Epidemiology of Malaria in Endemic Areas

Malaria infection is still to be considered a major public health problem in those 106 countries where the risk of contracting the infection with one or more of the Plasmodium species exists. According to estimates from the World Health Organization, over 200 million cases and about 655.000 deaths have occurred in 2010. Estimating the real health and social burden of the disease is a difficult task, because many of the malaria endemic countries have limited diagnostic resources, especially in rural settings where conditions with similar clinical picture may coexist in the same geographical areas. Moreover, asymptomatic parasitaemia may occur in high transmission areas after childhood, when anti-malaria semi-immunity occurs. Malaria endemicity and control activities are very complex issues, that are influenced by factors related to the host, to the parasite, to the vector, to the environment and to the health system capacity to fully implement available anti-malaria weapons such as rapid diagnostic tests, artemisinin-based combination treatment, impregnated bed-nets and insecticide residual spraying while waiting for an effective vaccine to be made available.

regions. 1,2 However, between 2005 and 2010 malaria cases decreased from 244 million to 216 million; moreover, malaria mortality rates showed a global reduction of 26% between 2000 and 2010.
Malaria in humans is caused by 5 Plasmodium parasites: Plasmodium falciparum, P. vivax, P. malariae, P. ovale and P. knowlesi. The current distribution of human-pathogenic Plasmodium species shows preponderance of P. falciparum in tropical Africa, while P. vivax prevails over P. falciparum in South America. Both P. falciparum and P. vivax are prevalent in south-eastern Asia and western Pacific. Although P. malariae may occur in all malarious areas, its prevalence is generally low. In tropical Africa, P. falciparum and P. malariae co-infection is sometimes encountered. P. ovale is widespread principally in tropical Africa whereas P. knowlesi infection occurs only in certain forested areas of South-East Asia.
Malaria burden is hard to estimate, particularly in low income countries where data collection and reporting quality is poor. Incomplete and discontinuous reports from single health facilities may alter final global malaria prevalence. Malaria cases are often under-diagnosed in hyper endemic countries, where mild symptoms of chronic malaria my possibly lead to misdiagnosis. On the contrary, over-diagnosis may also occur. In fact, not all reported malaria cases are confirmed by microscopy or others assay, such as rapid diagnostic tests (RDTs). Furthermore, in hyper endemic areas febrile illnesses from different causes might be misdiagnosed with malaria. 3 Anyway, WHO guidelines recommends that microscopy or RDTs should be used to confirm all malaria cases. Another issue is the lack of population denominator that makes the real incidence of malaria difficult to assess. Data emerging from WHO reports just estimate malaria incidence and mortality, reporting malarial cases and malarial death from the different WHO regions, collected by Ministries of Health of different countries. These data do not reflect the real incidence in the general population. Nevertheless, they are good indicators to assess malarial control programmes and to estimate the impact of malaria infection in health systems.

Malariometry
The term endemicity is a proxy to WHO  Malaria endemicity is a very complex issue, that is influenced by factors related to the man-host interactions (agricultural activities, nocturnal activities, migration movements, wars, limited resources), to the parasite (different species, sporogonic cycle length, drug susceptibility), to the vector (density, larvae breeding sites, temperature, receptivity, feeding pattern, longevity, insecticide susceptibility) and to the environment (physical -biological -socio-economic). The detailed analysis of all such variables is beyond the scope of this brief article that will only report available data, referring the interested reader to more exhaustive and valuable treatises on the topic.
Moreover, malaria incidence may fluctuate according to seasonality. Figure 2 shows the months of start and end of malaria transmission over the year in the African continent.
Different methods to classify malaria endemicity in a population exist. These methods includes (i) proportion of individuals in a population with a palpable enlargement of spleen (spleen rate [SR]), (ii) proportion of individuals in a population with a laboratory-confirmed parasite infection (parasite rate [PR]), (iii) number of infective bites per person (entomological inoculation rate [EIR]) and (iv) number of microscopically confirmed malaria cases detected during one year per unit population (annual parasite incidence [API]). 5 Proportion of individuals with splenomegaly (SR) in a given population was the first method used to assess malaria endemicity during a malariometric survey in 1848 in India, where spleen dimension was assessed in selected population age groups. Thus, malariometry attention was focused on clinical manifestations of malaria. On the basis of splenomegaly prevalence rates in children from 2 to 9 years old, 4 different endemicity areas can be distinguished: holo-endemic areas, where proportion of people with splenomegaly is above 75%; hyperendemic areas, where splenomegaly prevalence is between 51 and 75%; meso-endemic areas, with prevalence between 50 and 11%; hypo-endemic areas, where prevalence is below 11%. 5 Parasite rate (PR) assesses the proportion of individuals with microscopically confirmed presence of asexual parasites in peripheral blood. It's a technique that requires expert laboratory technicians and suffers of malaria seasonal variation.
Spleen and parasite rate are actually less used, whereas entomological inoculation rate (EIR) and annual parasite incidence (API) are utilized to prepare epidemiologic malaria maps that show malaria distribution in the world. Where data are unavailable, a model is required to predict malaria endemicity. [6][7][8][9][10] Many recent studies investigated a predictive framework known as model-based geostatistics (MBG) to asses malaria endemicity [11][12][13][14][15] and the prevalence of other vector-borne and intermediate host borne diseases.
Maps showing the global distribution of P. falciparum and P. vivax has recently been published by Malaria Atlas Project. These maps provide a geographical framework for monitoring malaria incidence and evaluation of impact on malaria control worldwide (Figures 3a and 3b).
P. falciparum malaria endemicity has been mapped considering national malaria reports, medical intelligence and biological rules of transmission, such as temperature and aridity, important for Anopheles vectors spreading 16,17 . Thus, in 2007, the world was stratified into three spatial representation: (i) areas without P. falciparum malaria risk, (ii) unstable risk areas (P. falciparum annual parasite incidence [PfAPI]: < 0.1 per 1.000 people per annum [pa]) and (iii) stable risk areas (PfAPI > 0.1 per 1.000 people  pa). 18 The global area at risk of stable P. falciparum malaria was quantified in 29.7 million km 2 , distributed into Africa (18.2 million km 2 , 61.1%), Americas (6.0 million km 2 , 20.3%) and Central and South East Asia regions (5.5 million km 2 , 18.6%). Of the 2.37 billion people exposed to P. falciparum transmission worldwide, 0.98 billion live in unstable risk areas, 16,17 whereas 1.383 billion live in stable risk areas, distributed into Africa (0.657 billion, 47.5%), Americas (0.041 billion, 2.9%) and Central and South East Asia (0.686 billion, 49.6%). Children are the most represented category, accounting for 32% of the population at risk in Americas and in Central and South East Asia. In Africa this percentage rise up to 43%.
P. vivax is transmitted in 95 tropical, subtropical and temperate countries. 19 People living at risk of P. vivax malaria infection are 2.85 billion, 91% living in Central and South East Asia region, 5.5% in America and 3,4% in Africa. As many as 57.1% of people exposed to P. vivax infection lives in unstable malaria areas.
Stable -unstable classification is another way to determine malaria endemicity. Macdonald defined malaria stability on the ground of the number of mosquitoes lifetime bites in the human host. 20 This vector-based index differentiated stable and unstable malaria. Vector-based classification is less used because of entomological-based metrics complexity, ethical concerns related to exposing human beings to malaria infection and measurement error issues. 5 Endemic Areas Distribution. Different malaria endemic areas have different epidemiological situations and also the feasible targets may differ. According to WHO, the following terminology should be adopted when referring to malaria endemic status: Malaria control: reducing the malaria disease burden to a level at which it is no longer a public health problem.
Malaria elimination: the interruption of local mosquito-borne malaria transmission; reduction to zero of the incidence of infection caused by human malaria parasites in a defined geographical area as a result of deliberate efforts; continued measures to prevent reestablishment of transmission are required.
Malaria eradication: permanent reduction to zero of the worldwide incidence of infection caused by a particular malaria parasite species. Intervention measures are no longer needed once eradication has been achieved.
Hereunder we summarize data from WHO Reports, showing malaria trends in different WHO Regions. Trends in malarial cases and deaths reflect control programmes, such as distribution of insecticide-treated nets (ITN), long lasting insecticidal nets (LLIN), use of indoor residual spraying (IRS) and artemisinin combination treatment (ACT).
On the ground of slide positivity rate (SPR) and of the population at risk of malaria, WHO distinguishes areas with advance malaria control activities in (I) preelimination phase, (II) elimination phase, (III) prevention of reintroduction and (IV) malaria-free stages as shown in table 2. 1 Most malaria cases and deaths occur in the African Region. As a consequence of implementation programs, high burden countries of African Region, such as Madagascar, Sao Tome and Principe, Eritrea, Rwanda and Zambia, showed a decrease in malaria cases up to 50% between 2000 and 2009. 21 Rwanda showed a decrease by 74% of confirmed malarial cases between 2005 and 2010 and slide positivity rate decreased from 35% to 9%. Moreover, number of malaria hospital admissions and malaria deaths showed a decrease of 65% and 55% respectively. Zanzibar, belonging to United Republic of Tanzania, showed a dramatic decrease of malaria admissions and deaths due not only to the efficacy of control strategies, but also to favourable geographic position. In lowtransmission countries of African Region control strategies have also been performed. Thanks to these strategies, Algeria is in the malaria elimination phase, Capo Verde in pre-elimination phase. 1 In 15 countries of the WHO Region of the Americas, where P. vivax is the most represented species, reductions of more than 50% in the number of the reported cases were observed. During 2010, malaria transmission occurred in 21 countries, of which 17 are in the control stage and 4 are in the preelimination stage. Bahamas and Jamaica are in the prevention of reintroduction phase. In Ecuador, malaria cases dropped from 105.000 in 2000 to 4.120 in 2009, a reduction of 96% due to IRS, LLINs distribution, strengthening of malaria diagnosis and treatment and also due to Global Found, UNICEF, USAID and government funds invested in malaria control. 1 In Plasmodium Species Distribution. Plasmodium species are differently distributed in the world. Prevalence of malaria cases and deaths differs during different seasons, as mentioned before. Prevalence data must be related to season and to endemicity of each countries.
The following paragraphs report available epidemiological data obtained in different geographical areas for the different Plasmodia species. While recognizing the value of the robust epidemiological research reviewed, we would like to underline that comparison among studies is virtually impossible because of lack of homogeneity in diagnostic methods, seasonality and methodological approach. The interested reader is then invited to refer to the selected articles for further information and details.  32 In South America P. vivax is the predominant species, followed by P. falciparum (25.7%). 33 Most of the malaria cases occur in Brazil; the others are distributed in 20 other countries of Central and South America. 33

b) Plasmodium vivax
In Central and Western Africa P. vivax infection is rare because of the high prevalence of the red blood cells Duffy negative phenotype in the population, interfering with P. vivax merozoite entry into the red blood cells. A large study carried out in nine African countries failed to detect P. vivax species. 23 Despite these studies, however, there are some evidence of P. vivax transmission in West and Central Africa. In Congo, specific P. vivax antibodies were researched in 409 samples from patients coming from an health center located on the west coast, where Duffy antigen is expected to be > 95%. Out of the 409 samples, 55 (13%) tested positive for specific P. vivax antibodies. 34 Another study from Kenya demonstrated the presence of P. vivax among mosquitoes; in addition, P. vivax DNA was amplified and sequenced in blood of two Duffy negative children. 35 In eastern and southern Africa only 5% of malaria infections are attributable to P. vivax. 36 In Asia, P. vivax and P. falciparum are the predominant species. 37  The real burden of P. ovale malaria is difficult to asses because its diagnosis is difficult. Plasmodium ovale may be encountered in sub-Saharan Africa and in Asia. 46 A recent study from Mozambique tested malaria prevalence among febrile patients: only 2 of 111 malaria positive patients presented P. ovale mono-infection, while 4 P. ovale and P. falciparum mixed infections were also detected. 47 In Congo, a cross-sectional, population-based cluster household survey of adults aged 15-59 years demonstrated that P. ovale parasitemia was rare and its prevalence in mono-infection was only 0,1%. 26 In a recent multicenter study, blood samples were collected from the indigenous population of nine African countries and malaria parasites were searched by PCR method. Of 1.737 samples, 67 were positive for P. ovale: 12 single infections, 51 mixed with P. falciparum and 4 triple infections with P. falciparum and P. malariae. When samples from Rwanda, Mozambique, Angola and Sao Tome were excluded, P. ovale infection represented 3,9% of all malaria cases. 23 Another study performed in Congo-Brazzaville, Uganda and Equatorial Guinea, concluded that two P. ovale species, P. o. curtisi and P. o. wallikeri, are both widespread in Africa; in Uganda and Equatorial Guinea the prevalence of P. ovale spp. in population-based samples was found to be between 1% and 6%. 48 There were positive for P. ovale. 29 Nested PCR detected P. ovale parasites in 1,3% of blood samples collected from 1.356 inhabitants of eight villages of Rattanakiri Province (Cambodia). 30 In a study performed in Myanmar, P. ovale was detected by PCR technique in 4,9% of malaria positive samples; most of cases were co-infections with P. falciparum, P. vivax and/or P. malariae. 49 (11,5%) were P. malariae positive as tested by microscopy. 25 During the rainy season, blood samples were collected from resident people of four village in Mulanda, Uganda. Of 709 malaria positive samples, 6% were positive for P. malariae. 24 In the state of Orissa, India, the prevalence of P. malariae detected by PCR method was 44,6% during the peak season of malaria incidence. 28 A recent case report was published demonstrating the presence of P. malariae in Bangladesh. 56 In Papua New Guinea, prevalence of P. malariae was detected by nested PCR, by quantitative PCR and by PCR-ligase detection reaction-fluorescent microsphere assay (PCR-LDR-FMA); the results were 3,3%, 4,7% and 7,7% respectively. 57 Plasmodium malariae infection was confirmed in Thailand 58 and in Yemen. 42 68 Seventy Anopheles species are able to transmit Plasmodium parasite to human hosts. 69 Anopheles mosquitoes breed in water and each species has its own breeding preference. Transmission is more intense in places where mosquito lifespan is longer (parasite has time to complete its development inside the mosquito) and where anthropophilic mosquitoes prevail. Forty-one of the 512 Anopheles species are defined by experts "Dominant Vector Species" (DVS). DVS are the most important malarial vector worldwide, providing the majority of human malaria cases. Characteristics of dominant vector species are their propensity for humans feeding, longevity, abundance and elevate vectorial capacity. 70 Many studies were recently conducted to define Anopheles distribution in Africa, 71,72 Americas, 73,74 Europe, 75 Central and South East Asia 76 and worldwide. 77 Africa has the most effective and efficient DVS of human malaria, the Anopheles gambiae complex; thus some areas account the highest entomological inoculation rates and the highest malaria prevalence worldwide. 17,78,79 There are 4 principal species belonging to An. gambiae complex: An. gambiae, An. arabiensis, An. merus and An. melas. Other three highly anthropophilic DVS are spread in African region: An. funestus, An. moucheti and An. nili. 80 Despite European and the Middle Eastern regions are low malaria transmission areas, existence of DVS may play a potential role in re-introduction of malaria. 78,81 An. atroparvus is mostly diffused in these regions. 80 Nine DVS were found in the Americas, where An. darlingi is considered one of the most efficient malaria vectors. 82 In Asian-Pacific region 19 DVS were found; some of them are predominant in the Arabian Peninsula (e.g. An. stephensi and the An. culicifacies complex), others in the Indian subcontinent, China and Korea (e.g. An. lesteri), in the Solomon Islands and Vanuatu (e.g. the An. farauti complex) and finally in Queensland and the Northern Territory of Australia (e.g. the An. farauti complex). Myanmar appeared to contain the greatest number of DVS, of wich An. aconitus, An. annularis, An. barbirostris complex, An. culicifacies complex, An. dirus complex, An. maculatus group, An. minimus complex, An. sinensis complex, An. stephensi, An. subpictus complex and, in some coastal site, An. sundaicus complex. 83 Environmental factors play an important role in vector distribution and malaria biodiversity. Climate seasonality, rainfall patterns, temperature, humidity, presence of vegetation and surface water all are directly related to the malaria transmission cycle. In addition, human activities such as agriculture, irrigation, deforestation, urbanization, population movements, dam/road constructions and wars are also connected to transmission levels and malaria epidemiology. 84

Conclusion.
Malaria is still considered a global health problem and a major killer. Morbidity and mortality burden of malaria could be reduced strengthening prevention, improving malaria diagnosis, using correct therapies based on artemisinin combination and adopting strategies aimed at preventing drug resistances.
Real malaria incidence is difficult to obtain. However, it is possible to make reliable estimates thanks to the data supplied by Ministries of Health of different countries and to accurate prevalence studies.
Determination of real incidence of malaria and determination of the real Plasmodium species distribution are two different issues that could help expert in eradication of malaria, the real and unique goal in the fight against malaria. .