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Distinct geographic distribution of subtypes of Haemophilus influenzae type b in Western Europe.

The spread within populations and the geographic distribution of Haemophilus influenzae type b (Hib) strains has hardly been investigated [1, 2], mainly because there have been no reliable and discriminating subtyping methods. Biotyping has been used for a long time for typing H. influenzae. Most Hib strains, however, are biotype I [3]. Recently, Barenkamp et al. [4] described 21 subtypes of strains isolated in the United States; his differences between subtypes were based on the outer membrane protein composition as analyzed by SDS-PAGE subtyping. Eighty-nine percent of the strains belong to six subtypes [4, 5]. Isolated strains of the most frequently encountered subtypes were equally distributed over the country, but distinct regional differences were found in the distribution of some of the less-common subtypes [6]. We found that 93% of the Hib strains from patients with meningitis in the Netherlands belong to one subtype (subtype 1) [7]. Strains with this subtype, which is identical to subtype 3L described by Barenkamp [4], were isolated from 15% of the cases in the United States [5]. In addition to SDS-PAGE subtyping, serotyping based on differences in lipopolysaccharides was introduced to further differentiate the Hib strains isolated in the Netherlands [7]. Based on biotyping, SDS-PAGE subtyping, and lipopolysaccharide serotyping, the Dutch strains appeared homogeneous [7]. These strains may be descendents from one ancestral clone [8]. In this paper we show that only a few subtypes of Hib are predominant in Western Europe and that the distribution of these subtypes shows distinct geographic differences. The significance of this finding for the understanding of the endemic spread of Hib disease is discussed.