Results and Discussion

Mapping of the geographical distribution of WNF in Israel (2000-2005) indicated that the majority of cases occurred in the densely populated area (Tel Aviv metropolis) along the seashore, and less in the north, which is less populated (Fig. 34.1, see also Paz and Albersheim, 2008). These results reflect the country's population dispersion. The warming tendency that had been detected in the Israeli hot season in past decades (e.g., Paz et al., 2007) was ongoing in the study period. A positive anomaly is apparent in the three examples in Fig. 34.2 (see also Paz and Albersheim, 2008) that illustrates the positive anomalies of the mean monthly temperatures from the perennial averages (maximum and minimum). In four out of the five months of March (2001-2005) an increase in the maximum temperature was found, mostly with an anomaly of more than 3°C (an extreme anomaly of more than 5.5°C was detected in March 2001). In the spring (May) the highest anomaly was detected in 2003, with more than 3°C above the perennial averages for both minimum and maximum temperatures. In four out of five summers, positive anomalies were detected in August with 0.5-1.5°C above the minimum and maximum averages.

Analysis of the distribution of mosquito hazard demonstrated that most mosquito concentrations were found in three main areas (see the locations in Figs. 34.1 and 34.2): (a) along the northern seashore, (b) within the center of Israel, and (c) throughout the southeastern borders between the Dead Sea and the Red Sea. Medium and severe hazards were found near the most densely populated areas (central and northern coastal areas). Medium to severe mosquito hazards appeared in the early spring, weeks before the extreme summer heat. A significant positive linkage with a 2-week lag (result of multinomial logistic regression) was detected between the standardized mean daily temperatures and the mosquito hazard levels (medium and sever hazards) in the North. The same tendency was found during a longer period in the Hadera district, north of Tel Aviv (Paz and Albersheim, 2008).

Precipitation amounts as well as rainfall distribution did not show a spatial coherence along the study period.

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Fig 34.1 WNF cases distribution in Israel, during the years 2000 (initial outbreak with 429 cases in humans), 2001 - 33 cases, 2002 - 7 cases, 2003 - 80 cases, 2004 - 38 cases, and 2005 -163 cases (Paz and Albersheim, 2008).

Fig 34.1 WNF cases distribution in Israel, during the years 2000 (initial outbreak with 429 cases in humans), 2001 - 33 cases, 2002 - 7 cases, 2003 - 80 cases, 2004 - 38 cases, and 2005 -163 cases (Paz and Albersheim, 2008).

Results of Spearman correlation calculation between WNF cases (hospital admission dates) and mean daily temperatures are presented in Table 34.1 (see also Paz and Albersheim, 2008) for three main districts. Values were grouped into 2-week lags (Lag 1 = 2 weeks, Lag 2 = 4 weeks, etc.). High positive significant correlations (r < 0.62) were found between temperature increase and a rise in WNF cases in the North during a lag of up to 8 weeks and for the Hadera district in a lag of up to 6 weeks. Lower correlation was detected for the Tel Aviv area in a lag of 4 to 6 weeks (r < 0.31). A similar tendency was found using Pearson cross-correlation calculations between WNF data (admission dates) and minimum/ maximum daily temperatures in Tel Aviv area in 2005 (the year with the highest number of WNF cases). Lag correlation analysis using Z-test found significant positive medium linkages (0.3 < r < 0.4). The strongest correlation was found after

~34 days for the daily minimum temperatures and after ~38 days for the daily maximum temperatures.

Fig. 34.2 Temperature anomalies in selected months (March 2001, April 2003, and August 2005). The black value refers to the monthly maximum temperature, and the gray value represents the monthly minimum temperature (Paz and Albersheim, 2008).

Significant lag correlations between daily rainfall amounts and WNF cases were not found. The disease frequency reduce in the years following the WNF outbreak of 2000 was probably a result of appropriate treatment by health and environmental authorities: standing water drying up, pest control systems use, and special programs to increase public awareness of the importance of draining standing water sources and using mosquito repellent. However, the risk was still present.

The warming tendency during the hot season in the EM over recent years (Paz, 2006; Paz et al., 2007) was ongoing during the study period. It was severe at the end of the winter and spring and less severe in mid-summer. The positive lag correlations between air temperature and mosquito hazard level/WNF cases indicate the significance of temperature increase at the beginning of the hot season, weeks before the disease's main appearance at mid-summer (Paz and Albersheim, 2008). This supports Paz (2006) who noted that an early extreme rise in temperature in the hot season could be a good indicator of increased vector populations.

Table 34.1 Results of Spearman correlation calculation between WNF cases in humans (hospital admission dates) and mean daily temperatures, along the period 2001-2005 in three main districts.

Spearman's

Rho p-value

North

LAG0

0.56

0.00

LAG1

0.62

0.00

LAG2

0.62

0.00

LAG3

0.51

0.01

LAG4

0.36

0.08

Hadera

LAG0

0.46

0.01

district

LAG1

0.59

0.00

LAG2

0.62

0.00

LAG3

0.45

0.01

LAG4

0.24

0.18

Tel Aviv

LAG0

0.27

0.13

LAG1

0.24

0.18

LAG2

0.30

0.09

LAG3

0.31

0.08

LAG4

0.28

0.12

Source: Paz and Albersheim (2008).

Source: Paz and Albersheim (2008).

Along the study period, most of the cases occurred in the Tel Aviv metropolis, a narrow coastal area, where the heat is very intense due to the combination of high temperatures and a high level of air humidity. This region has a high population density and suffers from the urban heat island effect, which tends to worsen the local heat conditions. Gibbs et al. (2006) also found that the risks associated with WNF outbreaks appear to increase in urban and suburban areas as a result of human activity.

The increase in mosquito hazards and in WNF cases in 2003 (Fig. 34.1) could be related to the extreme heat at the beginning of the spring, but also to the unusual increase in rainfall amount in the previous winter and spring. It is reasonable to assume that these precipitations could enlarge the standing water recourses at the beginning of the hot season. This supposition is different from previous studies (Epstein, 2005), which noted that WNF increases after an extreme dry period, since standing water pools become richer in organic materials. The current results claim that these conclusions fit regions with larger amounts of precipitation throughout a longer rainy season. However, in a Mediterranean climate type, when the rainfall regime consists of a long dry summer without any rainfall, the usual conditions are severe drought and dry pools during the summer. Therefore, an increase in rainfall amounts in the spring might be an important parameter that expands the mass of water ponds. This insight is similar to the finding of Hubalek and Halouzka (1999), who noted that heavy rains could increase the incidence of WNF in Europe. Therefore, in a Mediterranean climate type, two indicators may encourage the WNF appearance - rainy spring and/or severe heat.

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