Relationship of California group virus infection and mental retardation: seroepidemiological observations.

A plaque reduction neutralisation test was utilised to determine the prevalence of antibodies to California group virus (LaCrosse subtype) among 612 moderately to profoundly retarded subjects resident in two Wisconsin state institutions. Neutralising antibodies were detected in the sera of fifty-one (8 per cent) of the subjects tested. Age and sex differences in antibody distribution were not significant. When sero-logical data correlated with clinical case history records, antibody was found in the sera of 13 per cent (24/188) of a group in which mental retardation (MR) was attributed to "presumed psychological causes" and in 8 per cent (12/158) of a group for which the aetiology of MR was "encephalopathy". In a group of 254 subjects with MR of unknown aetiology, none of the sixty subjects with cranial anomaly and only two of the ninety-five (2.1 per cent) subjects with Down's syndrome were positive for antibody to CEV, while thirteen of ninety-nine (13.1 per cent) subjects diagnosed as "other cerebral defects" had CEV antibody. In a majority of the subjects with CEV neutralising antibody, MR was attributed to perinatal influences.

Isolation of La Crosse virus (California encephalitis group) from the chipmunk (Tamias striatus), an amplifier host.

La Crosse (LAC) virus was isolated from the blood of seven chipmunks (Tamias striatus) captured during the summer of 1970 in southwestern Wisconsin. With the exception of the original isolate obtained from human brain after fatal encephalitis, these represent the first known isolations of LAC virus from a naturally infected free-living vertebrate. The chipmunks were trapped and periodically recaptured in two study areas where 59 chipmunks became infected and developed neutralizing antibody during the summer cycle of virus transmission. All isolates were obtained from blood samples collected within a 7 week period between 11 July and 23 August, and all were from seronegative chipmunks; 6 of these were recaptured and found to be seropositive 2 to 3 weeks later. The isolates proved identical to each other in comparative micro-neutralization tests using BHK21 cell cultures and immune chipmunk serum or hyperimmune mouse ascitic fluids. Neutralization tests showed the isolates to be different from snowshoe hare, trivittatus, and Jamestown Canyon prototype virus strains but indistinguishable from the LAC prototype. Findings demonstrate multiplication and transmissibility of LAC virus in a naturally infected host and are consistent with the thesis that chipmunks are important amplifying hosts for LAC virus and that Aedes triseriatus mosquitoes serve as vectors in transmitting their infections. Ecological significance of the findings are discussed in regard to current perspectives.

Observations on a natural cycle of La Crosse virus (California group) in Southwestern Wisconsin.

Ecological studies were conducted to document the role of the eastern chipmunk (Tramias striatus) as a vertebrate host for La Crosse (LAC) virus in nature during late summer when Aedes triseriatus mosquitoes are most abundant. Movement, home range and density of chipmunk populations were determined by trap mark and recapture techniques on grid study areas. The temporal distribution of A. triseriatus was estimated by use of oviposition traps. Passive antibodies were found in spring-born juveniles captured prior to mid-July and in summer-born juveniles in September. Active antibodies neutralizing LAC virus were first detected in susceptible chipmunks in mid-July and 68 free-living and 4 sentinel animals developed antibodies during the study. Virus transmission continued at a high rate through August but was not detected in September. Chipmunk habitat was ranked for quality and populations of chipmunks. A. triseriatus were more abundant in study areas with most suitable chipmunk habitat. Populations of A. triseriatus were temporally associated with the elaboration of antibodies in chipmunks. In one study area, antibody prevalence rates in adult and spring-born juveniles reached 100% by September. Findings implicate A. triseriatus as the vector and establish chipmunks as important amplifying hosts in discontinuous foci of virus transmission.