An attempt at an epidemiological explanation - Epib.nl

General discussionexample of the above-mentioned course." Tbe gradual rise of MD in the 1980s inthe Netherlands contrasts sharply with the described steep rise of MD epidemics, andrenders the nomination t1epidemict1 for the current situation in the Netherlandsunlikely.In our opinion, another factor necessary for an exact definition of an epidemic ofMD pertains to the causative organism. N. meningitidis shows a marked heterogeneitywith regard to both the phenotype, which is determined by its surface structures, andtbe genotype.' 8 During an epidemic, one would expect to find an epidemic strain ofone single phenotype (or at the most a very limited number of different phenotypes)which belongs to a single genotype or a limited set of closely related genotypes. Apart(Tom B:4:P1.4, which was the most prevalent phenotype in the Netherlands in 1990(21 % of all cases) and belonged to a complex of closely related clones (lineage llI),'a variety of other phenotypes· and clones has been found, which gives little furthersupport for the label "epidemic" in the Netherlands in the late 1980s.In conclusion, the situation concerning MD in the Netherlands in the late 19805,with the gradual rise of the number of infections and a variety of meningococcalpbenotypes and genotypes, is not in accordance with tbe definitions of an epidemic,and can best be described as hyperendemic.Possible explanations for the increased incidenceExplanations for an increased incidence of an infectious disease in a particulargeographical area can be sought in (a combination of) the pathogen, the host, or theenvironment.With respect to the pathogen, a possible explanation for a rise of the incidencecould be the emergence of a virulent strain of N. meningitidis in a susceptiblepopulation. Such a strain could be imported from outside the country or result from amutation of a pre-existing strain. In both circumstances it is to be expected that thegenotype and phenotype of that particular strain are homogenous, at least immediatelyafter its appearance. Indeed, this has often been reported with regard to MD,including the 1966 epidemic in the Netherlands (strain B:2b:P1.2)' 10 As was poi ntedout by Caugant et aI., a new meningococcal clone complex (lineage Ill) appeared inthe Netherlands around 1980' Initially, this complex consisted of one single phenotype(B:4:P1.4), but later an increasing number of other pbenotypes was found in thiscomplex, mainly characterized by the subtype Pl.4 (Chapter 3). If it is assumed thatall Pl.4 isolates collected in the late 1980s belong to this new clone complex (Chapter3), approximately 30% of all cases of MD in 1990 in the Netherlands are due to tbeemergence of a new clone complex. The remaining part of the increase is due to avariety of other clones and/or phenotypes, suggesting that additional factors areresponsible for the increase.The susceptibility of tlle host is a crucial factor in contracting an infectious disease.It has been postulated that the waxing and waning of the specific immunity of thehost to the various antigens of the meningococcus in the general population isreflected in the frequently found cyclical occurrence of MD.I 6 11 The emergence in a87