Climatic variables and malaria transmission dynamics in Jimma town, South West Ethiopia

Abstract

Background: In Ethiopia, malaria is seasonal and unstable, causing frequent epidemics. It usually occurs at altitudes < 2,000 m above sea level. Occasionally, transmission of malaria occurs in areas previously free of malaria, including areas > 2,000 m above sea level. For transmission of malaria parasite, climatic factors are important determinants as well as non-climatic factors that can negate climatic influences. Indeed, there is a scarcity of information on the correlation between climatic variability and malaria transmission risk in Ethiopia in general and in the study area in particular. Therefore, the aim of this study was to determine the level of correlation between meteorological variables and malaria cases.

Methods: Time-series analysis was conducted using data on monthly meteorological variables and monthly total malaria in Jimma town, south west Ethiopia, for the period 2000-2009. All the data were entered and analyzed using SPSS-15 database program. Spearman correlation and linear regression analysis were used to asses association between the variables.

Results: During last ten years (2000-2009), a fluctuating trend of malaria transmission was observed with P.vivax becoming predominant species. Spearman correlation analysis showed that monthly minimum temperature, total rainfall and two measures of relative humidity were positively related with malaria but monthly maximum temperature negatively related. Also regression analysis suggested that monthly minimum (p = 0.008), monthly maximum temperature (p = 0.013) and monthly total rainfall (p = 0.040), at one month lagged effect, were significant meteorological factors for transmission of malaria in the study area.

Conclusion: Malaria incidences in the last decade seem to have a significant association with meteorological variables. In future, prospective and multidisciplinary cooperative research involving researchers from the fields of parasitology, epidemiology, botany, agriculture and climatology is necessary to identify the real effect of meteorological factors on vector- borne diseases like malaria.

Figure 1

Annual trends of total malaria cases in Jimma town, 2000-2009. The figure shows that an increase in malaria cases occurrence from 2003-2005 with peak cases occurring in 2005 and malaria cases were reduced the following three consecutive years (2006-2008) but a remarkable increase in 2009.

Figure 2

Relative annual trends of P. falciparum and P. vivax malaria in Jimma town, 2000-2009. The figure demonstrates that a high fluctuation of malaria cases at species level in the study area. With the exception of some years, a high predominance of P. falciparum over P. vivax was observed. Thus the remarkable increment in total malaria cases was mainly due to the increment of P. falciparum rather than P. vivax except that by the year 2009 P. vivax was found to be highly diagnosed relative to P. falciparum which indicates that currently there was a trend shift from P. falciparum to P. vivax malaria in the study area.

Figure 3

Monthly and seasonal variation of total malaria cases in Jimma town, 2000-2009. The figure indicates that in the study area malaria cases occurred in almost every month of the year. The highest peak of malaria cases in almost all years was observed during September with exception that in 2009 the highest cases were observed during October. Again the season with the highest average total malaria cases occurrence was spring (September, October and November ) and the minimum malaria cases was observed during winter (December, January and February ).

Figure 4

Annual meteorological factors variation in Jimma town, 2000 -2009. The figure demonstrates that in the town, annual mean temperature ranged from as low as 19.5°C in 2005 to as high as 20.1°C in 2009 and a slight fluctuating trend of temperature through the years 2000 to 2009 was observed. It also shows a high fluctuating trend of rainfall was reported through the years 2000 to 2009 with 1586.7 mm mean annual rainfall and maximum total rainfall was observed in 2006 (1859.9 mm) and the minimum rainfall was observed in 2003 (1285.2 mm). In the last ten years, a fluctuating trend of relative humidity was observed at three different hours (0600, 1200 and 1800).

Figure 5

Relationship between monthly malaria cases and meteorological variables in Jimma town, 2000-2009. The figure demonstrates association between monthly malaria cases and meteorological variables (temperature, rainfall and relative humidity) in the same graph. There was fluctuation of malaria cases with rainfall fluctuation in the figure. However, other meteorological variables were less likely fluctuates.