An attempt at an epidemiological explanation - Epib.nl

Chapter 1by electrophoretic typing." RFLP typing has not been used for genotyping ofmeningococcal isolates in the Netherlands.Phenotypic similarity does not always imply genotypic similarity, and isolates thatare genetically closely related may be of different phenotypes .' ~4l A newly-appearingmeningococcal strain in a certain geographical area, however, is often homogenouswith regard to both the genotype and phenotype during the first few years after itsappearance.41 43Meningococcal diseaseEpidemiologyThe natural habitat of the meningococcus is the human nasopharynx.' Thebacterium is transmitted from person to person hy small droplets, and colonizationusually leads to an asymptomatic carrier state, and occasionally to a clinical syndromeof meningitis or septicemia. l 24445 Asymptomatic carriers are the main source ofinfection,14S Carrier rates in the open population range from 5% to 11%.4446-49 Theserogroups B, C, Y and W-135 are common among carriers, and meningococci of theserogroups X. Z and 29E, as well as nongroupable (nonencapsulated) meningococci,are found almost exclusively among carriers.44 46 49 Carriage of meningococci ofserogroup B is usually characterized by long duration (median 9 months) and lowacquisition rates (i.e. the incidence of carriage).'" Serogroup A, however, has beenfound to be less prevalent among carriers, but has demonstrated high acquisitionrates and a rapid turn_over. 14748Meningococcal disease is endemic in both developing and industrialized couotries.«In western Europe most cases of MD are reported during the winter months,but in (sub)tropical areas the incidence of MD is highest during or at the end of thehot, dry season.' .. 50 Desiccation of the mucosal surface during these periods has beenassumed to facilitate the invasion of nasopharyngeal meningococci l In most developingcountries the majority of the cases of MD is due to meningococci of serogroup A,whereas in Europe and the Americas the serogroups Band C prevail. 17 2.526 28-3144.56-S4The uncommon serogroups (X, Y, Z, 29E, W-135) and nongroupahle isolates areconsidered as opportunistic pathogens, because they seem to cause disease predominantlyamong predisposed patients, e.g. in2muno-compromised patients." ss Duringendemic periods many different phenotypes and clones are found among serogroup Band C isolates,l7 26 30 313543 These two serogroups share many serotypes, subtypes andimmunotypes." Serogroup A isolates are rather homogenous with regard to both thephenotype and genotype.'''''''7''Epidemics of MD have heen reported regularly world-wide.""" l4 3031 .. 50 SI " 57·6\Epidemics are mainly due to a particular meningococcal clone or a limited number ofclosely related clones.25 34 36 37 4143 54 60 61 In most developing countries epidemics areusually caused by meningococci of serogroup A, but in Europe and the Americas bymeningococci of the serogroups B or C. The appearance of a new meningococcal8