Serotyping of Haemophilus pleuropneumoniae in the Netherlands: with emphasis on heterogeneity within serotype 1 and (proposed) serotype 9

Four hundred and forty-three Dutch field isolates of Haemophilus pleuropneumoniae were serotyped by rapid slide agglutination (RSA) using specific antisera against serotypes 1 to 5 and against the recently proposed types 6 to 9. The predominant serotypes were 9 (49%) and 2 (32%). Serotypes 1, 3, 5, 7 and 8 were isolated in small numbers: together they accounted for 3% of the total. Five percent of the isolates were not typable either due to autoagglutination or because they were not agglutinated by any of the available antisera. The remaining 49 strains (11%) agglutinated in more than one antiserum and could therefore not be properly classified. Forty-four of these 49 strains agglutinated in both anti type 1 and anti type 9 serum. Antigenic relationships between serotype 1, serotype 9 and isolates reacting with both antisera were studied using immunodiffusion and RSA with adsorbed sera. Serotype 9 strains appeared not to be a homogenous group. Isolates agglutinating exclusively in anti type 9 serum can be divided into two groups: one closely related and another hardly related to serotype 1. Serotype 9 reference strain 13261 belongs to the latter. Type 1 + 9 strains have antigens in common with serotypes 1 and 9, but they also have their own specific antigenic material. Such strains are proposed as a new serotype 10.