Canadian invasive pneumococcal disease outbreak characterised

By Joanna Lyford, Senior medwireNews Reporter

Canadian scientists have reported details of an outbreak of serotype 12F invasive pneumococcal disease (IPD) that occurred in Winnipeg between 2009 and 2011 and predominantly affected people who were homeless and/or engaged in illegal drug use.

Writing in Clinical Infectious Diseases, the researchers say this is the largest documented community outbreak of serotype 12F IPD and the first reported outbreak in a marginalised urban population in Canada.

“Further comparative genetic studies of the outbreak strains may offer insights into their virulence and connections with 12F strains implicated in previous 12F outbreaks in the United States and Canada, whereas nasopharyngeal carriage studies among high-risk populations may provide clues to the origin of this outbreak”, write Salaheddin Mahmud (University of Manitoba, Winnipeg, Canada) and co-authors.

Between September 2009 and January 2011, 169 cases of IPD were identified in the region.

Rates of IPD in Winnipeg, Manitoba’s capital city, were twice as high in 2010 as in previous years (14.1 vs around 8.0 per 100,000), with a significant increase in the proportion of cases caused by serotype 12F, which had previously been very rare (2.0% to 29.0% between 2007 and 2010).

Mahmud’s team undertook an epidemiological and microbiological analysis of these cases, finding that 58% of affected individuals were men. The average age of patients was 42 years (range 0–94 years) and around half of cases occurred in two neighbouring inner-city communities.

Homelessness and substance abuse were common among IPD cases, at 12% and 14%, respectively. Ten percent of patients developed meningitis and 73% were hospitalised. The overall case fatality rate during the study period was 10%.

Compared with other serotypes, 12F cases were more likely to be adults aged 18 to 54 years, to live in inner-city communities, to be homeless or report substance abuse, and less likely to have a pre-existing chronic disease.

Multi-locus sequencing typing determined that all 12F isolates belonged to a single sequence type and formed two clusters, one of which represented 59% of cases. Again, cases in this cluster were more likely to be male, aged 18 to 54 years, and to report homelessness, alcohol abuse and substance abuse – mostly crack cocaine and intravenous drugs.

Despite these similarities, however, obvious epidemiological links such as sharing the same space could not be identified among these cases.

Mahmud et al say it is not clear what caused the outbreak, but note that several possible explanations have been suggested, including agent, host and environmental factors. They conclude: “This outbreak underscores the importance of continued monitoring of IPD incidence and S. pneumoniae serotype distribution.”

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