Seroepidemiology of Saffold Cardiovirus Type 2

To the Editor: Saffold virus (SAFV) is a new human virus belonging to the genus Cardiovirus of the family Picornaviridae (1–6). The virus has also been named human Theiler's-like cardiovirus (4). To date, 8 SAFV genotypes are known, based on molecular variation in the P1 region, which codes for the viral capsid (5). SAFVs are detected in respiratory and fecal samples from infants and children <6 years of age. The virus is generally detected at low frequency (0.5%–3%) in the general population, but in Afghanistan and Pakistan higher incidences have been reported (10%–12%) (5). Detection is mainly based on molecular techniques because virus isolation is cumbersome. Other than gastroenteritis, no clear disease manifestations are known (1– 8), although recently, researchers in Japan have suggested that for a select group of children, SAFV infection may cause pharyngitis and tonsillitis (7). In rodents, however, cardioviruses are serious pathogens that can cause myocarditis, meningoencephalitis, and pancreatitis. Previously, we have shown that SAFV-3 infections are ubiquitous and are transmitted early in life (6). This conclusion was based on antibody testing by virus neutralization, a highly specifi c test that can discriminate serotypes within a single species, but formal proof for existence of serotypes was lacking. Extrapolated to other genotypes, the fi nding would indicate a high infection rate for SAFVs, which is at odds with the low detection frequency reported in most studies. Most SAFV-2 isolates, from our laboratory and others, grow poorly in cell culture, which hampers reading of neutralization. Recently, however, a SAFV-2 strain was isolated in Finland that grows well in HeLa cells and shows clear cytopathic effects (SAFV-2-FIN2008, GenBank accession no. FR682076; S. Blomgvist et al., unpub. data). This strain enabled us to set up a virus neutralization test similar to that described for SAFV-3 (6). For comparison, we used strain SAFV-3(NL2007) (GenBank accession no. FM207487), which was isolated in Nijmegen (6). The virus neutralization test was performed on HeLa cells with 100 TCID 50 of virus per serum dilution. Virus–serum mixtures were incubated for 1 hour at 37°C and overnight at 7°C to stabilize virus–antibody complexes (6). Human serum samples submitted previously were tested by using 3-fold dilution steps and duplicate testing (6). The results are presented in the Table. A low seroprevalence was found for SAFV-2 and SAFV-3 at 9 months of age, which is at the nadir of immunoglobulin G levels in infants. At the age of …