Tick Species Associated with SFTS in China
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SFTS or Severe Fever with Thrombocytopenia Syndrome - a tickborne Phlebovirus - was first discovered in China in 2009, but has also been found in Japan and Korea. It is genetically similar to the recently discovered Heartland Virus (see MMWR: Heartland Virus Disease — United States, 2012–2013) that has been detected in roughly a dozen people here in the United States.
Phleboviruses are part of the very large family Bunyaviridae.
To date over three hundred Bunyaviruses have been identified around the world, with rodents often cited as carriers. While not all Bunyaviruses are dangerous to humans (some only infect plants), the Bunyavirus family include such nasties as Crimean-Congo hemorrhagic fever (CCHF), Hantaviruses, and Rift Valley Fever.
Most are spread via arthropod vectors (ticks, mosquitoes & sand flies), with the notable exception of Hantaviruses (see Hantaviruses Revisited), which are primarily spread via virus-laden feces and urine of rodents.
In 2011, we got our first good look at the disease when the NEJM published a study Fever with Thrombocytopenia Associated with a Novel Bunyavirus in China. STFS has previously been associated with a 12% mortality rate in China. Since then we’ve seen reports both from Japan and South Korea (see Korean CDC On SFTS Cases) of fatal cases as well.
Like with CCHF, under the right conditions, human-to-human transmission of SFTS (via blood, mucus or other bodily fluids) has been documented. (see Person-to-person transmission of severe fever with thrombocytopenia Syndrome virus)
As a relatively newly discovered pathogen, we are just beginning to learn about its prevalence, both in ticks and in humans in SFTS endemic areas.
Today, we’ve excerpts from two dispatches published in the CDC’s EID Journal that help in that regard.
The first, looks at the various species and the rate of SFTS infection among ticks collected South Korean. They found SFTSV in 5.7% of H. longicornis ticks tested, which represented 81.2% of the ticks sampled. Other species tested were also found to carry the virus, but it isn’t yet known whether they can transmit it to humans.
Severe Fever with Thrombocytopenia Syndrome Virus in Ticks Collected from Humans, South Korea, 2013
Seok-Min Yun1, Wook-Gyo Lee1, Jungsang Ryou, Sung-Chan Yang, Sun-Whan Park, Jong Yeol Roh, Ye-Ji Lee, Chan Park, and Myung Guk Han
Author affiliations: Korea Centers for Disease Control and Prevention, Cheongwon-gun, South Korea
Abstract
We investigated the infection rate for severe fever with thrombocytopenia syndrome virus (SFTSV) among ticks collected from humans during May–October 2013 in South Korea. Haemaphysalis longicornis ticks have been considered the SFTSV vector. However, we detected the virus in H. longicornis, Amblyomma testudinarium, and Ixodes nipponensis ticks, indicating additional potential SFTSV vectors.
The second study, this time out of China, is a seroprevalence study of healthy individuals, looking for evidence of prior infection with SFTS, without a history of severe illness.
Antibodies against Severe Fever with Thrombocytopenia Syndrome Virus in Healthy Persons, China, 2013
Lei Zhang1, Jimin Sun1, Jie Yan, Huakun Lv, Chengliang Chai, Yi Sun, Bin Shao, Jianmin Jiang, Zhiping Chen, and Yanjun Zhang
Abstract
In June 2013, a subclinical infection with severe fever with thrombocytopenia syndrome virus (SFTSV) was detected in Zhejiang Province, China, prompting seroprevalence studies in 6 districts within the province. Of 986 healthy persons tested, 71 had IgG antibodies against SFTSV. This finding suggests that most natural infections with SFTSV are mild or subclinical.
Conclusions
SFTSV can cause severe disease and high rates of death among infected hospitalized patients. The virus also has the limited ability to be transmited from person to person through contact with contaminated blood, but secondary cases are generally less severe and have so far not resulted in fatalities (7–9). Nonetheless, there is great public health concern regarding SFTSV.
Our seroprevalence study was prompted by the identification of a subclinical, secondary infection that was most likely caused by person-to-person transmission of the virus from an infected family member with a fatal case of SFTS. We found an overall SFTSV seroprevalence of 7.2% among 986 healthy persons who reported no symptoms associated with SFTS. Because the seropositive participants in our study did not have contact with persons with diagnosed cases of SFTS, their infections most likely occurred through natural exposure. From this, we conclude that SFTSV infections are widespread in rural areas of Zhejiang Province, and only a small percentage of the infections result in clinical disease.
Like a lot of viruses, infection with SFTS appears to carry with it a wide range of illness – ranging from subclinical to mild to occasionally life-threatening. Very few viruses are universally fatal (rabies comes close, though).
This is a pattern we see with everything from influenza to West Nile Virus to MERS. It is also a fact of life that surveillance is far more likely to pick up severe cases than mild ones, and that can sometimes skew our perception of just how deadly an emerging pathogen really is.
You don’t have to travel to Asia to be exposed to a potentially serious tickborne disease. The CDC maintains a long (and growing) list of of tickborne pathogens available locally, including:
Babesiosis , Ehrlichiosis, Lyme disease, Rickettsia parkeri Rickettsiosis, Rocky Mountain Spotted Fever (RMSF), STARI (Southern Tick-Associated Rash Illness), Tickborne relapsing fever (TBRF), Tularemia, and 364D Rickettsiosis.
Whether a new and emerging threat, or simply our ability to finally recognize a long-time nemesis, it makes sense to take precautions against ticks and other vector-borne diseases.
For help in that regard, the following CDC website offers advice on:
And for more on tickborne pathogens, you may wish to revisit:
EID Journal: Seroprevalence Of B. Miyamotoi In N.E. United States
CDC: Estimate Of Yearly Lyme Disease Diagnoses In The United States
