Mortality rates from coronary heart disease are lower in Swedish men and among Finnish migrants who have lived in Sweden for over 20 years than in men living in Finland. Sero-epidemiological studies, investigations of atheromatous plaque specimens, in vitro animal models and anti-chlamydial antibiotic trials have given support to the hypothesis that Chlamydia pneumoniae (Cpn) has a role in atherosclerosis. We investigated whether men with a similar genetic background but living permanently in Finland or Sweden have differences in the prevalence of Cpn seropositivity, and whether chronic Cpn infection is associated with markers of subclinical atherosclerosis. We measured anti-Cpn antibodies and ultrasonographic markers of subclinical atherosclerosis, including carotid intima-media thickness, carotid artery compliance and brachial artery flow-mediated dilatation, in a population of 76 migrant-discordant male twin pairs (152 men). The number of men with seropositivity to Cpn infection (defined as IgA⩾1:64 and IgG⩾1:128) was greater in Finland than in Sweden (21.5% compared with 10.5%; P=0.046). Cpn seropositivity accompanied by elevated C-reactive protein (CRP) levels (>1 mg/l) was associated with attenuated brachial artery flow-mediated dilatation (3.3±0.3%, compared with 5.5±0.4% in men with no signs of Cpn infection; P<0.001).Thus, among Finnish twin brothers discordant for migration to Sweden, the prevalence of Cpn seropositivity is higher for those living in Finland, and men with Cpn seropositivity combined with elevated CRP levels had attenuated endothelial function. These findings offer insight into the mechanism whereby chronic Cpn infection may increase the risk of coronary heart disease.

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