**3.4 Effects of seasonality on** *Plasmodium* **Transmission**

Tables 2 and 3 summarise the mean biting rate, infection rate and the entomological inoculation rate (EIR) of *An. gambiae* and *An. funestus* in the two study villages in 2002, 2003 and 2005.

#### **3.4.1 Relationship between season and biting and infection rates**

In Zatta, significantly higher *An. gambiae s. s.* biting rates were recorded in the dry seasons of 2002 and 2005 when irrigated rice farming was practiced, compared to the dry season of 2003 when irrigated rice farming was interrupted (LRT comparing 2002 with 2003: 13.79, LRT comparing 2005 with 2003: 20.50; both *P* <0.001). In 2003, there was no seasonal difference in the biting rate of *An. gambiae s. s.* (LRT = 0.13; *P* = 0.900) and *An. funestus s. s.* (LRT = 0.17, *P* = 0.879). In Tiémélékro, in 2002 (LRT = 1.84; *P* = 0.069) and 2005 (LRT = 0.56; *P* = 0.455), there were no significant differences in *An. gambiae s. s.* biting rates between the dry and the rainy season. In 2003, the biting rate was significantly higher in the long rainy season (LRT = 3.87, *P* <0.001). Regarding *An. funestus s. s.* biting rates, those recorded in the dry season of 2002 (LRT = 6.15) and 2003 (LRT = 4.50) were significantly higher than those recorded in the rainy season (both *P* <0.001). The difference in the biting rates between the dry and rainy season in 2005 also showed statistical significance (LRT = 3.26; *P* = 0.031).

### **3.4.2 Relationship between season and** *Plasmodium* **transmission**

In both villages, higher EIRs of *An. gambiae s. s.* were usually recorded in the rainy season. For example, in Zatta, the EIR of *An. gambiae s. s.* recorded in the rainy seasons of 2002 and 2005 were 458 and 365 infective bites per person per season (ib/p/s), respectively. In 2003, when irrigated rice farming was interrupted in Zatta, *P. falciparum* transmission by *An. gambiae s. s.* and *An. funestus s. s.* only occurred during the rainy season. In Tiémélékro, in the rainy seasons of 2003 and 2005, the number of infective bites recorded for *An. gambiae s. s.* (357 and 208 ib/p/s, respectively) were 3-14 times higher than in the dry seasons (25 and 77 ib/p/s in the respective years). In 2002, in contrast, the EIR of *An. gambiae s. s.* recorded in the dry season was 2.5 times higher than the one recorded in the rainy season.

The highest EIRs of *An. funestus s. s.* were usually noted during the dry season. In Tiémélékro, this species was the primary *P. falciparum* transmitter during the dry season of 2005 when 207 ib/p/s were recorded. With regard to infection rates, with the exception of the 2005 infection rate of *An. funestus s. s.* recorded in Tiémélékro (χ2 = 4.47, *P* = 0.035), no significant differences were observed between seasons, neither for *An. gambiae s. s.* nor for *An. funestus s. s.* in any village.

Effects of Irrigated Rice Fields and Seasonality on

parameter

Malaria vector Entomological

*An. gambiae*

*An. funestus*

**4. Discussion** 

*Plasmodium* Transmission in West Africa, Particularly in Central Côte d'Ivoire 163

season

Infection rate 4.1 (268) 1.7-6.5 2.6 (531) 1.3-4.0 1.27 0.260 Parity rate 72.5 (240) 66.8-78.2 52.4 (597) 48.4-56.4 28.34 <0.001

Infection rate 2.6 (35) 0.0-8.7 7.9 (467) 5.4-10.4 1.19 0.274 Parity rate 59.0 (39) 42.8-75.1 58.6 (449) 54.0-63.1 0.002 0.961

Infection rate 10.9 (52) 3.7-17.3 6.8 (163) 2.6-10.6 1.47 0.226 Parity rate 48.3 (103) 33.3-63.4 81.1 (201) 75.5-86.7 30.19 <0.001

Infection rate 3.1 (97) 0.0-6.6 7.7 (26) 0.0-18.7 1.11 0.292 Parity rate 65.4 (185) 58.5-72.3 50.0 (22) 27.3-72.7 2.02 0.155

Infection rate 3.6 (55) 0.0-8.7 9.1 (66) 1.9-16.2 1.45 0.229 Parity rate 75.0 (60) 63.7-86.3 69.0 (42) 54.4-83.6 0.44 0.507

Infection rate 3.8 (131) 1.2-5.6 17.6 (11) 0.0-37.8 4.47 0.035 Parity rate 65.1 (203) 50.6-73.2 91.7 (12) 73.3-100.0 3.61 0.057

χ² or LRT

P value

Dry season Rainy

2002\* Biting rate 19.6 18.1-21.1 12.6 11.2-14.1 1.84 0.069

2003\*\* Biting rate 5.2 3.8-7.0 24.7 21.6-28.3 3.87 <0.001

2005\*\* Biting rate 3.9 2.1-5.7 16.7 9.1-24.4 10.56 <0.001

2002 Biting rate 5.0 4.1-5.9 0.7 0.4-1.1 6.15 <0.001

2003 Biting rate 8.4 6.6-10.5 4.0 2.9-5.4 4.50 <0.001

2005 Biting rate 29.9 20.9-38.9 1.0 0.6-1.9 3.26 0.031

Total EIR 146 - 60 -

Total EIR 25 - 357 -

Total EIR 77 - 208 -

Total EIR 55 - 67 -

Total EIR 28 - 9 -

Total EIR 207 - 32

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

In brackets are the number of malaria vectors analyzed; LRT (likelihood ratio test) \*Vegetable farming is performed intensively with 2 production cycles per year \*\*Vegetable farming is performed intensively with 1 production cycle per year

Table 3. Monthly average biting rate, infection rate, parity rate and entomological

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

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

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


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

aIrrigated rice farming performed in a synchronized manner

bInterruption of rice cultivation

cIrrigated rice farming performed in a synchronized manner

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


Effects of Irrigated Rice Fields and Seasonality on *Plasmodium* Transmission in West Africa, Particularly in Central Côte d'Ivoire 163

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

\*Vegetable farming is performed intensively with 2 production cycles per year

\*\*Vegetable farming is performed intensively with 1 production cycle per year

Table 3. Monthly average biting rate, infection rate, parity rate and entomological inoculation rate (EIR) of *An. gambiae* and *An. funestus* during the dry season and the rainy season in 2002, 2003 and 2005 in Tiémélékro, central Côte d'Ivoire
