The first identification and retrospective study of Severe Fever with Thrombocytopenia Syndrome in Japan

Abstract

Background: Severe fever with thrombocytopenia syndrome (SFTS) is caused by SFTS virus (SFTSV), a novel bunyavirus reported to be endemic in central and northeastern China. This article describes the first identified patient with SFTS and a retrospective study on SFTS in Japan.

Methods: Virologic and pathologic examinations were performed on the patient's samples. Laboratory diagnosis of SFTS was made by isolation/genome amplification and/or the detection of anti-SFTSV immunoglobulin G antibody in sera. Physicians were alerted to the initial diagnosis and asked whether they had previously treated patients with symptoms similar to those of SFTS.

Results: A female patient who died in 2012 received a diagnosis of SFTS. Ten additional patients with SFTS were then retrospectively identified. All patients were aged ≥50 years and lived in western Japan. Six cases were fatal. The ratio of males to females was 8:3. SFTSV was isolated from 8 patients. Phylogenetic analyses indicated that all of the Japanese SFTSV isolates formed a genotype independent to those from China. Most patients showed symptoms due to hemorrhage, possibly because of disseminated intravascular coagulation and/or hemophagocytosis.

Conclusions: SFTS has been endemic to Japan, and SFTSV has been circulating naturally within the country.

Keywords: Hemophagocytosis; SFTS; SFTS virus: Japan; Severe fever with thrombocytopenia syndrome; bunyavirus; tick borne virus infection.

Figure 1.

Reactivity of patient sera samples to severe fever with thrombocytopenia syndrome virus (SFTSV). A, Detection of SFTSV antigen in Vero cells by indirect immunofluorescence assay (IFA) with rabbit anti-SFTSV recombinant nucleoprotein (rNP) serum. B, Virions in the culture supernatant detected by electron microscopy (bar in the image indicates the length of 100 nm, ID: SPL004). C, Positive indirect IFA results of the serum collected from a surviving patient (ID: SPL030) in the convalescent phase of SFTSV HB29. D, Neutralizing antibody activity was induced in the serum collected from a patient (ID: SPL032) in the convalescent phase of SFTSV YG1 and SFTSV HB29.

Figure 2.

Macroscopic and microscopic findings under hematoxylin-eosin staining. A, Gross appearance of the swollen right axillary lymph node (3.5 × 2.0 cm). B and C, Microscopic findings in the right axillary lymph node. The basic architecture of the lymph node has been replaced by massive necrosis. The necrotic regions contain histiocytes, immunoblasts, nuclear debris, and eosinophilic ghosts but no neutrophils. D and E, Marked erythrophagocytosis in the bone marrow (D) and spleen (E). F, Acute subepithelial hemorrhage in the renal pelvis.

Figure 3.

Microscopic and immunohistochemical tissue images. Hematoxylin-eosin staining (A, C, and E) and subsequent immunohistochemical analysis reveal the presence of severe fever with thrombocytopenia syndrome virus (SFTSV) NP (B, D, F, G, H, I, and J) in the right axillary (A and B), right cervical (C and D), and the mediastinal (E and F) lymph nodes. A, C, and E, Infiltration by immunoblasts and prominent hemophagocytosis was generally observed in the right axillary, right cervical, mediastinal, hilar, and abdominal lymph nodes; however, necrosis was only observed in the right axillary and cervical lymph nodes. B, D, and F, Viral antigen–positive cells were detected in the right axillary and cervical lymph nodes. Positive signals for SFTSV NP antigen were detected in the cytoplasm of blastic cells (B; inset). Inset shows a higher magnification (40×). In contrast, no signals were detected in the mediastinal lymph nodes, regardless of immunoblast infiltration and hemophagocytosis. G, H, I, and J, Immunohistochemical analysis of SFTSV NP was performed in the bone marrow (G), adrenal glands (H), liver (I), and spleen (J). A few SFTSV antigen–positive cells were observed in these tissues, with no notable cytopathic effects or necrosis (magnification, 20×; inset, 40×).

Figure 4.

Detection of severe fever with thrombocytopenia syndrome virus (SFTSV) RNA in the right cervical lymph node by the in situ hybridization AT-tailing method. A, SFTSV genomic RNA was detected in the right cervical lymph node by the in situ hybridization AT-tailing method and a sense probe. SFTSV genomic RNA was detected in the cytoplasm of the blastic cells. B, The in situ hybridization AT-tailing method with an anti-sense probe detected a few cells in the right cervical lymph node that were positive for SFTSV messenger RNA (mRNA). SFTSV mRNA was also detected in the cytoplasm of blastic cells. C, No signals were detected in the right axillary lymph node by the in situ hybridization AT-tailing method with an irrelevant probe (negative control). D, A SFTSV sense probe detected no signals in lymph node sections showing necrotizing lymphadenitis without SFTSV infection.

Figure 5.

Chronological (A), age-based (B), geographic (C), and seasonal (D) distributions of patients with retrospectively diagnosed severe fever with thrombocytopenia syndrome (SFTS) in Japan. Black and gray bars in panels A–C indicate patients with and those without SFTS, respectively. The red and blue circles in panel D indicate the areas where patients with and those without SFTS were located.

Figure 6.

Phylogenetic trees showing the phylogenetic positions of severe fever with thrombocytopenia syndrome virus (SFTSV) strains in Japan, compared with other known strains. Trees are based on the S segment (A; left panel), M segment (B; middle panel), and L segment (C; right panel). Heartland virus, Uukuniemi virus, Toscana virus, and Rift Valley fever virus are included in the phylogenetic analyses.