Malaria is one of the serious tropical diseases caused by protozoan parasites transmitted by the bite of female Anopheles mosquitoes. Malaria control is an increasingly important focus for the international body concerned with public health and disease control. In Africa ITNs and IRS are both effective for malaria vector control. Insecticide-treated nets are being promoted throughout Africa as a fundamental preventive strategy to Roll Back Malaria [1]. However, the control of malaria in Africa is less successful because of the occurrence of drug resistant parasites and insecticide resistant vectors [2].

Compared with placebo treated, the wide use of ITNs was shown to reduce incidence and provide significant protection against mortality attributed to malaria, clinical attacks of malaria and malaria infection [3]. In addition, in a small field trial in a Kenyan school [4] reported a reduction of 97.3% in malaria attack for children who slept under bed nets.

Currently, there are a number of possible precipitating epidemic factors of malaria in addition to environmental or climatological factors, chloroquine-resistance of Plasmodium falciparum, high scale population movement (due to resettlement and labor forces in agro-industrial schemes) and deterioration of vector control operations [5].

The nationally-adopted malaria prevention and control strategies in Ethiopia include early diagnosis and prompt treatment, selective vector control strategy using ITNs and IRS [6]. Dichloro-diphenyl-trichloroethane (DDT) has been used in Ethiopia for nearly five decades and is still the insecticide of choice. Most spraying operations are carried out during June and July to prevent potential malaria epidemics that can occur during September to November after the heavy rains and in January and February to prevent transmission following the small rains [7]. Indoor residual spraying with DDT can be used for insecticides on the wall of dwellings up to six months [8]. However, the increase and spread of insecticide resistance, the high degree of replastering rate of sprayed houses in the context of expanded urbanization, and an increased number of natural resource development projects, has necessitated the utilization of all appropriate technological and management techniques in an integrated approach to bring about an effective degree of vector suppression [9].

Widespread use of ITNs by the entire community results in a decline in human-mosquito contact, a decrease in the number of mosquitoes, and a reduction in malaria transmission, which will lead to a decline in malaria-related morbidity and mortality [7]. In addition, ITNs provide protection against nuisance mosquitoes and kill head lice and bedbugs. A study carried out for the implementation of insecticide-treated mosquito nets in malaria control in Ethiopia showed that the community reported unaffordability of ITNs to most households and improper use of nets. However, acceptability and willingness to use ITNs for malaria prevention was very high [10].

The control of malaria and other vector borne diseases in Ethiopia until recently relied up on case treatment and vector control through application of water dispersible residual insecticides, such as DDT, to the interior surfaces of walls, ceilings and roofs of houses. Currently, in line with the strategies set by the Roll Back Malaria, wide-scale use of insecticide-treated nets is added to the existing control methods. The objective of this study was to assess the impact of ITNs/IRS combined malaria control strategies currently used in Aneno Shisho kebele, Adami Tulu District with regard to malaria transmission and on the Anopheles vector species.

The overall prevalence of malaria in the first survey (October/November 2006) and second survey (April 2007) was 8.6% in Jela Aluto, 4.4% in Kamo Gerbi and 1.3% in Aneno Shisho. This data was found during no malaria control under taken in Jela Aluto, while there was ITN in use in Kamo Gerbi and both ITN and IRS in use in Aneno Shisho. Regarding elevations among the three kebeles, there were no significant differences. In connection with this in Ethiopia areas below 2,000 metres altitude are considered malarious. However, when microclimate and weather conditions are favourable, malaria transmission is reported to occur in areas, higher than 2,000 metres above sea level [15]. Also, the distances among the three kebeles were far from each other as indicated in Figure 1, and then there is no Anopheles flight. People also confined to their own kebeles for their subsistence agriculture and cattle herding. Furthermore, in each kebeles there were drainage ditches, swamps, hoof prints, and cattle sheds, which have contributed to the creation of breeding areas. Therefore, the significant difference in the prevalence among the three kebeles was related to the introduction of ITNs and IRS that had direct impact on the malaria transmission. The highest prevalence of malaria was detected in <5years of children in Jela aluto (26.2%), followed by Kamo Gerbi (6%) and Aneno Shisho (1.2%) as shown in Figure 2. This could be associated with the ITN and IRS coverage in Aneno Shisho, ITN coverage in Kamo Gerbi and neither of the two malaria control measures in Jela Aluto as obtained from the KAP survey.

As the survey undertaken the dwellings of Aneno Shisho were sprayed with DDT in June and July 2006, just before the major transmission season and for the second round in January and February 2007, just before the minor transmission season through health center of Adami Tulu District. In connection with this, in Africa IRS has been focused at selected ecological zones such as those experiencing unstable transmission [16]. Aneno Shisho and Kamo Gerbi kebeles had received two PermaNets per household without any cost disseminated through health center of Adami Tulu District. During this study conducted in the District between October-December 2006 and April-May 2007, the total density of An. gambiae s.l., An. pharoensis and An. coustani collected from the three study kebeles were 15(2.7%), 187(33.4%) and 357(63.8%), respectively (Figure 3). Treated-mosquito nets have been shown to provide strong protection against malaria and are currently widely applied as a vector control measure particularly in Sub-Saharan Africa [17]. Similarly, assessment of the effects of ITNs and IRS in malaria control showed the importance of implementation of ITNs and insecticide spraying as a malaria control strategy in malarious areas of Adami Tulu District.

In relation to the relative density of Anopheles species Oct/Nov 2006, the malaria prevalence was 8.6% in Jela Aluto where there was no malaria control; on the other hand in Kamo Gerbi and Aneno Shisho kebeles malaria prevalence in Oct/Nov 2006 to April 2007 was reduced in comparison with the corresponding period in Jela Aluto. The findings obtained from studies carried out in the same District revealed that the prevalence of P. falciparum and P. vivax were 30.2% and 6.2%, respectively [18]. The use of ITNs is suggestive of the fact that reduction of man-vector contact and significantly reduced the transmission of malaria in the area.

The use of insecticide-treated bed nets was recommended for the area as an option for reducing contact with Anopheles arabiensis[11]. This was based on the experience of pyrethroid-treated bed net that has been shown to be an effective tool in the reduction of malaria mortality and morbidity in some countries, and had reduced malaria transmission with large-scale use [19]. On the other hand, in a comparative study done in neighbouring Kenya showed that sleeping under an ITN reduced the risk of infection by 63% and sleeping in a room sprayed with insecticide reduced the risk by 75% [20]. This study recommends the use of IRS as a more effective and cheaper option in communities with low and seasonal risks of infection.

This is suggestive of the use of insecticide-treated bed nets can be considered an option for reducing contact with An. gambiae s.l., but would be ineffective against An. pharoensis, as it has been shown that most man-vector contact with this species occurs outdoors during the early hours of the evening [11]. The average density of Anopheles mosquito species in the insecticide-treated kebeles of Aneno Shisho and Kamo Gerbi was lower than the density in the untreated Jela Aluto area. The difference can be attributed to the insecticide malaria control.

The density of An. gambiae s.l. and An. pharoensis were higher from October to December than from April to May. These results indicated the similarity with the national pattern of malaria transmission where high number of malaria cases occurs during the major transmission season from September through December following the heavy rains [21]. It has been reported that An. arabiensis fed predominantly indoors than outdoors [3]. Furthermore, the predominantly endophagic behaviour of An. arabiensis had reported in different African countries including Ethiopia [22]. From the results of this study area, the use of insecticide-treated bed nets can be considered effective for reducing man-vector contact with An. gambiae s.l.

The exophagic behaviour of An. pharoensis has been documented in Zeway [11]. Furthermore, a study in Gambella, Ethiopia had also shown this species to be feeding indoors and outdoors in equal proportions [23] indicating eco-geographic variations in its feeding behaviour. Anopheles pharoensis may play significant role in the transmission of malaria during the dry season as suggested by its higher density that tends to increase whereby the density of An. gambiae s.l. is known to drop. That is, An. pharoensis may take the role of main malaria vector during the dry season [11]. Anopheles coustani which was collected from outdoors (cattle sheds) was previously reported as common in Akaki, out district of Addis Ababa [24].

Out of a total of 380 specimens (15 An. gambiae s.l., 187 An. pharoensis and 178 An. coustani) tested for the presence of P. falciparum and P. vivax sporozoite. The sporozoite ELISA did not demonstrated infection; this finding is coincided with the study carried out in the same District that from a total of 334 An. arabiensis and 272 An. pharoensis tested for the sporozoite antigens; none were found to be infected [11]. In contrast, 262 An. gambiae s.l. and 436 An. pharoensis were assayed by ELISA. Of these sporozoite rates of 0.76% (P. falciparum) for An. gambiae s.l. and 0.47% (P. vivax) for An. pharoensis were reported in the Gambella area of western Ethiopia (19). Moreover, malaria sporozoite rates were determined by ELISA, out of An. arabiensis, 0.5% infective with P. falciparum and 1.76% with P. vivax were found in Southern Ethiopia from Sille [25]. Therefore, although the sporozoite ELISA did not demonstrated infection, their abundance, which coincided with increasing malaria prevalence in this study area, may indirectly support the role of these species as malaria vectors at Adami Tulu District.

The significantly high abundance of adult Anopheles mosquito population in Jela Aluto compared to Kamo Gerbi and Aneno Shisho suggests that the use of ITN and/or insecticide spraying had reduced the indoor resting mosquito vectors. Thus, malaria control programs must also include vector control to reduce drug pressure in the endemic kebele. In particular, insecticide-impregnated bed nets were found to substantially reduce occurrence of malaria in the population, especially in the wet season in Ghana [26]. Moreover, insecticides repel mosquitoes and by so doing reduce the number of mosquitoes entering the sprayed room [27]. In contrast to the present study, insecticide-treated nets and insecticide sprays have been shown to affect the indoor resting habit of mosquitoes by increasing the rate of exophily [28]. This was also shown in Ethiopia from Gambella [23].

The combined effect of IRS and ITN that reduces mosquito human contact was directly associated with low malaria prevalence in the present study. A comparable finding was reported from Eritrea whereby a combination of IRS, larvacidal measures and malaria case management was used to combat malaria [29]. It was noted that the drop in infection rates was closely related to the amount of DDT used and the number of insecticide-treated bed nets distributed.

In the present study, the ITN/IRS control measures did not fully control malaria transmission as low level prevalence were registered albeit in a significantly lower level in the ITN/IRS kebele. The reason for this may be due to inadequate coverage of households with ITNs whereby each household received only two PermaNets and only children slept inside the nets in majority of the cases, leaving the adults exposed to high risk of infection. This was reflected by the relatively higher malaria prevalence in adults in the kebeles that received ITNs. Furthermore, insecticide resistance in the region can be the cause for the relatively high Anopheles catch as it was indicated [11], which would maintain malaria transmission in the study locations.

In conclusion, based on the findings of the study, the prevalence of malaria and the number of Anopheline mosquito species was significantly higher in the kebeles of Adami Tulu District where no malaria control measures were in use. Therefore, the concurrent use of ITNs and IRS was a much more effective malaria control measure. In order to have adequate knowledge about the mode of malaria transmission health education is very important in addition to disseminating ITNs and/or insecticide spray since the level of awareness was not very high even in those communities that received the control interventions.