Information on the incidence of disease is required to determine the needs for treatment of malaria. Data on treatment needs can be compared to existing levels of service provision to identify underserved populations and, in situations of resource constraint, to target interventions to high priority areas. Data on changes in disease incidence can help to judge the success of program implementation, and help to determine whether programs are performing as expected or whether adjustments in the scale or in the blend of strategies are required. In highly endemic settings in Africa incidence rates are typically as high as 1.5 cases per person per year for children under 5 years old, or 1 case in every 10 adults per year. Outside of Africa incidence rates are generally less than 1 case per 100 or 1 case per 1,000 population per year.
Information on malaria death rates can also help to judge the success of program implementation, and may point to failures of programs in terms of prevention of malaria or access to effective treatment. In many countries in Africa one child out of seven will die before their fifth birthday and one in five of these deaths will be due to malaria.
Malaria is not only important in its own right but the disease can contribute to deaths arising from other conditions. In addition, malaria imposes an economic burden on families particularly those who are least able to pay for prevention and treatment and most affected by loss of income due to the disease. The disease also represents a financial burden to malaria-endemic countries that must use scarce resources to fund bednets, insecticides and drugs in an effort to control the disease.
Estimates of incidence and death rates are critically dependent on the information provided to WHO by NMCPs, and on the data available in published household surveys. Estimates of the number of malaria cases are particularly sensitive to the completeness of health facility reporting. If Ministries of Health keep accurate records of the number of surveillance reports received and expected from health facilities then adjustments can be made for missing reports. However, if this information is not rigorously recorded, and the stated reporting completeness differs from reality then the number of malaria cases will be misestimated.
Where estimates of incidence are derived from an estimate of the number of people living at different levels of risk, uncertainty arises because: (i) the delimitation of only two risk categories (high and low) does not provide for a fine categorization of malaria risk (ii) the longitudinal studies used to determine typical incidence or death rates were not designed to be representative of the levels of endemicity they purport to describe, are small in number, and show a wide variation in measured case incidence, with few, if any, studies in urban areas and low-risk rural areas which required rates to be inferred; (iii) the adjustments made to take into account the effects of interventions on case incidence are based on a relatively small number of clinical trials.
In areas of high transmission, parasite prevalence measured through nationally representative household surveys can provide an indication of the risk of infection and trends in disease burden. However, this indicator needs to be treated with caution because many infections may be asymptomatic and not reflect a diseased state, and the indicator does not always reflect changes over time since at high levels of transmission intensity, moderate reductions in inoculation rates do not necessarily translate to reductions in prevalence. Parasite prevalence is less relevant in areas of low transmission intensity where parasite prevalence rates are less than 5% and more difficult to measure precisely.
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