5. EPIDEMIOLOGY OF MENINGOCOCCAL DISEASE

5.1 Global picture

Meningococcal disease occurs sporadically in small clusters throughout the world. In temperate climates there is a seasonal pattern to disease with an increased incidence in winter and early spring. Changes in patterns of disease and serogroups are characteristic of meningococcus and highlight the importance of ongoing surveillance. Serogroups B and C account for a large majority of cases in Europe and the Americas. Serogroup C is also responsible for large outbreaks in Africa, South America and Asia. In the African meningitis belt serogroup A predominates with smaller epidemics caused by serogroup C and recently with serogroup W135. Serogroup A is usually the cause of meningococcal disease in Asia. In South Africa the pattern of meningococcal disease is characterised by sporadic cases throughout the year with occasional small clusters and a definite seasonal increase in winter and early spring.

5.2 The African Meningitis Belt

The highest burden of meningococcal disease occurs in the “Meningitis Belt”, an area stretching from Senegal in the west to Ethiopia in the east (Figure 1). This region has an estimated total population of 300 million people and is characterized by particular climatic and demographic conditions. During the dry season (December to June) dust storms increase the risk of upper respiratory tract viral infections. In addition, large numbers of people travel back and forth to the Hajj and to regional markets, so that crowding is exacerbated. Serogroup A diseases predominates and seasonal incidence rates vary between 1 and 20/ 100 000 annually. Every 8 to 12 years, with the waning of herd immunity, attack rates increase to 100 to 800 per 100 000 population with some communities reporting rates as high as 1000 per 100 000. The spread of a new epidemic-prone strain of serogroup W135 has been linked with the Hajj pilgrimage, causing high morbidity and mortality, particularly in West Africa. Following these Hajj- related outbreaks, since 2000, vaccination with a quadrivalent meningococcal vaccine has become mandatory for travellers to Saudi Arabia.

During endemic periods the highest attack rates are observed in young children, while during epidemics, older children, teenagers and young adults are also affected. In 1996, the meningitis belt experienced the largest recorded outbreak of epidemic meningitis in history, with over 250 000 cases and 25 000 deaths reported. Between that crisis and 2004, over 223 000 new cases of meningococcal meningitis were reported to the World Health Organization. In 2002, the outbreaks occurring in Burkina Faso, Ethiopia and Niger accounted for about 65% of the total cases reported in the African continent.

5.3 Meningococcal Disease in South Africa

It is important to recognise the difference between the epidemics that occur in the “meningitis belt” and the sporadic seasonal increase in cases seen in South Africa. Increases in sporadic cases and outbreaks of meningococcal disease tend to occur in late winter and early spring in South Africa and outbreaks may especially occur in mines, correctional and detention facilities, academic institutions, and displaced communities. The total number of cases notified in South Africa has decreased steadily from around 2000 cases in 1972 to less than 500 cases in 2005. Under-notification after laboratory confirmation is the key factor.

The National Institute for Communicable Diseases (NICD) data on laboratory-confirmed cases indicate high incidences in the Gauteng and the Western Cape provinces. The incidence rates as reported are highest in the less than five-year-old age group. In the Western Cape serogroup B tends to be the most common serogroup. Outbreaks have been linked to the mining areas of Gauteng and North West provinces with serogroup A, and to a lesser extent, serogroup C predominating. As identified by the Respiratory and Meningeal Pathogens Reference Unit (RMPRU) of the National Institute for Communicable Diseases (NICD), since 2003, an increase in the number of cases of serogroup W135 has been reported in Gauteng province. This has been associated with a decrease in serogroup A disease.

5.4 Carriage of Meningococci

About 5 to 10% of people carry meningococci in their nasopharynx, very few will become ill due to the organism to some extent depending on risk factors mentioned above. Transmission of meningococci is higher in closed populations such educational institutions, prisons, army camps and is facilitated by climatic and living conditions such as winter, crowding and poor ventilation. In households and closed populations the carriage rate is significantly higher (20-70%). Carriage is more often in adolescents and young adults, lasts about 3 to 4 months and results in an immunological response and generates herd immunity.