The Epidemiology of Malaria Outbreaks

The Epidemiology of Malaria Outbreaks: What do we really know ?

• Key determinants of malaria endemicity/epidemics

• The Uganda malaria epidemic 1997/98

Determinants of Malaria

Parasite Vector

Host

Environment


Environment

ImmunityTransmission

• Competent vector species

• Suitable breeding sites

• Sufficient temperature• Humidity, soil moisture• Reservoir of parasites

Temporary (rain) A. gambiae s.l.

Permanent (rain, drought) A. funestus.

• Repeated exposure• Mature immune system

Anti-disease No syptoms

Anti-parasite Low Parasites

• Age• Pregnancy• Disease/Nutrition


ImmunityTransmission

Hypoendemic

Mesoendemic

Hyperendemic

Holoendemic

No Malaria

• In a stable malaria situation an equilibrium exists• This may, however, include strong seasonality


• Gradual, long-term change >> new equilibrium• Sudden, temporary change >> outbreak

• Any change in the determinants of malaria will lead to a disturbance of the equilibrium

• This may occur at any level of endemicity

Epidemic Outbreak

Children, pregnant women

Total population

• When does seasonal variation become an epidemic ?


• Since epidemic in the past have rarely been anticipated, usually only routine data from health services exist

• We have no clear understanding how the various clinical parameters (out-patient attendance, severe malaria cases, slide positivity rate) relate to each other and to the dynamic of transmission

• This knowledge is important for early detection as well as control

Documentation of Epidemics

The Uganda Malaria Epidemic 1997/98

• Recognition only through press/politicians• Retrospective analysis• El Niño associated


Monthly rainfall around epidemic years, Kabale District


Clinical malaria cases in health facilities Kabale District Endemo-epidemic channel and 1998 data


Relationship between altitude and malaria endemicity

Village parasite rate Zalueta 1959 Kigezi Kilian 1994-1997 Kabarole

Probability of being parasitemic, Kabarole, 8,000 children 0-10 years

The Uganda Malaria Epidemic 1997/98Intensity of epidemic at decreasing levels of altitude

• Malaria outbreaks/epidemics are part of the rather complex epidemiology of malaria

• There is no sharp distinction between endemic and epidemic prone situations

• Which factors trigger epidemics in which way is not yet fully understood

The Epidemiology of Malaria Outbreaks:

J. Najera, P. Langi, A. Talisuna, J. Cox

The Epidemiology of Malaria Outbreaks

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