Fever with thrombocytopenia associated with a novel bunyavirus in China

Abstract

Background: Heightened surveillance of acute febrile illness in China since 2009 has led to the identification of a severe fever with thrombocytopenia syndrome (SFTS) with an unknown cause. Infection with Anaplasma phagocytophilum has been suggested as a cause, but the pathogen has not been detected in most patients on laboratory testing.

Methods: We obtained blood samples from patients with the case definition of SFTS in six provinces in China. The blood samples were used to isolate the causal pathogen by inoculation of cell culture and for detection of viral RNA on polymerase-chain-reaction assay. The pathogen was characterized on electron microscopy and nucleic acid sequencing. We used enzyme-linked immunosorbent assay, indirect immunofluorescence assay, and neutralization testing to analyze the level of virus-specific antibody in patients' serum samples.

Results: We isolated a novel virus, designated SFTS bunyavirus, from patients who presented with fever, thrombocytopenia, leukocytopenia, and multiorgan dysfunction. RNA sequence analysis revealed that the virus was a newly identified member of the genus phlebovirus in the Bunyaviridae family. Electron-microscopical examination revealed virions with the morphologic characteristics of a bunyavirus. The presence of the virus was confirmed in 171 patients with SFTS from six provinces by detection of viral RNA, specific antibodies to the virus in blood, or both. Serologic assays showed a virus-specific immune response in all 35 pairs of serum samples collected from patients during the acute and convalescent phases of the illness.

Conclusions: A novel phlebovirus was identified in patients with a life-threatening illness associated with fever and thrombocytopenia in China. (Funded by the China Mega-Project for Infectious Diseases and others.).

Figure 1. Geographic Distribution of SFTS in Mainland China

Areas where SFTS surveillance was carried out and SFTS bunyavirus was isolated from patients are shown in red.

Figure 2. Morphologic Features of SFTS Bunyavirus

Panel A shows virus-induced cellular changes that are visible on light microscopy (cytopathic effect) in DH82 cells infected with the severe fever with thrombocytopenia syndrome bunyavirus (SFTSV) 8 days after inoculation. The virusinfected DH82 cells are differentiated into macrophages with elongated pseudopodia with visible granular particles. Panel B shows virus grown in Vero cells and detected on immunofluorescence assay from a serum sample obtained from a patient with SFTSV infection. Panel C shows negatively stained virions purified from SFTSV-infected Vero cells. Panel D shows an image obtained on transmission electron microscopy of virus-infected DH82 cells (arrows).

Figure 3. Phylogenetic Analysis of SFTS Bunyavirus and Other Phleboviruses

The phylogenetic trees that are shown were generated by MEGA4 software from aligned amino acid sequences of six strains of phleboviruses, including the newly identified severe fever with thrombocytopenia syndrome bunyavirus (SFTSV). Coding regions for RNA-dependent RNA polymerase (RdRP) in the L segment (Panel A), for glycoproteins Gn and Gc in the M segment (Panel B), for N protein (NP) in the S segment (Panel C), and for NSs protein in the S segment (Panel D) were analyzed with the neighbor-joining method with the use of Poisson correction and complete deletion of gaps. Bootstrap testing (2000 replicates) was performed, and the bootstrap values are indicated. Sequences are identified by their GenBank accession numbers, followed by the virus name, according to the International Committee on Taxonomy of Viruses. The red dots indicate clusters of SFTSV strains. The scale bars in each panel indicate 0.1 substitutions per site. PTV denotes Punta Toro virus, RVFV Rift Valley fever virus, SFNV Sandfly fever Naples virus, SFSV Sandfly fever Sicilian virus, and UUKV Uukuniemi virus.