Invasive Disease caused by Haemophilus influenzae serotype a, an emerging pathogen in Alaska
METHODS: We identified all cases of invasive Haemophilus influenza disease in AK from 1983–2012 through Alaska state-wide laboratory-based surveillance. Serotyping was performed by slide agglutination and polymerase chain reaction. Multi-locus sequence typing was performed. Medical charts were reviewed using standardized forms to characterize clinical presentation.
RESULTS: During the study period, isolates from 912 cases of invasive Hi disease were serotyped, 35 (3.8%) of these were Hia. The first identified Hia case occurred in 2002. From 1983-2001, none of the 30 encapsulated non-type b Hi strains were Hia, compared to 35 of 73 (48%) isolated from 2002-2012 (p<0.001). Median age of Hia cases was 0.7 year (range 0.3 - 48 years); 91% of cases occurred in children <5 years; 3 Hia cases, all among infants, were fatal. The most common clinical presentations included: meningitis (40%), pneumonia with bacteremia (26%), and septic arthritis (17%). Overall annual incidence of Hia disease (2002-2012) among children < 5 years of age was 5.4 per 100,000; 30 cases occurred in Alaska Native (AN) children (18 per 100,000) vs. 2 cases in non-AN children (0.5 per 100,000) (Risk Ratio = 36, p < 0.001). Eighteen cases occurred in southwestern Alaska from December 2009 through December 2012 (in children < 5 years of age, a rate of 141 cases per 100,000 population).
CONCLUSIONS: More than 10 years after introduction of Hib conjugate vaccine, Hia has emerged as a significant invasive bacterial disease in Alaska, with the highest rates among AN children. Further research is needed to investigate severity of illness and long term sequelae, risk factors, and regional differences in rates to inform development of prevention strategies. An evaluation of oropharyngeal carriage, transmission patterns and the potential role for chemoprophylaxis is underway.