The four human cases reported in Thailand identified in Bangkok and Chachoengsao Province between 2019 and 2020, indicate that SFTSV has been spreading in the country. Therefore, we examined the seroprevalence of anti-SFTSV IgG antibodies in 1162 human samples collected from 940 healthy blood donors in Bangkok with a median age of 41 years (range 17–68 years) and from 222 individuals with dengue-like symptoms in Nong Khai Province, with a median age of 15 years (range 8 months-67 years). The results of the first screening for ELISA anti-SFTSV IgG antibodies revealed that 12 (1.1%) human samples were positive, 5 in donors (3 company employees and 2 civil servants) and 7 in patients. The median age of the positive individuals was 40 years (range 16–57 years), with a prevalence of males 10/12 (83.3%). After confirmation by VNT, none of the samples tested positive, indicating the absence of neutralizing antibodies (Table 1). Seroprevalence studies have also been performed in other countries, including China (0.2%-10.5%)28,29, Japan (0.0-4.2%)29–34, South Korea (1.9–7.7%)35,35–39, Pakistan (2.5%)15, and Vietnam (3.6%)16. The detection methods used were predominantly ELISA tests. Given that the seroprevalence rate in this current study was determined to be 1.1% using ELISA, this figure aligns with some studies from China, Japan, and South Korea. Further studies must be conducted to determine whether any viruses antigenically related to SFTSV are present in Thailand. For instance, Guertu virus (GTV), which was recently isolated from D. nuttalli ticks in China, is closely related to SFTSV and can also infect humans40.
Table 1
Human seroprevalence of severe fever with thrombocytopenia syndrome and molecular analysis of Rhipicephalus sanguineus ticks in Thailand.
Sample | Collection | Location | Province | SFTSV |
ELISA | VNT | qPCRa |
No. total | No. pos | No. total | No. pos | No. total | No. pos |
Human sera | Blood donors | Thai Red Cross | Bangkok | 940 | 5 | 5 | 0 | NA | NA |
Human sera | Dengue suspected | Phonphisai Hospital | Nong Khai | 222 | 7 | 7 | 0 | NA | NA |
Rh. sanguineus ticks | Dogs | Tha Takiap Subdistrict | Chachoengsao | NA | NA | NA | NA | 433 (50 pools) | 0 |
Rh. sanguineus eggs | Rh. sanguineus ticks | Tha Takiap Subdistrict | Chachoengsao | NA | NA | NA | NA | 12 pools | 0 |
aPathogens: SFTSV, Crimean-Congo hemorrhagic fever, Coxiella spp., Bartonella spp., and Rickettsia spp.
Abbreviations: ELISA: Enzyme-linked immunosorbent assay; NA: Not applicable; pos: Positive; qPCR: Real-time polymerase chain reaction; SFTSV: Severe fever with thrombocytopenia syndrome virus; VNT: Virus neutralization test
Severe fever with thrombocytopenia syndrome in domesticated animals has recently gained public attention for its potential role as an infection source to humans. For instance, the first human case of SFTS in Thailand was a 70-year old woman suspected of having been infected by cats, as all seven of her cats died13. In Thailand, the presence of stray or neglected companion dogs, and the widespread popularity of dog ownership, pose an important source of infection41, as clinical signs of SFTSV-infected dogs are relatively mild, making the diagnosis difficult and barely attracting enough attention to their owners and veterinary professionals32. Moreover, a SFTSV seroprevalence study published in 2023 and conducted in dogs, demonstrated a total seroprevalence of 16.6% (ELISA and seroneutralization) in the provinces of Bangkok, Chachoengsao, Samut Prakan, Rayong, and Chonburi, suggesting that infection among the canine population may be similar to those in endemic regions of China4. Rh. sanguineus tick infestation in dogs is high (e.g. north-eastern Thailand 80.0%), which represents public health significance due to its greater likelihood of spreading pathogens. Since it requires different hosts for each blood meal, and in the era of globalization and climate changes, it has been suggested that ticks exposed to high temperatures attached more to humans, potentially increasing the risk of transmission of certain zoonotic pathogens11. Therefore, we collected 433 Rhipicephalus sanguineus sensu lato ticks from dogs in rural communities of Chachoengsao Province, Thailand, where one human case of SFTSV was identified14. Two hundred fifty-three (58.4%) ticks were male, 55 (12.7%) were females and 125 (28.9%) engorged female ticks. Almost all were adults, except for four female nymphs. In addition, 12 tubes of eggs were collected. All pool of ticks and eggs were tested for the detection of SFTSV (Table 1). However, none of the pools were positive for RNA detection. As Haemaphysalis longicornis serves as the primary vector, and considering that livestock are common hosts for this tick42, we also investigated the presence of tick on livestock in the same area. However, the use of insecticides limited the collection.
Rh. sanguineus ticks have been identified as responsible for transmitting various well-known canine pathogens, including Babesia vogeli, Babesia canis, Hepatozoon canis, and Mycoplasma haemocanis, as well as pathogens capable of infecting both humans and dogs, such as Ehrlichia canis, Anaplasma platys, and several zoonotic Rickettsioses11,43,44. In this study, we tested less common but important pathogens that cause important health problems in humans, mainly transmitted by other tick species, but also found in Rh. sanguineus ticks, including CCHF, Coxiella spp., Bartonella spp., and Rickettsia spp. However, we did not detect RNA or DNA from the collected tick samples.
This study has some limitations. Firstly, the seroprevalence study involved a disproportionately higher number of samples from urban areas compared to rural ones, with only a few samples from farmers. The sample size was not large enough to accurately reflect the entire country’s population. Additionally, the study did not consider the influence of socioeconomic factors, outdoor activities, or history of tick-bites. Secondly, molecular identification was performed on a limited number of ticks, focusing on one tick and animal species, and was confined to a specific area.