The fight against malaria in Sub-Saharan Africa has to be transectoral and tackled from the clinical, social and vectorial points of view, involving the local health structure (primary health care systems, national anti-malaria and vector control programmes) and international support programmes [1–3].

The vector control campaigns are based on environmental sanitation and suitable environmental management, the implementation of educational programmes and the use of insecticides, either to impregnate fabrics (mosquito nets, curtains....) or to spray (indoors and outdoors). The majority of these directives are currently the basic pillars of the fight against vectors, except for the use of pesticides, which has to be highly regulated and their handling subject to strict control, according to the recommendations of the WHO [4].

Despite the good results from controlling the transmission during the 50 s and 60 s with widespread spraying activities with insecticides from the organochlorine family (such as DDT) in various regions of the world [5], this strategy has not proven to be as effective in highly endemic African countries and has led to the emergence of factors that limit the success of the vector control campaigns: (i) appearance of vector populations that are physiologically resistant to organochlorine insecticides, and, in certain countries, resistance to pyrethroid and carbamate insecticides [1, 6, 7], (ii) social rejection of fumigation campaigns, mainly inside the home, and to toxic side effects from their use [8], (iii) changes in the feeding and resting behaviour of the vector populations exposed to surfaces treated by excito-repellent insecticides (DDT, pyrethroids) [8, 9] and (iv) damage to species that act as natural predators of the vectors [10].

Insecticide-treated nets (ITNs) appeared to be an alternative to indoor residual spraying (IRS) and proved to be a good tool to reduce the morbidity-mortality among the most vulnerable groups (children and pregnant women) [11–15]. The first trials resulted in widespread control campaigns based on the use of ITNs [16–18]. In general, these campaigns led to the conclusion that ITNs are the ideal tool to control malaria transmission, in particular in countries with a high endemicity, provided that they are used correctly and that a high population coverage is reached [19].

Despite what has already been stated, control campaigns based on IRS are being implemented in many countries of Sub-Saharan Africa. These are obtaining excellent results, in particular in those countries where the transmission of malaria is not constant or seasonal [20, 21].

On the island of Bioko (Equatorial Guinea), ITNs have been the main tool used to control malaria over the last 13 years (1991–2004). At the start of 2004, a private initiative, funded by oil companies, was set up under the aegis of Equatorial Guinea's Ministry of Health, to implement a huge campaign to control malaria on the island. The campaign was based on IRS with pyrethroids.

The aim of this study was to assess the impact of the two control strategies currently used on the island of Bioko with regards to Plasmodium infection and anaemia in children under five years of age.

During the study conducted in February 2004, the number of selected households was 168: 143 in the urban area and 25 in the rural area, and 168 children under 5 were surveyed. In February 2005, the number of households selected for the study came to 279: 150 in the urban area and 129 in the rural area, and 433 children were surveyed.

In contrast to to the first survey, a team member checked the quality of the usage of mosquito nets during the 2005 survey: whether the nets were correctly hung or not, if them were in the correct position and whether or not they had any holes. Only 4% (12) of the mosquito nets were not hung. With respect to hanging, 13 were incorrectly hung or were too small compared to the size of the bed (5%). 25% of the mosquito nets checked had holes.

The transmission of malaria in Equatorial Guinea is stable. There may be some seasonal variations in the transmission model, but there were no significant variations from one year to another [24]. Therefore, the fact that there was significant difference in the infection prevalence from one year to the next may be attributed to the introduction of a new variable that had a direct impact on the transmission. When the percentage of ITNs users among children was analysed, there was no significant difference between 2004 and 2005, although the percentage of users was slightly higher in 2004. This data should be interpreted with caution, as there was no control during the first survey, to check whether or not a child was really protected by a mosquito net while he slept or to establish the condition of the net. During the second survey, it was noted that a small percentage of the ITNs were not hanging at all or were not hung correctly, which suggests that the percentage of children protected by a mosquito net in the 2004 survey was overestimated. It is logical to believe that the dramatic reduction noted in the CPR over these two years is due to the implementation of the widespread indoor spraying campaign that took place during 2004, subsequent to the first survey. During a historical review of the malaria control activities based on IRS in countries in southern Africa [20], a dramatic reduction was noted in the case incidence, in particular shortly afterwards these campaigns had been implemented. In all the cases, even in unstable and low endemic malaria conditions, the transmission was never completely eradicated [27, 28]. Similar results were noted in pilot projects started between 1950 and 1960 in tropical countries, such as Cameroon, Rwanda, Burundi, Tanzania, Kenya and Uganda [29, 30]. Apart from the malaria indexes being reduced, there was always a dramatic reduction in the vector populations.

A factor to be taken into account with respect to the success of the control campaigns based on IRS would be the resting habit of the vector species [1, 31]. On the island of Bioko, An. gambiae s.l. and An. funestus are the species responsible for the transmission. During a longitudinal study conducted on the island between 1998 and 1999, it was noted that both species showed markedly endophilic behaviour (resting indoors) during the dry season and exophilic behaviour (resting outdoors) during the rainy season [25]. As the dwellings were sprayed for the first time at the end of the dry and the start of the rainy season, it was guaranteed that the vector populations would be in contact with the recently treated surfaces.

On the other hand, a clear association between sleeping or not under a mosquito bed net and Plasmodium infection was noted in both surveys. This data coincides with the results obtained in all the experimental studies conducted with ITNs [11, 12, 14, 15, 18]. The fact that no major differences were found in the CPR between those children who slept under a mosquito net without holes and hung correctly and those that slept under a mosquito net with holes and/or badly hung, could be due to their being treated with insecticide. The majority of the tests conducted with mosquito bed nets have shown that the degree of protection of the treated mosquito nets is greater than that of the non-treated, even when they have holes [32]. In the present study, data relating to the re-impregnation or purchase date of the mosquito net was not gathered, which would have been information that could have corroborated the above hypothesis.

The difference found in the risk of infection between the different age groups could be associated to differences in the immunological status and the use of ITNs. It is known that, during the first months of life, the risk of infection is lower because there is still a degree of immunity from the mother. The risk of infection first increases with age and then decreases when the individual himself reaches a degree of immunity due to the numerous contacts with the parasite [33]. With respect to the use of mosquito nets, it was noted in the two surveys that the proportion of mosquito net users among children under one year of age is higher than the proportion of users in the group between three to years of age, which could also explain the difference in the CPR between those two age groups.

With respect to the prevalence of anaemia, a significant reduction was not noted as the result of using ITNs or IRS, contrary to what was seen in other studies [14, 15]. The type of study carried out was probably not suitable to determine the impact of these two strategies on the blood haemoglobin levels. In areas of intense transmission, the majority of the cases of severe anaemia due to malaria would be associated with episodes of malaria, due to re-infections or to a poor reaction to the treatment [34, 36]. Therefore, a longitudinal study would be more appropriate to evaluate the impact of these two strategies with respect to haemoglobin levels [15, 35, 36].

IRS and ITNs on the island of Bioko have proven to be effective control strategies. In some trials carried out with ITNs, where high population coverage was reached, a similar community protection effect was noted to that obtained using IRS [14, 15, 37]. The health impact of the two strategies is fully proven. The choice of one or other strategy is, above all, a question of operational feasibility and availability of local resources [5]. A further two fundamental aspects when selecting one or the other would be: the degree of social acceptance or "prestige" of both strategies, and the cost effectiveness and sustainability in the medium- to long-term [5]. With respect to cost-effectiveness, there are various comparative studies that favour one or the other strategy [38–42]. Spraying, with one or two fumigation cycles, has been shown to be cheaper in the majority of the trials, when compared with the distribution of mosquito nets [39, 41]. As the number of fumigation cycles increases (one or two per annum, depending on the endemicity), the cost of the strategy increases in comparison to re-treating mosquito nets [42].

The sustainability of any control strategy is conditioned by the availability of skilled national human resources and by the degree of economic investment, whether from private or state resources. The presence of economic interests in an area of high endemicity (e.g. the oil industry on the island of Bioko) fosters the implementation of control programmes with a high investment level, mainly with private resources [38]. The disappearance of those economic interests or the occurrence of a variety of internal phenomena (war, changes in government, etc...) may result in the control activities being interrupted. In the case of IRS, it has been noted that an interruption of activity may lead to the appearance of serious epidemic outbreaks, as was the case of South Africa in 1996 [43].