Genetic and Experimental investigations of the pathogenicity of Sindbis virus genotypes.

Abstract: Sindbis virus (SINV) is a virus transmitted by mosquitoes. It has a wide geographical distribution covering most of Europe, Africa, Asia, Australia and New Zeeland. To date, six main different types of SINV are known (genotypes), but only genotype I (SINV-I) has been linked to human disease. SINV-I is present in Europe and Africa but has only been reported to cause outbreaks in northern Europe (Finland and Sweden) and South Africa. It is interesting to note that SINV does not cause any outbreaks in other parts of Europe or around the world; for example, no known cases have been observed in Germany. In this study, we aimed to characterize differences among SINV genotypes from different parts of the world, that can potentially explain why outbreaks of disease only occur in certain regions. This was done by full genome sequencing, as well as experiments where different SINV viruses were allowed to infect cells. The full genome sequencing will add to the existing knowledge about SINV genotypes and will help to establish a relationship between the divergence or the mutations associated with the SINV isolates. The experiments will help us understand the growth and infectivity of the different genotypes. Analysis of the sequencing showed that SINV formed five very distinct groups confirming earlier proposed division into genotypes. SINV isolates originating from Africa, Germany, Finland, and Sweden were all groped within one cluster. The result further supported the hypothesis that SINV-I was introduced to northern Europe from Central Africa. We compared the similarity of nucleotides and amino acids for all proteins coded for by the virus, and they showed high levels of similarity within genotypes as well as in between genotypes, with certain point-mutations in nucleotides and few amino acid substitutions in peptide sequences. In our growth experiments, we found considerable differences in the growth pattern of SINV genotypes. SINV-I strain from Germany followed the growth pattern of SINV-I strains from Finland and Sweden which are associated with causing the disease. Interestingly, even though on a molecular level SINV-II and SINV-III seemed to be similar, they differ in their growth rate and how much infectious virus particles they produce. These findings were very interesting because, to date, there are no human cases of SINV infection in Germany, and the antibody prevalence in the human population is 0,1% as compared to 3% in Sweden and 5.2% in Finland. Our results suggest that the German SINV strain is not less infectious than the Swedish and Finnish strains, thus the absence of cases and low antibody prevalence in Germany is likely to be due to other factors than the virus itself. Such factors could be related to the transmission ecology, meaning that the virus might not be transmitted as efficiently in central Europe as in the north. One suggested reason is that the vector mosquito species (Culex torrentium) that most efficiently transmit SINV is less common in central Europe than in the north.