severe fever with thrombocytopenia syndrome
What are the main symptoms of severe fever with thrombocytopenia syndrome (SFTS)?
How is SFTS transmitted?
What is the cause of SFTS?
What animals are reservoirs for SFTS virus?
What is the typical course of SFTS infection?
severe fever with thrombocytopenia syndrome (SFTS), rapidly progressing tick-borne viral disease characterized primarily by the sudden onset of high fever and a significantly reduced number of platelets in the blood, a condition known as thrombocytopenia. These features are accompanied by leukopenia (low white blood cell count), fatigue, muscle pain, and gastrointestinal distress, marked by vomiting and diarrhea. SFTS progresses rapidly, with hemorrhage and multiple organ failure developing within days of infection and potentially leading to death. The disease carries a fatality rate estimated at between 5 and 30 percent, influenced by factors such as the individual’s age, the amount of virus present in the body (viral load), and the availability of timely medical care.
SFTS was discovered in 2009 in China, where it has since become endemic. Cases also have been reported in Japan and South Korea, where ticks that transmit the disease thrive. Cases of SFTS typically peak in the spring and summer, when people are more likely to spend time outdoors and the ticks that transmit the disease are more active.
Cause and transmission
SFTS is caused by SFTS virus (SFTSV), a type of RNA bunyavirus classified in the family Phenuiviridae. Although SFTSV is similar to other human hemorrhagic viruses in that it infects immune cells, it is unique for its tendency to attack B cells, as opposed to more common targets, such as dendritic cells, macrophages, and monocytes. SFTSV has a particular affinity for plasmablasts, a type of activated B cell that is crucial to launching early antibody responses. By invading and replicating inside these cells, SFTSV ultimately destroys them, disrupting antibody production and opening the way for the virus to multiply unchecked.
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Several species of hard-bodied ticks transmit SFTS, including primarily the Asian longhorned tick (Haemaphysalis longicornis), as well as Amblyomma testudinarium, Ixodes nipponensis, and Rhipicephalus microplus. Ticks acquire SFTSV after feeding on an infected animal. Livestock and other domesticated animals, such as cattle, cats, dogs, goats, and sheep, are major reservoirs for SFTSV. Wild animals, including various rodents, deer, wild boar, and migratory birds, also carry the virus. Collectively, these reservoirs and vectors create an ecological niche for SFTSV, facilitating its spread.
Humans become infected through the bite of a tick carrying SFTSV. Transmission is also possible through contact with contaminated bodily fluids, such as blood or saliva, from infected animals or humans. Thus, the risk of infection is increased for those who come in direct contact with animals. It is also elevated for those who work outdoors in or who are close to dense vegetation, since the ticks that transmit the disease are often found in vegetated areas, particularly in rural regions.
Course of infection
SFTSV has an incubation period of anywhere from 5 to 14 days, with an average of 9 days, before symptoms first emerge. SFTS typically begins with flulike symptoms, such as fever, headache, and muscle aches. These symptoms often are accompanied by vomiting and diarrhea. This phase lasts about a week. The second week of illness is considered the most dangerous, as the condition can rapidly worsen in this period, particularly between days 8 and 10. Progressive symptoms may include multiple organ failure, marked by kidney and liver dysfunction; neurological symptoms, including altered mental state and coma; hemorrhagic manifestations, such as disseminated intravascular coagulation (a severe blood clotting disorder); and myocarditis (inflammation of the heart muscle), which is a major risk factor for mortality.
Diagnosis and treatment
Early laboratory findings commonly show reduced platelet and white blood cell counts. Infection is confirmed by molecular analysis of serum samples, typically by reverse transcription-polymerase chain reaction (RT-PCR), which can detect viral RNA; serological testing that reveals a four-fold increase in anti-SFTSV immunoglobulin G is also indicative of infection. These approaches also help to distinguish SFTS from other tick-borne diseases, such as anaplasmosis and scrub typhus (tsutsugamushi disease).
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Treatment for SFTS is supportive, with the administration of intravenous fluids and antipyretic drugs to control fever. Platelet transfusions and blood products may be used to manage thrombocytopenia and bleeding. In some instances, antiviral drugs may be given; for example, favipiravir, when administered early in the course of infection, can potentially improve the chances of patient survival.