An attempt at an epidemiological explanation - Epib.nl

General discussionPossible changes in the virulence of N. meningitidisFrom 1980 to 1990 the proportion of isolates cultured from the blood alone(blood-strains) increased from 8% to 21 %, which may indicate an increased virulenceof the causative meningococci (Chapter 2). A similar, but more pronounced increasehas been found in the percentage of notifications of meningococcal septicemia in theNetherlands, which increased from 19% in 1980 to 47% in 1990 (figures from theDepartment of the Chief Medical Officer of Health). Another indication for anincreased meningococcal virulence could be the case-fatality rate (CFR) of 7.7%during the period 1989-1990 (Chapter 5), which was higher than the CFR of 5.1 % inthe period 1959-1981. 12 This difference might be explained by the increase in theproportion of blood-strains during the 1980s, but after adjustment for this covariate adifference still is present. A more likely explanation for this difference is probably anascertainment bias. We have assessed the outcome of disease in a prospective survey,whereas the survey of the period 1959-1981 was retrospective, and could have failedto detect all deceased patients. This may have led to an underestimation of the CFRduring the period 1959-1981. 12In Chapter 5 it was pointed out that, after adjustment for confounding by hostfactors, the outcome of MD was not associated with specific meningococcal surfacecharacteristics. In addition, the increase of the proportion of blood-strains could notbe attributed to the occurrence of a specific (new) meningococcal phenotype. Thus,no relation could be found between the apparent increase in virulence and the meningococcalsurface antigens. it might well be possible that this increase is due toextraneous factors which are not related to the meningococcus, e.g. more bloodcultureswere carried out in the late 1980s than in the early 1980s. However, theexistence of another virulence factor, which is shared by meningococci of the variousphenotypes, can not be excluded. This factor might be the amount of endotoxinreleasefrom the meningococcus,13 which could be investigated by comparing isolatesof fatal and non-fatal cases, or those of septicemic and meningitic cases.Shift in the age-distributiOlt of patients with meningococcal diseaseFrom 1980 to 1990 a shift in the age-distribution of patients with MD fromyounger to older age-categories was noted (Chapter 2). Similar shifts have beenfound in the past in the Netherlands and also in other countries, and have beenattributed to coinciding changes in the distribution of meningococcal phenotypes. 4 14 15The association has been explained as follows: immunity to the meningococcus isacquired by (repeated) colonization and/or carriage of both pathogenic and nonpathogenicNeisseria spp." Children