Seroepidemiological Studies of Echovirus Type 30 Infection

Abstract

A large outbreak of echovirus type 30 infection occurred throughout Japan in 1983. During the epidemic we isolated echovirus type 30 from patients with aseptic meningitis and the representative isolate was designated as the Fukui 609/83 strain (3). The strain was identified as an antigenic variant having a strain-specific neutralization determinant(s) in addition to a type-specific antigen(s) in common with the prototype Bastianni strain. Interestingly, most convalescent phase sera obtained from the patients with this virus infection showed a two-to fourfold higher neutralizing antibody response to the Fukui strain than to the Bastianni strain. This finding suggests that the outbreak in 1983 might have been caused by the Fukui strain rather than the prototype strain. To determine the prevalence of the two strains in detail, we carried out seroepidemiological studies retrospectively.The serum samples were obtained by random sampling from individuals visiting the Fukui Prefectural Central Hospital before and after the epidemic in 1983. Twenty serum samples were obtained from persons in each of the following age groups : 0-2, 3-5, 6-9, 10-12, 13-15, 16-19, 20-29, 30-39, 40-49, and 50-59 years. Sera from babies less than 6 months old were not used. Sera collected in 1977 and 1979 were also examined, since an outbreak of this virus infection was reported to have occurred in Aichi Prefecture in 1978 (4). All sera were tested for neutralizing antibody against the Fukui 609/83 and the Bastianni strains of echovirus type 30. Neutralization tests were carried out by the microtiter method after inactivation of the sera at 56 C for 30 min. A 0.025-ml volume of a suspension containing 100 TCID50 of the virus was mixed with an equal volume of serial twofold diluted serum and incubated at 37 C for 90 min. The mixtures were then kept at 4 C overnight and inoculated onto FL cell monolayers. The neutralizing antibody titer was expressed as the reciprocal of the highest dilution of serum inhibiting viral cytopathic effect. Figure 1 shows the prevalence of antibody to echovirus type 30 in the different age groups before and after the epidemic. In September 1977 no children younger than 15 years old were seropositive (Fig. 1A). However, two years later seropositive children appeared, and 15 % of the children 6 to 9 years of age showed high neutralizing antibody titers (Fig. 1B), suggesting that sporadic infection with echovirus type 30 did occur during the two years although an attempt to isolate this virus was unsuccessful. On the other hand, adults had a high rate of seropositivity in 935