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General Distribution of Malaria

General Distribution of Malaria

Malaria is presently endemic in a broad band around the equator in areas of America, many parts of Asia, and much of Africa. However, it is in Sub-Saharan Africa where 85-90% of malaria fatalities occur (Layne, 2007).

The geographic distribution of malaria within large regions is complex, and malaria-afflicted and malaria-free areas are often close to each other (Greenwood and Mutabingwa, 2002). Out of the four Plasmodium species P.vivax has the most extensive range and is the dominant species in temperate zones, extending over a wide range from 64oN to 30oS. P. malariae, although widespread is comparatively rare, has a localized distribution and much lower incidence
than P vivax and P.falciparum. In certain areas in the sub-tropical and temperate regions, P. malaria may be the predominant species (Aguwa, 1996).

In most tropical and sub-tropical regions, P.falciparum is the dominant species, but is seldom encountered in the northern parts of temperate zones. It has been found in the extreme south of the United States and is also present in southern Europe. Its distribution in temperate zones is governed by the favorable temperature for the completion of its sporogonic cycle in the mosquito. Its optimal developmental temperature of 30o C only occurs for a short period during the summer in most areas in temperate regions. The distribution of P.ovale has not been fully determined. Its incidence is low and its distribution is localized. It has been reported in many parts of Africa, Asia, the Philippines, Central America and the West coast of South America. It is usually found coexisting with one or more of the other species (Aguwa, 1996).
In drier areas, outbreaks of malaria can be predicted with reasonable accuracy by mapping rainfall (Grover-Kopec et al., 2005). Malaria is more common in rural areas than in the cities. For example, the cities of Vietnam, Laos and Cambodia are essentially malaria-free, but the disease is present in many rural regions of these countries. Whereas in Africa, malaria is present in both rural and urban areas, the risk is lower in the larger cities (Kieser et al., 2004). In a work done by Cerutti et al., (2007) to determine the frequency of positive cases diagnosed as P.vivax malaria from April 2001 to March 2004 in Brazil, it was reported that 2/3 of them live in rural areas (64.6%). Those who lived near towns or in the capital admitted having visited the rural area for at least once in the preceding 30 days (Cerutti et al., 2007)

The occupational distribution of malaria cases involved farmers most and students next. Cerutti junior observed that most of the students help their families on the land both during seeding and harvest, thus increasing the figure for occupational contact with rural areas to 59.6% (Cerutti junior et al., 2007)

  Distribution with respect to time and season indicated that the prevalence of malaria parasite showed a bi-modal distribution with two peaks at the onset and late parts of the rainy season in Nigeria. The rainy season which occurs from April to October each year had an average prevalence of 55% while the dry season recorded an average prevalence of 26.92% (Okocha et al., 2005). The prevalence shows a sudden rise with the onset of rains in March and April. As the rains become heavier and more frequent, the flood water probably flush out the vectors in their breeding sites leading to drops in the parasite population. At the later part of the rainy season in July to September, when the rains become less frequent and with less volume of runoff water and possibly with less disruption of the breeding of the mosquito vectors, a steep rise in parasite rate occurs, before falling steeply thereafter as the dry season sets in (Okocha et al., 2005).

In the sex distribution, males were generally found to be more infected with malaria parasites than females (Bonilla et al., 1993). This trend was probably due to higher exposure of males to the vector because of nocturnal and occupational activities. However, in a study carried out in the Caribbean and Ghana, more females were infected (Vlassoff et al., 1994). This finding contradicts the general reports of researchers on the sex distribution of malaria parasite infection.



In age distribution of malaria, Uneke (2006) noted that the prevalence of malaria decreased with age in the South-Eastern Nigeria. Although the reason for this was not apparently clear, it is however established that in malaria endemic areas of sub Saharan Africa, the younger members of the community are more disposed to the infection than older individuals (WHO, 2003). The reason for this is probably due to their greater involvement into nocturnal activities that enhance contact with mosquitoes, which may be social or occupational in nature. Researchers have also shown that children below the age of five years are mostly at risk of being infected with malaria parasite. This may probably be due to their poorly developed immune system. In addition, the use of insecticide treated bed nets is a very rare occurrence among young men in their 20s in South-Eastern Nigeria and this trend may have influenced the prevalence of the infection among this age group. (Uneke et al., 2006)

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