SCIENTIFIC CORRESPONDENCE Biotypes and serotypes of Haemophilus influenzae ocular isolates

Aim: To determine which subtypes of Haemophilus influenzae are most commonly associated with ocular disease, and whether the site of ocular H influenzae infection is correlated with specific subtypes of the organism. Methods: The biotypes and serotypes of ocular H influenzae isolates collected at the Francis I Proctor Foundation between March 1989 and January 2000 were examined. A total of 62 ocular isolates were retrieved from frozen storage and plated on chocolate agar. Biotypes were assigned based upon the ability of the isolates to produce indole, urease, and ornithine decarboxylase. Capsular subtypes a–f were determined by slide agglutination using commercially available subtype specific antisera. Identified biotypes and serotypes were then analysed with regard to site of infection. Results: Patient age ranged from 1 to 92 years with a median age of 45 years. 38 (61%) of the isolates were biotype II, 23 (37%) were biotype III, and one (2%) was biotype VII. All of the isolates were non-encapsulated and thus serologically non-typable. H influenzae biotype II was found in 28 of 48 (58%) conjunctivitis cases, five of eight (63%) keratitis cases, and two of two (100%) endophthalmitis cases. Biotype III was found in 20 of 48 (42%) conjunctivitis cases, two of eight (25%) keratitis cases, and a single case of dacryocystitis. Biotype VII was associated with one of eight (13%) keratitis cases. Conclusion: Most ocular H influenzae isolates appear to be serologically non-typable strains from biotypes II and III, less virulent subtypes that frequently colonise the nasopharynx. In addition, the site of ocular H influenzae infections appears to be largely independent of species subtype.