The relationship between malaria transmission intensity and overall incidence of the disease has remained complex. Morbidity rates of the disease seem to be insensitive to differences in transmission over a wide range of intermediate values. Efforts towards control of the disease include reduction of human-mosquito contact using insecticide treated nets or materials. This and other methods such as the transmission blocking vaccine depend on reduction in transmission intensity. However, there seems to be a conflict with these strategies because there are concerns that bed nets, by delaying the acquisition of immunity, may merely shift the burden of disease to an older age group, resulting in little or no long-term benefit or potentially detrimental consequences for the incidence of severe disease-the so-called rebound effect [1, 2]. There is also evidence that low to intermediate levels of transmission result in higher incidence of severe disease [3, 4]. Supporting these evidences, are suggestions that at high transmission intensities exposure to the parasite occurs very early in life, permitting the development of acquired immunity which protects the individual by other mechanisms (e.g. maternal antibodies) from severe disease [4]. On the other hand, data from Ghana, suggest that a tenfold decrease or increase in malaria transmission is associated only with a twofold decrease or increase in malaria morbidity [5]. In that study, number of malaria attacks were compared between Accra (1 infective bite per person per year), Kumasi (20 infective bites per person per year) and Cape coast (200 infective bites per person per year). Despite these major differences in transmission intensity, the cumulative number of malaria attacks by the age of 60 years was pretty similar -30, 62 and 43 respectively. This deduction is corroborated by findings from Nigeria where each 10 fold increase in the Entomological Inoculation Rate (EIR) corresponded with a 1.6 fold increase of incidence of clinical malaria [6]. Quantifying the relationship between transmission levels and the incidence of clinical attacks, it has also been noted that for low levels of transmission i.e. between 0.001 and 0.1 infective bites per person per year, the incidence of malaria attacks is probably directly proportional to the level of transmission in adults as in children [7]. But for higher levels of transmission like 1, 10, 100 and 1000 infective bites per person per year, the data suggests that malaria morbidity in Africa varies, with higher clinical incidences among children than adults [7].

These conflicts create situations where useful policies and effective planning and implementation of strategies for sustainable malaria control become difficult. It therefore becomes paramount to clearly understand how transmission intensity affects morbidity for proper planning, effective implementation and evaluation of interventions.

This study ascertains the relationship between malaria transmission intensity and patterns of morbidity in holoendemic areas of Imo River Basin of Nigeria.

This study demonstrates that malaria transmission in the study area is high. The variation in malaria transmission intensity between the study communities was not very significant, although slightly higher mean entomological inoculation rates (EIRm) were observed in some communities located at lower altitudes. The proximity of such communities to rivers and river beds and the ability of the vector mosquitoes to easily reach human populations from their resting habitats may be implicated in this phenomenon [2, 11]. The importance of altitude in malaria transmission have been documented [7]. As one ascends into higher altitudes, temperature gradually decreases rendering the weather conditions unfavorable for malaria transmission [11]. It is very important to look into the problems of malaria transmission in the study area and other parts of Southeastern Nigeria, taking into serious consideration the peculiar ecology, climatology and other factors and human activities which encourage mosquito breeding and malaria transmission. It is also important to note that these human activities directly affect the social, cultural and economic lives of the people.

Malaria morbidity rates as recorded in health facilities were relatively high compared with what is obtained in some other parts of Nigeria [6]. However, Similar observations have also been made in other parts of Africa[11, 12]. This phenomenon is particularly disturbing in the present study and may be attributed to factors such as late acquisition of natural immunity, malnutrition or nutritional status and late recognition and treatment of malaria [13].

A significant association between transmission intensity and morbidity rates was found in this study. This is consistent with previous reports that high morbidity rates were found in communities with high malaria transmission in Tanzania [11] and that high entomological inoculation rates were associated with higher malaria morbidities in western Kenya [12]. This trend provides evidence for a likely benefit of interventions aimed at reducing transmission intensity for low and middle altitude areas [13]. However the reverse has been predicted for higher altitude communities were low transmission intensity tends to correspond with higher morbidity rates [14]. This may be explained by concerns that low transmission intensity by delaying acquisition of natural immunity against malaria may merely shift the burden of the disease to an older age group on subsequent exposure to transmission of the disease [14, 15]. Furthermore, it has been observed that the degree of malaria immunity acquired by individuals living in endemic areas depend on the amount of exposure to infections and genetically determined immunological responses [16]. In areas of high stable transmission such as the study area, the incidence of the clinical malaria peaks is between 1 and 5 years of age and then declines rapidly as effective immune responses develop [14]. The epidemiological situation of malaria in the World remains a major threat to public health. In Africa, the global malaria eradication programme of the 1950s was not implemented due to high malaria endemicity, poor infrastructure and lack of financial resources. After the failure of the global eradication approach in 1992, WHO changed to a malaria control strategy based on early diagnosis and prompt treatment, implementation of selective, sustainable, preventive measures including vector control and strengthening of local capacities for assessment of malaria situation and its determinants in the affected countries. In 2004, the World Health Organization estimated the global incidence of malaria at 300-500 clinical cases annually, causing 1.5 to 2.7 million deaths each year. Today, more than 90% of malaria morbidity and mortality occur in Sub-Saharan Africa (SSA), where malaria accounts for an estimated 25% of all childhood mortality below the age of five. Recent studies suggest that this percentage might even be higher because of the contribution of malaria as an indirect cause of death [8].

The epidemiological picture of malaria is worsening with the spread of Plasmodium falciparum resistance to existing first line drugs and vector resistance to insecticides. This therefore makes transmission reduction strategies very important. This study ascertained high malaria morbidity rates and severe malaria morbidity indicators to be relatively higher in communities located at higher altitudes with higher transmission intensities. This calls for scaling up of the insecticide treated bed nets strategy especially in high altitude rural areas. In addition more concerted effort should be made towards the production of more effective transmission blocking vaccines.