4. PATHOGENESIS OF DISEASE

Humans are the only natural host of meningococcus. The transmission of N. meningitidis is directly from person to person by droplet spread or intimate contact with nasopharyngeal secretions. Nasopharyngeal carriage of meningococci is much more common than invasive meningococcal disease. Nasopharyngeal carriers rather than patients with meningococcal disease are generally the source of new infections.

Studies in the United States and the United Kingdom show that between 5-10% of the population carry N. meningitidis at any given time. Carriage rates increase from about 2% in children under five to 25% in the late teens. Carriage is increased in smokers, overcrowded households, new military recruits and in first year residents of university hostels. Asymptomatic carriage of meningococcus is an immunising event and systemic immunity (serum antibodies) develops about 14 days after acquisition of meningococci. Invasive disease develops in the minority of people who carry meningococcus and usually occurs in the first 3 to 5 days after acquisition of meningococci. In these individuals the organisms in the nasopharynx evade the immune system and this result in blood stream invasion and dissemination especially to the brain.

The incubation period is 3 - 4 days (range 2 to10).