Genetic and pathogenic diversity of severe fever with thrombocytopenia syndrome virus (SFTSV) in South Korea

Abstract

To investigate nationwide severe fever with thrombocytopenia syndrome virus (SFTSV) infection status, we isolated SFTSVs from patients with suspected severe fever with thrombocytopenia syndrome (SFTS) in 207 hospitals throughout South Korea between 2013 and April 2017. A total of 116 SFTSVs were isolated from 3137 SFTS-suspected patients, with an overall 21.6% case fatality rate. Genetic characterization revealed that at least 6 genotypes of SFTSVs were co-circulating in South Korea, with multiple reassortments among them. Of these, the genotype B-2 strains were the most prevalent, followed by the A and F genotypes. Clinical and epidemiologic investigations revealed that genotype B strains were associated with the highest case fatality rate, while genotype A caused only one fatality among 10 patients. Further, ferret infection studies demonstrated varying clinical manifestations and case mortality rates with different strains of SFTSV, which suggests this virus could exhibit genotype-dependent pathogenicity.

Keywords: Infectious disease; Platelets; Virology.

Figure 1. Distribution of SFTS cases between 2013 and 2016.

(A) Suspected, confirmed, and isolated human cases of SFTS in South Korea from 2013 to 2016. (B) Age distribution of confirmed SFTS cases. (C) Number of SFTS fatalities in confirmed cases. (D) Ratio of nonfatal to fatal cases by sex and year. M, male; F, female. Asterisks indicate statistically significant differences in mortality rates between each age group of infected patients as determined by 2-tailed Mantel-Cox or Mann-Whitney U test (*P < 0.05).

Figure 2. Phylogenetic analysis based on the complete ORF sequences of L, M, and S gene segments of Korean SFTSVs compared with those from China and Japan.

(A) L segment. (B) M segment. (C) S segment. Compressed versions of the ML trees based on the Kimura 2-parameter model were constructed and tested by bootstrap analysis with 1000 replications. The scale bars (0.005) indicate the number of nucleotide substitutions per site, and the phylogenetic branches were supported with greater than 70% bootstrap values. The SFTSV strains from China, Japan, and South Korea are marked with black, red, and blue closed circles, respectively. The full versions of the ML trees in this study are shown in Supplemental Figure 2.

Figure 3. Genotypes of Korean SFTSVs isolated from human sera.

The SFTSVs were assigned to different genotypes based on the genetic origin of each segment as determined by phylogenic analysis. The SFTSVs were differentiated into at least 15 genotypes, including genetic reassortant viruses. Gene segments from top to bottom are L, M, and S. The bars indicate pure groups of genotypes (A) A (red), B-1 (yellow), B-2 (black), B-3 (light blue), C (purple), D (blue), E (pink), F (green); and reassortant groups of genotypes (B) R-1 to R-9, including the unique clustering genotype (gray).

Figure 4. Prevalence and fatality rates of each genotype by year.

(A) Incidence of each genotype of SFTSV from 2012 to 2017. (B) Ratio of nonfatal to fatal cases for each genotype. The case fatality percentage (%) is shown at the top of each bar. (C) Ratio of nonfatal to fatal cases by age group. Asterisks indicate statistically significant differences in mortality rates between each genotype (B) or aged group (C) determined by Fisher’s exact test (*P < 0.05).

Figure 5. Pathogenicity of each randomly selected SFTSV pure genotype (A, B-1, B-2) in ferrets.

Young adult (<2 years, n = 5; blue) or aged female and male ferrets (>4 years, n = 5; red) were inoculated with 10^5.0 FFU/mL of each randomly selected SFTSV strain (A) A, (B) B-1, and (C) B-2. Survival, viral titers in serum, and platelet concentration were assessed. The normal range of platelets (171.7 × 10³/μL to 1280.6 × 10³/μL) is marked by dashed lines in each panel. Both viral titers and platelet concentrations are presented as mean ± SEM. Pound sign (#) indicates no sample due to death of all ferrets of this group. Asterisks (*) indicate statistically significant difference between infected young adult and infected aged ferrets per dpi as determined by the Mantel-Cox method (survival) or 2-way ANOVA with Šidák’s comparison (viral copy number and platelets). Crosses (†) indicate significant difference in platelet count between noninfected aged adult and infected aged ferret as calculated by 2-way ANOVA with Tukey’s test. *P < 0.05, †P < 0.05, ††P < 0.001, ***P < 0.0001.

Figure 6. Pathogenicity of each randomly selected SFTSV pure genotype (B-3, D, F) in ferrets.

Young adult (<2 years, n = 5; blue) or aged female and male ferrets (>4 years, n = 5; red) were inoculated with 10^5.0 FFU/mL of each randomly selected SFTSV strain (A) B-3, (B) D, or (C) F. Survival, viral titers in serum, and platelet concentration were assessed. The normal range of platelets (171.7 × 10³/μL to 1280.6 × 10³/μL) is marked by dashed lines in each panel. Both viral titers and platelet concentrations are presented as mean ± SEM. Pound sign (#) indicates no sample due to death of all ferrets of this group. Asterisks (*) indicate statistically significant difference between infected young adult and aged infected ferrets per dpi as determined by Mantel-Cox method (survival) or 2-way ANOVA with Šidák’s comparison (viral copy number and platelets). Crosses (†) indicate significant difference in platelet count between noninfected aged adult and infected aged ferrets as calculated by 2-way ANOVA with Tukey’s test. *P < 0.05 and **P < 0.001, ††P < 0.001, ***P < 0.0001.