40 percent of the world's population live in areas with malaria risk (see Fig1). There are an estimated 300-500 million cases of malaria each year, resulting in up to 2.7 million deaths. Most of these deaths are due to infection with Plasmodium falciparum and occur among children and pregnant women in the developing world. In Africa, every 12 seconds a child dies from malaria.
Fig1. Malaria endemic areas of the world, 2001
Plasmodium vivax has the widest geographical range; it is prevalent in many temperate zones, but also in the subtropics and tropics.
Plasmodium falciparum is the commonest species throughout the tropics and subtropics.
Plasmodium malariae is patchily present in the same areas as P.falciparum, but much less common.
Plasmodium ovale is found mainly in tropical Africa, but also occasionally in West Pacific.
In order to better understand the malaria epidemiology, a number of epidemiological terms and concepts need to be clarified. Table1 summarizes some key terms used in malaria epidemiology.
Table1. Key terms used in malaria epidemiology
||- Malaria is natural to an area or country
||- Locally contracted malaria
||- Malaria acquired outside the specified area in which it was found
||- Secondary case contracted locally, but derived from an imported case
||- Accidental infection by blood transfusion, needles, organ transplant etc.
|Anophelism without malaria
||- A situation when anopheles mosquito is present in the area but there is no case of human malaria (e.g. malaria-free Soviet Armenia in 1963-1994)
Malaria endemicity indicates the amount of malaria in a given region, whereas malaria epidemic refers to the periodic or sharp increase in that amount. The presence of peripheral parasitemia has been used to determine the degree of malaria endemicity in certain community. Such information is best collected through random community-based samples. In 1950s WHO suggested using the spleen rates (percent of children with enlarged spleen) as a proxy of malaria endemicity. Based on both the parasite and spleen rates, malaria endemicity has been classified as hypoendemic, mesoendemic, hyperendemic and holoendemic (see Table2).
Table2. Classification of endemicity
||<= 10% of children aged 2-9 years
||<= 10% of children aged 2-9 years but may be higher for part of the year
||Areas where there is a little transmission and the effects, during the average year, upon the general population are unimportant
||11-50% of children aged 2-9 years
||11-50% of children aged 2-9 years
||Typically found among rural communities in subtropical zones where wide geographical variations in transmission risk exist
||Constantly > 50% in children aged 2-9 years; also high in adults (>25%)
||Constantly > 50% among children aged 2-9
||Areas where transmission is intense but seasonal; immunity is insufficient in all age groups
||Constantly > 75% in children aged 2-9 years, but low in adults
||Constantly > 75% among infants aged 0-11 months
||Intense transmission resulting in a considerable degree of immunity outside early childhood
Depending of the intensity of transmission, malaria can be stable or unstable, reflecting differing epidemiological scenarios.
Stable malaria implies the overall balanced presence of malaria in certain community with persistently high prevalence of infection, insensitive to environmental changes. Under stable endemic conditions, variation of malaria transmission from year to year is minimal, although seasonal fluctuations may take place.
Unstable malaria implies a great variability of malaria rates in space and time. The background immunity in the community is low, and therefore there is a high risk of malaria epidemic.
Periods when malaria incidence is low alternate irregularly with times of high incidence.
Unstable malaria is very specific for P.vivax, although sharp outbreaks may also occur with P.falciparum.
Malaria transmission may be influenced by various factors including climate, local ecology, existing control measures etc. Temperature affects development of both the vector and the parasite. Temperature, required for parasite sporogony inside mosquito, predicts how soon the mosquito becomes infective after taking a blood meal. It varies in between malaria species. Optimum temperature for sporogony is between 250C and 300C. It stops below 160C. At 250C duration of sporogonic development is 10 days for P.vivax, 12 days for P.falciparum, 16 days for P.ovale, and 28 days for P.malariae (see Fig2).
Altitude is correlated with the temperature and thus also affects malaria transmission.
The amount of precipitation during the transmission season is another potential environmental factor that together with the temperature influences malaria transmission.
Fig2. Duration of sporogonic development of plasmodium in Anopheles mosquito in relation to temperature
Man made environmental changes such as dams, water reservoirs, irrigation systems, deforestation etc. may greatly increase the breeding sites of mosquito, thereby triggering epidemic. Another factors affecting malaria transmission include population migration, urbanization and other socio-economic phenomena along with personal mosquito protection, residual spraying, use of antimalarials etc.
In malaria epidemic 3 borderline periods can be distinguished:
- pre-epidemic increase of transmission
- higher gametocyte rate in the population
- greater density and infectivity of Anopheles mosquito
- epidemic wave
- sharp rise of disease incidence
- post-epidemic period
- incidence decrease to usual level
The rise of the epidemic wave is usually faster in P.vivax outbreaks than in those with P.falciparum, although the severity of the latter is much greater.
One of the greatest malaria epidemics of the last century struck the Former USSR after the World War I with more than 10 million cases in 1923-26 and at least 60,000 deaths.
Due to effective disease eradication campaigns, malaria, malaria was nearly forgotten in European Region by the 1980s. Since the early 1990s, however, the epidemiologic situation of malaria has deteriorated considerably due to political instability and socio-economic burden in many countries of the Former USSR. Rising number of autochthonous cases were reported in Tajikistan, Azerbaijan, Armenia, Georgia, Uzbekistan etc.
Meantime, countries of the European Union not affected by malaria still report more than 12,000 malaria cases annually among travelers. Similarly, in the US, Centers for Disease Control and Prevention (CDC) report about 1,500 annual cases of imported malaria. It has to be mentioned that sometimes infected vectors can be transported from the malaria endemic areas and become the cause of so called "airport", "baggage" or "taxi-rank" malaria infections.
The issue of imported malaria has been recently compounded by the emergence and spread of multidrug-resistance.
© 2002. Malaria in Armenia.
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