The transmission of malaria can be determined by climatic or non- climatic factors. But even leaving these non-climatic issues aside, the effect of climate itself on the intrinsic probability of malaria transmission, remains controversial. So, climate variability that impacts on the incubation rate of Plasmodium and breeding activities of Anopheles is considered one of the important environmental contributors to malaria transmission dynamics [13, 26].
For example, temperature rise is expected to increase transmission and prevalence of malaria by reducing the interval between mosquito blood meals, thus decreasing the time to produce new generations and by shortening the incubation period of the parasite in the mosquitoes. Sporogonic cycles take about 9 to 10 days at temperatures of 28°C but higher than 30°c and below 16°C have negative impact on parasite development . Also the minimum temperature for P. falciparum and P.vivax parasite development approximates to 18°C and 15°C, respectively and the daily survival of the vector is dependent on temperature as well. At temperatures between 16°C and 36°C, the daily survival is about 90%. The highest proportion of vectors surviving the incubation period is observed at temperatures between 28° - 32°C. So, temperature of 20°C to 30°C and relative humidity greater than 60% are optimal for Anopheles survive long enough to acquire and transmit the parasite .
The result of our study revealed that during the last ten years, a fluctuating trend of occurrence of malaria cases was observed in Jimma town. 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 was observed. Except for the year 2009, the remarkable increment of total malaria cases was mainly due to an increase of P.falciparum with little increase of P.vivax. But for 2009 total malaria cases, P.vivax contributed more than P.falciparum. Resistance of P. falciparum to the commonly used drug (chloroquine) during these years may have contributed to total malaria case occurrence [27–29]. In those years with remarkable malaria increase, from all meteorological variables only annual rainfall was increased and showed a positive relationship but it was not statistically significant (p = 0.069).
The occurrence of malaria was reduced during three consecutive years from 2006-2008. It coincides with the increased availability of the new drug Coartem for P.falciparum malaria at national and local level. A decrease in malaria cases occurrence after the 2005 maximum occurrence was observed. The increased attention to malaria control and preventive activities by different responsible bodies, increased awareness of the community on use of ITNs and other malaria control activities, increased accessibility of ITNs to community, increment of budget for malaria control and prevention activities (personal communication and data not shown) might contributed the decrement in malaria case occurrence in addition to meteorological factors.
It is likely that the excessive flooding due to heavy rains in 2006 might have also impaired mosquito breeding and flushed out the mosquito larvae . From all the months, the highest monthly malaria cases occurrence was observed during October in 2009 which was the main contributor for 2009 annual malaria cases increase. In this month a small increase of minimum temperature during September after summer rainfall, because correlation analysis at one month lagged effect minimum temperature, was the first meteorological variable that significantly affects malaria transmission in the study area. Some other non-climatic factors, such as road constructions and some other activities in the town which increased the number of breeding sites of mosquitoes might have contributed to the peak malaria occurrence (personal observation) or it might be due to increase of temperature from 11.7°C in 2008 to 12.2°C in 2009 or annual rainfall was increased from 1551 mm in 2008 to 1770.9 mm in 2009 or might be due to resistance of P. vivax for currently available drug (chloroquine) in the market [30, 31].
According to correlation findings, monthly maximum and mean temperature at zero month effect and maximum temperature and relative humidity at o6oo at one month lagged effect were negatively correlated but other meteorological factors were positively related with total monthly malaria case occurrence. The finding implies that meteorological variables can affect malaria transmission either positively or negatively even if the correlation was less likely linear. This finding contradicts with the findings in Shuchen County, China  and Highlands of Madagascar  which showed that all meteorological variables were positively correlated with malaria. The present study was undertaken at different altitude in Jimma. Correlation between malaria and climate vary with altitudes . The correlation coefficient for the association between monthly malaria cases and some meteorological factors was greater than other meteorological factors. This indicates that one meteorological factor plays greater role in malaria cases occurrence or transmission than others which coincide with the finding from Dehradun, Uttaranchal, India  Shuchen county, China , Rwanda , Madagascar  and east Africa Highlands .
In this study, the correlation coefficient for the association between monthly mean minimum temperature and monthly malaria cases was greater than that of the correlation coefficient for the association between any other measured meteorological variables and monthly malaria cases. Our results suggest that mean minimum temperature was the most significant factor that correlated with malaria transmission dynamics in the study area. The results of a similar study conducted in Rwanda suggested that monthly malaria cases occurrence or incidence in high altitude regions is related to change in minimum temperature, while in low altitude zone rainfall and mean minimum temperature was the most significant meteorological factor . It is also similar to the findings in Madagascar ; China  and east African Highlands ; all suggested that minimum temperature was most significant factor for malaria transmission over other meteorological factors. Therefore a rise of temperature, especially minimum temperature, would enhance the survival of Plasmodium and Anopheles during different seasons and thus accelerate the transmission dynamics of malaria and spread it into populations that are currently malaria free and immunologically naïve.
The monthly total rainfall was the most significant factor that determines malaria transmission in the study area after to minimum temperature. Rainfall plays an important role in malaria epidemiology because water not only provides the medium for the aquatic stage of the mosquitoes' life cycle but also increases the relative humidity and then the longevity of the adult mosquitoes. In some Sub-Saharan countries, for example, malaria transmission is restricted largely to the rainy seasons . However, the effect of rainfall on the transmission of malaria is very complicated, varying with the circumstances of particular geographical regions and depending on the local habits of mosquitoes. Rainfall may prove beneficial to mosquito breeding if moderate, but it may destroy breeding sites and flush out the mosquito larvae when it is excessive . This study indicates that total monthly rainfall was associated with occurrence of malaria in the town with a month lag effect. The same results can also found in Shucher County, China  and New Halfa, Eastern Sudan .
On the other hand, the correlation coefficients for the linear regression between the monthly mean and maximum temperature and monthly malaria cases were negative. This finding was similar to a study in India . This is important in the hot months, in which an increase in temperature would limit vector and parasite survival and therefore cause a decrease in malaria transmission rates. This finding contradicts which the findings in Shucher County, China which concluded that an increase in monthly maximum temperature should cause an increase rather than a decrease in malaria rates . This variation could be due to differences in local climatic condition in China and Jimma. That is the large number of months in Jimma that are hotter than months in China - this makes sense in the hot months, in which an increase in temperature would limit vector and parasite survival and therefore may cause a decrease in malaria transmission rate. The most likely explanation for the finding that increases in temperatures is correlated with a decrease in malaria cases is the significant autocorrelation between monthly temperature and relative humidity. This hypothesis is supported by the finding of high negative correlation between temperature and relative humidity. This indicates that, for a given amount of moisture in air, an increase in temperature cause a decrease in relative humidity, which can limit Anopheles survival. The correlation between maximum temperature and rainfall may also lead to an explanation of the negative correlation coefficient between maximum temperature and malaria cases occurrence. The negative correlation between maximum temperature and rainfall in hotter months may decrease Anopheles breeding or increase dryness which may be the limiting factor for malaria transmission.
Both the correlation and regression analyses suggests that temperature, rainfall and relative humidity act on monthly malaria case total occurrence with a lag of one month. Although all meteorological variables were less likely to predicts the occurrence of malaria in Jimma town. This finding contradicts the findings in Dehradun, Uttaranchal, India  Shuchen County, China , Rwanda , Madagascar  and east Africa Highlands  which concluded that at one month lagged effect meteorological variables were highly likely correlated with malaria occurrence and the prediction was higher than this finding with higher R square value. This variation might be due to the fact that this study was conducted in lowlands in which malaria is endemic. In lowlands, the factors that contribute to malaria transmission dynamics are microclimate variation due to anthropogenic effects and other non- climatic factors like, health system, population growth, population movement and others [13, 26].
At zero month time effect none of meteorological variables were statistically significantly correlated with monthly P.falciparm cases occurrence and total monthly malaria cases but there were statistically significant positive correlation between monthly P.vivax cases and minimum temperature and statistically significant negative correlation between monthly P.vivax case occurrence and maximum temperature. This might be due to the fact that P.vivax requires a little lower temperature than P.falciparum. Thus, minimum temperature variability could have more effect on P. vivax than P.falciparm by shortening the extrinsic phase and little change in maximum temperature have more negative effect on the development of P.vivax than P. falciparum.
Seasonality and year (time trend) played a role in the transmission of malaria in the town. From the time series analysis, none of the measured meteorological variables were able to predict malaria transmission dynamics. In general, there was a fluctuation in malaria cases during the last ten years. Many factors might be responsible for seasonal changes, e.g., climatic variables, ecologic and environmental factors, host and vector characteristics, and social and economic determinants such as change in health care infrastructure. Thus the range of vector borne disease is not solely determined by meteorological variability [32–35]. Social, biological and economic factors such as mosquito control measures, population immunity, local ecological environment (vegetation, introduction of irrigation ), governmental policy, availability of health facilitates and drug resistance have also an impact of malaria transmission dynamics. Also in the study there were different malaria control activities in each year like insecticide spraying, elimination of mosquito breeding sites, health education about malaria, distribution of ITNs and some malaria drugs and other activities to decrease mortality and morbidity of malaria. A limitation of the study was data on some of non- climatic factors which were available in the study area from annual reports but due to its incompleteness it was not included in regression analysis.