Modelling of endemic carriage of Haemophilus influenzae in Aboriginal infants in Northern Australia

Aboriginal infants and children in rural communities in Northern Australia have high rates of nasopharyngeal carriage of nonencapsulated Haemophilus i nfluenzae (NCHi), with positive swab rates of 76%. In this population, the acquisition of NCHi from soon after birth is associated with the onset of o titis media and with muco-purulent nasal discharge, while the long-term per sistence of NCHi carriage is associated with the acquisition and turnover o f large numbers of antigenically diverse strains. Mathematical models have been fitted to data on the acquisition and loss of encapsulated strains of H. influenzae and 43 different strains of NCHi in 10 children followed from early infancy for up to 2 years. Subject to plausible assumptions, the pre ferred model estimated the mean time to acquisition of a H. influenzae stra in to be 7 days after first becoming exposed after birth. For an infant alr eady carrying H. influenzae, each additional strain was acquired after a me an waiting period of 45 days. On average, 1.50 different strains of H. infl uenzae were detected in four colonies routinely typed from each positive sw ab, but it was estimated that another 2.55 strains were 'hidden' behind the se more frequent strains. With an average of 4.05 strains per carrier, it w as estimated that each strain was carried for an average of 137 days, altho ugh detected on only 37% of occasions. Thus we have developed mathematical models that provide estimates for duration of colonisation, time to colonis ation, and number of colonising strains in a population in which H. influen zae is highly endemic, characterised by sequential and concurrent carriage of multiple strains in each infant. (C) 2001 Federation of European Microbi ological Societies. Published by Elsevier Science B.V. All rights reserved.