The hMPV seropositivity rate in Croatian individuals of 77.4% found in our study is similar to the rate found in a Japanese serologic study performed in 2003 (8
) on patients without respiratory symptoms and in Uruguayan study from 2005 (10
) performed on healthy donor adults and children without any underlying pathology. In these studies, hMPV seropositivity rates were reported to be 72.5% and 80%, respectively. The seropositivity rate of 100% was found in the population of subjects older than 20 years, in contrast to Dutch and Japanese studies where hMPV seroprevalence reached 100% in the 5-10 age group (1
). In addition, the results of serologic tests performed in the Netherlands showed that 25% of infants aged 6 months to 1 year were seropositive, increasing to 55% in children 1-2 years old and 70% in those 2-5 years old. Our study demonstrated slightly lower seroprevalence in the first two age groups (18.7%, 45.5%) and a higher seroprevalence in the 2-5 age group (85%).
In our study, 17 of 21 infants under 6 months of age (81%) had hMPV antibodies suggesting the presence of transplacentally derived maternal antibodies. In accordance with this is the lowest prevalence of 18.7%, found in the age group between 6 months and 1 year, when maternal antibodies would be expected to be at their nadir.
Seroprevalence increased with age consistent with increased exposure to hMPV. High titers of hMPV specific antibodies indicate recently acquired infection. Therefore, the highest proportion of titers ≥1:512 was found in the1-2 age group, when infants lack transplacentally derived maternal antibodies and are exposed to hMPV (this may be largely IgM). These findings indicate that the most of hMPV primary infections occur in early childhood (1-2 years of age).
Our results suggest that hMPV infection is present in Croatia, confirming the previous reports of hMPV worldwide distribution (7