Low Replication Efficiency of a Japanese Rabbit Hepatitis E Virus Strain in the Human Hepatocarcinoma Cell Line PLC/PRF/5

A Japanese rabbit hepatitis E virus (HEV) strain, JP-59, has been identified in a feral rabbit. When this virus was transmitted to a Japanese white rabbit, it caused persistent HEV infection. The JP-59 strain shares an <87.5% nucleotide sequence identity with other rabbit HEV strains. Herein, to isolate JP-59 by cell culture, we used a 10% stool suspension recovered from a JP-59-infected Japanese white rabbit and contained 1.1 × 10⁷ copies/mL of the viral RNA and using it to infect a human hepatocarcinoma cell line, PLC/PRF/5. No sign of virus replication was observed. Although long-term virus replication was observed in PLC/PRF/5 cells inoculated with the concentrated and purified JP-59 containing a high titer of viral RNA (5.1 × 10⁸ copies/mL), the viral RNA of JP-59c that was recovered from the cell culture supernatants was <7.1 × 10⁴ copies/mL during the experiment. The JP-59c strain did not infect PLC/PRF/5 cells, but its intravenous inoculation caused persistent infection in rabbits. The nucleotide sequence analyses of the virus genomes demonstrated that a total of 18 nucleotide changes accompanying three amino acid mutations occurred in the strain JP-59c compared to the original strain JP-59. These results indicate that a high viral RNA titer was required for JP-59 to infect PLC/PRF/5 cells, but its replication capability was extremely low. In addition, the ability of rabbit HEVs to multiply in PLC/PRF/5 cells varied depending on the rabbit HEV strains. The investigations of cell lines that are broadly susceptible to rabbit HEV and that allow the efficient propagation of the virus are thus needed.

Simple and rapid detection of severe fever with thrombocytopenia syndrome virus in cats by reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay using a dried reagent

Severe fever with thrombocytopenia syndrome virus (SFTSV) causes lethal hemorrhagic diseases in human, cats, and dogs. Several human cases involving direct transmission of SFTSV from diseased animals have been reported. Therefore, rapid diagnosis in veterinary clinics is important for preventing animal-to-human transmission. Previously, we developed a simplified reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay for human that does not require RNA extraction for detecting the SFTSV genome. In this study, we improved the simplified RT-LAMP assay for cats by introducing a dried reaction reagent and investigated the applicability of this method for diagnosing SFTS in cats. SFTSV RNA was detected in 11 of 12 cats naturally infected with SFTSV by RT-LAMP assay using both liquid and dried reagents. The RT-LAMP assay using liquid and dried reagents was also applicable to the detection of SFTSV genes 3-4 days after challenge in cats experimentally infected with SFTSV. The minimum copy number of SFTSV genes for 100% detection using the RT-LAMP assay with liquid and dried reagents was 4.3 × 10⁴ and 9.6 × 10⁴ copies/mL, respectively. Although the RT-LAMP assay using the dried reagent was less sensitive than that using the liquid reagent, it was sufficiently sensitive to detect SFTSV genes in cats with acute-phase SFTS. As the simplified RT-LAMP assay using a dried reagent enables detection of SFTSV genes more readily than the assay using a liquid reagent, it is applicable for use in veterinary clinics.

Lethal Disease in Dogs Naturally Infected with Severe Fever with Thrombocytopenia Syndrome Virus

Severe fever with the thrombocytopenia syndrome virus (SFTSV) causes fatal disease in humans, cats, and cheetahs. In this study, the information on seven dogs with SFTS was summarized. All dogs showed anorexia, high fever, leukopenia, and thrombocytopenia, two dogs showed vomiting and loose stool, and five dogs had tick parasites. All dogs also had a history of outdoor activity. The SFTSV gene was detected in all dogs. Remarkably, three dogs (43%) died. SFTSV was isolated from six dogs and the complete genomes were determined. A significant increase in anti-SFTSV-IgG antibodies was observed in two dogs after recovery, and anti-SFTSV-IgM antibodies were detected in four dogs in the acute phase. Using an ELISA cut-off value of 0.410 to discriminate between SFTSV-negative and positive dogs, the detection of anti-SFTSV-IgM antibodies was useful for the diagnosis of dogs with acute-phase SFTS. Four out of the ninety-eight SFTSV-negative dogs possessed high anti-SFTSV IgG antibody titers, indicating that some dogs can recover from SFTSV infection. In conclusion, SFTSV is lethal in some dogs, but many dogs recover from SFTSV infection.

Risk assessment of infection with severe fever with thrombocytopenia syndrome virus based on a 10-year serosurveillance in Yamaguchi Prefecture

In Japan, the first patient with severe fever with thrombocytopenia syndrome was reported in Yamaguchi in 2012. To understand the severe fever with thrombocytopenia syndrome virus (SFTSV) infection in this region, a retrospective surveillance in sika deer and wild boars in Yamaguchi was conducted using a virus-neutralizing (VN) test. The result revealed that 510 of the 789 sika deer and 199 of the 517 wild boars were positive for anti-SFTSV antibodies. Interestingly, seroprevalence in sika deer increased significantly from 2010–2013 to 2015–2020. The SFTSV gene was detected in one of the 229 serum samples collected from sika deer, but not from wild boars. In conclusion, SFTSV had spread among wild animals before 2012 and expanded gradually around 2013–2015 in Yamaguchi.

Roles of raccoons in the transmission cycle of severe fever with thrombocytopenia syndrome virus

The present study investigated severe fever with thrombocytopenia syndrome virus (SFTSV) infection in raccoons in Wakayama Prefecture from 2007 to 2019. To perform surveillance, an enzyme-linked immunosorbent assay (ELISA) was established, and the sensitivity and specificity of the ELISA were 100% in comparison with a 50% focus-reduction neutralization assay. Using the established ELISA, we performed serosurveillance of SFTSV infection in 2,299 raccoons in Tanabe region, Wakayama Prefecture from 2007 to 2019. The first anti-SFTSV-positive raccoon was captured in October 2009. The seroprevalence of SFTSV infection was <10% between April 2009 and March 2013, 23.9% between April 2013 and March 2014, 37.5% between April, 2014 and March 2015, and over 50% from April 2015. Next, we performed detection of SFTSV genes in sera of raccoons captured in Wakayama Prefecture after April 2013. The results indicated that 2.4% of raccoons were positive for SFTSV genes and that the frequency of SFTSV infection among raccoons between January and March (0.7%) was lower than that between April and June (3.4%). In addition, virus genes were detected from many specimens, including sera and feces of two raccoons, and viral antigens were detected in lymphoid cells in lymphoid follicles in the colon by immunohistochemical staining. In conclusion, SFTSV had recently invaded the area and had rapidly spread among wild animals. The first patient in this area was reported in June 2014, indicating that raccoons are good sentinels for assessing the risk of SFTSV in humans.

A serological survey and characterization of Getah virus in domestic pigs in Thailand, 2017-2018

Getah virus (GETV) is a mosquito-borne RNA virus belonging to the family Togaviridae, genus Alphavirus. GETV infection causes diarrhoea and death in piglets, and reproductive failure and abortion in sows. This study conducted a serological survey of GETV infection among domestic pig populations in Thailand. ELISA was used to analyse 1,188 pig serum samples collected from 11 provinces of Thailand during 2017-2018, with 23.1% of the samples being positive for anti-GETV antibodies. The positive ratio of anti-GETV antibodies was significantly higher in nursery (67.9%) and older stages (84.5%) of pigs than in finishing stage (14.2%). Furthermore, we successfully isolated GETV from one pig serum, designated as GETV strain GETV/SW/Thailand/2017, and determined the complete genome sequence (11,689 nt). Phylogenetic analysis demonstrated that our isolate was different from the recent GETV group spreading among pig populations in East Asia and formed a cluster with two GETV strains, namely YN12031 (China, 2015) and LEIV16275Mar (Far-East Russia, 2007). We concluded that two different GETV groups are currently spreading among pig populations in Asian countries.