bonnet *et al.*, 2002).

With regard to *An. funestus*, the highest EIRs were usually observed during the dry season. Indeed, *An. funestus* is often abundant and has high EIR during dry season compared to the rainy season (Fontenille et al., 1997; Manga et al., 1997). *An. funestus* was identified as the main malaria vector in the Guinean climatic region, in East Africa and Madagascar (Robert et al., 1985; Severini et al., 1990). As shown in our study, despite the presence of irrigated rice field, there is a great variability in the annual EIR values and seasonality would seem to play a key role (Mabaso et al., 2007).

Finally, an important finding of our study is that in Zatta, where irrigated rice farming was interrupted in 2003/2004, *Plasmodium* prevalence rates and the number of presumptive malaria cases decreased. This observation is corroborated by a significant decrease in the EIR from 2002 to 2003 (Koudou et al., 2005) and a significant increase from 2003 to 2005 (Koudou et al., 2007). This study demonstrated also that irrigated rice cultivation is associated with elevated malaria prevalence rates, as well as high numbers of presumptive malaria cases, as seen in Burundi (Coosemans, 1985), Kenya (Githeko et al*.*, 1993) and Madagascar (Marrama et al*.*, 1995). However, research carried out in Tanzania showed that irrigated rice farming was not associated with a higher risk of malaria. One important reason for this observation is that farmers engaged in irrigated rice have the opportunity to gain some extra money, part of which is spent for protective measures against malaria. A reduced risk of malaria despite enhanced rice production has been termed 'paddies paradox' (Ijumba & Lindsay, 2001).
