Infection Route Impacts the Pathogenesis of Severe Fever with Thrombocytopenia Syndrome Virus in Ferrets

Abstract

The threat of severe fever with thrombocytopenia syndrome (SFTS) to public health has been increasing due to the rapid spread of the ticks that carry the causative viral agent. The SFTS virus (SFTSV) was first identified in China and subsequently detected in neighboring countries, including South Korea, Japan, and Vietnam. In addition to the tick-mediated infection, human-to-human transmission has been recently reported with a high mortality rate; however, differential study of the pathogen has been limited by the route of infection. In this study, we investigated the pathogenic potential of SFTSV based on the infection route in aged ferrets, which show clinical signs similar to that of human infections. Ferrets inoculated with SFTSV via the intramuscular and subcutaneous routes show clinical signs comparable to those of severe human infections, with a mortality rate of 100%. Contrastingly, intravascularly infected ferrets exhibit a comparatively lower mortality rate of 25%, although their early clinical signs are similar to those observed following infection via the other routes. These results indicate that the infection route could influence the onset of SFTS symptoms and the pathogenicity of SFTSV. Thus, infection route should be considered in future studies on the pathogenesis of SFTSV infection.

Keywords: SFTSV; aged ferret; animal model; infection routes.

Figure 1

Virulence of severe fever with thrombocytopenia syndrome virus (SFTSV) in aged ferrets via different routes of infection. Eight ferrets per group were inoculated with SFTSV via the intramuscular (IM), subcutaneous (SC), or intravenous (IV) route. Animals were then assessed every other day for (a) temperature changes; (b) relative weight; (c) Kaplan–Meier survival; (d) and viral copy numbers in serum. (e) Viral copy numbers in spleen, liver, kidney, intestine, and brain were examined at 6 dpi. The data for the mock, IM, SC, and IV groups are shown in black, green, blue, and red, respectively. Data are presented as mean ± s.e.m (a,b,e) or min to max (d), and titers below the limit of detection are shown as 0.3 log₁₀ RNA copy numbers/mL or 0.3 log₁₀ RNA copy numbers/g (dashed lines). An asterisk (*) indicates statistical significance when data are compared to those for the mock group (* p < 0.05; ** p < 0.01; *** p < 0.001; and **** p < 0.0001). A section sign (§) indicates statistical significance when data are compared among the virus-infected groups (§ p < 0.05, §§ p < 0.01, §§§ p < 0.001, §§§§ p < 0.0001) with respect to viral copy numbers in serum. Statistical significance was determined by two-way ANOVA with Tukey’s comparison test (a,b,d,e) or rank (Mantel–Cox) test (c). A sharp symbol (#) indicates that no samples were collected because ferrets in that group died.

Figure 2

Hematological analysis of aged ferrets after inoculation with SFTSV via the IM, IV, and SC routes. Blood was collected every other day from the IM (green), SC (blue), and IV (red) groups for hematological analysis. (a) Platelet count; (b) white blood cell (WBC) count; (c) alanine aminotransferase (ALT) levels; and (d) aspartate aminotransferase (AST) levels. Data are presented as min to max. An asterisk (*) indicates statistical significance when data are compared to those for the mock group by two-way ANOVA with Tukey’s comparison test (* p < 0.05; ** p < 0.01; *** p < 0.001; and **** p < 0.0001). A sharp symbol (#) indicates that no samples were collected because ferrets in the group died. The middle part of the dashed line is the normal value of the hematological parameters. Reference values for platelet, WBC, ALT, and AST are 171.7–1280.6 × 10³/µL, 2.5–16.7 × 10³/µL, 49–242.8U/L, and 40.1–142.7U/L, respectively.

Figure 3

Histopathological observations of tissues from ferrets infected via IM, SC, and IV routes with PBS or 10⁵ FFU SFTSV. The spleen (a–d) and liver (e–h) samples were harvested at 6 dpi in the mock (a,e), IM (b,f), SC (c,g), and IV (d,h) groups. Bars, 100 μm.