**3.3 Effects of agricultural practices (irrigated rice fields & vegetable farming) on**  *Plasmodium* **prevalence and clinical malaria cases**

#### **3.3.1 Irrigated rice fields and** *Plasmodium* **prevalence**

In both villages, the peak prevalence of *P. falciparum* was generally observed in children aged 3-6 years. There were three exceptions: in Tiémélékro, the peak prevalence of *P. falciparum* during the May 2005 survey was found in the youngest age group (≤ 2 years), whereas in Zatta, the highest prevalence in the baseline survey (June 2002) and the second last survey (May 2005) was observed in children aged 7-15 years.

In June 2002, similarly high *P. falciparum* prevalence rates were observed in Zatta (85.4%) and Tiémélékro (86.1%). In Zatta, a significant decrease in the mean *P. falciparum* prevalence rate occurred from 2002 to 2003 (58.4%; χ2 = 42.33, degree of freedom (df) = 1; *P* < 0.001). There was a significant increase from 2003 to 2005 (66.0%; χ2 = 14.78, df = 1, *P* = 0.012). In Tiémélékro, the *P. falciparum* prevalence rate in June 2003 (78.2%) was significantly lower than during the June 2002 survey (χ2 = 4.92, df = 1; *P* = 0.027). The annual *P. falciparum* prevalence rate decreased significantly from 2003 (70.7%) to 2005 (60.4%; χ2 = 17.27, df = 1; *P* < 0.001).

#### **3.3.2 Fever cases and asymptomatic carriers, stratified by parasite density**

Table 1 shows how many of the children examined with parasitaemia in the 2003 surveys were either asymptomatic carriers or presented with a fever. There was a strong seasonal variation in the proportion of fever cases among individuals with parasitaemia. In Zatta, for example, the proportion of fever cases among *Plasmodium*-positive individuals was significantly higher towards the end of the rainy season (August) when compared to the dry season (March) (22.1% **versus** 9.9%; χ2 = 9.90, df = 1; *P* = 0.002). In Tiémélékro, considerably higher frequencies of fever cases among *Plasmodium*-positive individuals were recorded during the peak rainy

Effects of Irrigated Rice Fields and Seasonality on

and 2005.

3.26; *P* = 0.031).

the rainy season.

*An. funestus s. s.* in any village.

0.005) and 8.9% from 2004 to 2005 (IRR = 0.911, *P* = 0.008).

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

monthly number of infective bites per person (IRR = 1.007; *P* = 0.776).

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

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

*Plasmodium* Transmission in West Africa, Particularly in Central Côte d'Ivoire 161

In Tiémélékro, the yearly numbers of presumptive malaria cases were 2089, 1858, 1655 and 1541. Thus, we observed significant decreases in the yearly number of presumptive cases by 11.1% from 2002 to 2003 (IRR = 0.889, *P* < 0.001), 9.0% from 2003 to 2004 (IRR = 0.910, *P* =

As in the case of Zatta, the monthly number of presumptive cases was not related to the

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

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 =

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 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

season in June (27.3%) and towards the end of the rainy season in August (25.5%) when compared to the dry season in March (15.9%; *P* < 0.05 for both comparisons).

In Zatta, all individuals with a high level of parasitaemia (≥ 5000 parasites/μl blood) presented with fever, accounting for a highly significant difference between the proportion of asymptomatic carriers and fever cases in this parasitaemia class (*P* < 0.001). Similarly, there was a highly significant association between the fever cases and high parasitaemia in the three surveys carried out in 2003 in Tiémélékro (*P* < 0.05). No statistically significant difference was found in children with lower parasitaemias (1000-5000 parasites/µl of blood), neither in Zatta (March: χ2 = 1.53; df = 1; *P* = 0.068 and August: χ2 = 0.116; df = 1; *P* = 0.733) nor in Tiémélékro (March: χ2 = 0.18; df = 1; *P* = 0.671, June: χ2 = 2.23; df = 1; *P* = 0.135 and August: χ2 = 0.001; df = 1; *P* = 0.973).

#### **3.3.3 Annual variation of presumptive cases and malaria transmission**

In Zatta, 966, 812, 693 and 884 presumptive cases were recorded in 2002, 2003, 2004 and 2005, respectively. The annual number of presumptive malaria cases decreased significantly by 15.1% and 14.7%, respectively, from 2002 to 2003 (IRR = 0.841, *P* < 0.001) and from 2003 to 2004 (IRR = 0.853, *P* = 0.002). An opposite trend was observed from 2004 to 2005; the number of presumptive malaria cases increased significantly by 27.5% (IRR = 1.276, *P* < 0.001). The monthly number of presumptive cases was not related to the monthly number of infective bites per person (IRR = 0.994, *P* = 0.827).


n.a.: not assessed

a No survey carried out in June 2003 in Zatta due to unstable sociopolitical situation

Table 1. Number (%) of children infected with *P. falciparum* who were asymptomatic carriers or presented with fever, stratified by different levels of parasitaemia, in the two study villages of Tiémélékro and Zatta, central Côte d'Ivoire.

season in June (27.3%) and towards the end of the rainy season in August (25.5%) when

In Zatta, all individuals with a high level of parasitaemia (≥ 5000 parasites/μl blood) presented with fever, accounting for a highly significant difference between the proportion of asymptomatic carriers and fever cases in this parasitaemia class (*P* < 0.001). Similarly, there was a highly significant association between the fever cases and high parasitaemia in the three surveys carried out in 2003 in Tiémélékro (*P* < 0.05). No statistically significant difference was found in children with lower parasitaemias (1000-5000 parasites/µl of blood), neither in Zatta (March: χ2 = 1.53; df = 1; *P* = 0.068 and August: χ2 = 0.116; df = 1; *P* = 0.733) nor in Tiémélékro (March: χ2 = 0.18; df = 1; *P* = 0.671, June: χ2 = 2.23; df = 1; *P* = 0.135 and

In Zatta, 966, 812, 693 and 884 presumptive cases were recorded in 2002, 2003, 2004 and 2005, respectively. The annual number of presumptive malaria cases decreased significantly by 15.1% and 14.7%, respectively, from 2002 to 2003 (IRR = 0.841, *P* < 0.001) and from 2003 to 2004 (IRR = 0.853, *P* = 0.002). An opposite trend was observed from 2004 to 2005; the number of presumptive malaria cases increased significantly by 27.5% (IRR = 1.276, *P* < 0.001). The monthly number of presumptive cases was not related to the monthly number of

Tiémélékro Zatta

March 2003 < 1000 69 (54.8%) 13 (10.3%) 135 (73.4%) 9 (4.2%)

June 2003a < 1000 86 (52.1%) 34 (20.6%) n.a. n.a.

a No survey carried out in June 2003 in Zatta due to unstable sociopolitical situation

villages of Tiémélékro and Zatta, central Côte d'Ivoire.

August 2003 < 1000 99 (50.5%) 22 (11.2%) 78 (57.4%) 12 (8.8%)

Table 1. Number (%) of children infected with *P. falciparum* who were asymptomatic carriers or presented with fever, stratified by different levels of parasitaemia, in the two study

No. (%) of children with fever

1000-5000 37 (29.4%) 6 (4.8%) 57 (26.8%) 9 (4.2%) ≥ 5000 0 (0) 1 (0.8%) 0 (0) 3 (1.4%) Total 106 (84.1%) 20 (15.9%) 192 (90.1%) 21 (9.9%)

1000-5000 32 (19.4%) 7 (4.2%) n.a. n.a. ≥ 5000 2 (1.2%) 4 (2.4%) n.a. n.a. Total 120 (72.7%) 45 (27.3%) n.a. n.a.

1000-5000 46 (23.5%) 16 (8.2%) 28 (20.6%) 7 (5.1%) ≥ 5000 1 (0.5%) 12 (6.1%) 0 (0) 11 (8.1%) Total 146 (74.5%) 50 (25.5%) 106 (77.9%) 30 (22.1%)

54.0% (372/689) 16.7% (115/689) 49.8% (298/598) 8.5% (51/598)

No. (%) of asymptomatic carriers

No. (%) of children with fever

compared to the dry season in March (15.9%; *P* < 0.05 for both comparisons).

**3.3.3 Annual variation of presumptive cases and malaria transmission** 

No. (%) of asymptomatic carriers

August: χ2 = 0.001; df = 1; *P* = 0.973).

Date of survey *P. falciparum*

Overall 2003 (number of positive children/ number of total children)

n.a.: not assessed

infective bites per person (IRR = 0.994, *P* = 0.827).

parasitaemia (parasites/μl blood)

In Tiémélékro, the yearly numbers of presumptive malaria cases were 2089, 1858, 1655 and 1541. Thus, we observed significant decreases in the yearly number of presumptive cases by 11.1% from 2002 to 2003 (IRR = 0.889, *P* < 0.001), 9.0% from 2003 to 2004 (IRR = 0.910, *P* = 0.005) and 8.9% from 2004 to 2005 (IRR = 0.911, *P* = 0.008).

As in the case of Zatta, the monthly number of presumptive cases was not related to the monthly number of infective bites per person (IRR = 1.007; *P* = 0.776).
