Τετάρτη, 16 Μαρτίου 2016

Periodic waves of meningitis epidemics occur across sub-Saharan Africa, wreaking havoc in the region


meningvax.org
 
Studies indicate that epidemic meningitis has been present in Africa for about 100 years. The disease is most prevalent in the sub-Saharan meningitis belt, an area that stretches from Senegal and the Gambia in the West to Ethiopia in the East and has an at-risk population of about 430 million. Epidemics occur in the dry season (December to June), and an epidemic wave can last two to three years, dying out during the intervening rainy seasons.


The African meningitis belt. Source: Control of epidemic meningococcal disease, WHO practical guidelines, World Health Organization, 1998, 2nd edition, WHO/EMC/BAC/98.3

Size of epidemics

 

The size of these epidemics can be enormous and place an immediate and great burden on the health systems of meningitis belt countries. In major African epidemics, the attack rate ranges from 100 to 800 per 100,000 population, but individual communities have reported rates as high as 1/100. 
Over one million cases of meningitis have been reported in Africa since 1988. In 1996–1997, the largest epidemic wave ever recorded in history swept across Africa, causing over 250,000 cases and 25,000 deaths. The true disease burden is likely to be higher than statistics suggest because routine reporting systems break down during epidemics. In addition, many people die before reaching a health center and thus remain unrecorded in official statistics.

Epidemic cycles

Since the 1940s, epidemic cycles have been detected every 8 to 12 years, but two troubling phenomena have been observed since the early 1980s: the intervals between epidemics have become shorter and more irregular, and the meningitis belt seems to be extending further south, touching regions that had been spared until now, such as Angola, Burundi, the Democratic Republic of the Congo, Rwanda (Great Lakes region), and Zambia. At this point, it is not possible to tell with certainty if these changes are real or if they result from enhanced disease surveillance in the region.
Although the pattern of these epidemic cycles is not entirely understood, several risk factors have been associated with the development of epidemics in the meningitis belt. They include:
  • Medical conditions: immunological susceptibility of the population.
  • Demographic conditions: travel and large population displacements due to pilgrimages and traditional markets at regional level.
  • Socioeconomic conditions: overcrowding and poor living conditions.
  • Climatic conditions: drought and dust storms.
  • Concurrent infections: acute respiratory infections.

Bacteria groups associated with epidemics

Group A meningococci have been the main cause of meningitis epidemics in Africa and account for about 80 to 85 percent of all cases. In 2002 there was an exceptional major outbreak of group W135 meningococcal meningitis in Burkina Faso. Since then, enhanced surveillance activities show a dramatic fall in cases of W135 disease, isolated clusters of group X and group C cases, and a major return of epidemic disease due to group A throughout the belt.
The most recent large-scale meningitis epidemic in the African meningitis belt occurred in 2009, when more than 80,000 cases were reported. The number of meningitis cases reported during the 2009 epidemic season was the largest since 1996, and laboratory testing showed that most cases were due to group A meningococci.
Detailed meningitis surveillance reports are compiled on a regular basis.

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