I'm really scared. Tick ​​infection "severe fever with thrombocytopenia syndrome (SFTS)"


Ticks are familiar to hunters who deal with beasts and mountain hikers. It is completely different from the mites that live in the home.

Ticks are relatively large mites covered with a hard outer skin, and live mainly outdoors, such as in forests and grasslands. Ticks can also be found around urban areas, and are common in parks with many trees and bushes.

This article describes the characteristics and types of ticks and the infectious disease they carry, "severe fever with thrombocytopenia syndrome (SFTS)".

Have you ever been bitten by a tick?

Ticks basically lurk among the leaf litter on the ground to wait for wild animals, or lie in ambush on the tips of leaves.

Ticks have a sensory organ called Hurler's organ, which responds to carbon dioxide emitted by mammals, body temperature, body odor, and physical vibrations. When an animal passes by, it senses it and jumps up on it, sticking to it and sucking blood. When it becomes a large individual, it will bite even through thick clothing.

For hunters who often walk animal trails, being targeted by ticks may be destiny in a sense.

Blood-sucking by ticks may not be as short as mosquitoes, but may last from several days to two weeks. Therefore, the blood-sucking behavior of ticks is sometimes called "kochaku" to distinguish them.

The size of the tick is 3 to 8 mm before sucking blood, and swells up to 10 to 20 mm after sucking blood, depending on the type. Ticks tend to bite where they bite depending on the species, but in humans, they tend to bite behind the knees, genitals, chest, and armpits.

A tick's saliva contains an anesthetic, and many people are unaware that they are sucking blood. Depending on the individual, the site of the tick bite may become very itchy for several weeks. It may also blister and suppurate.

By the way, if you pick off a tick that has bitten you, the mouthparts may be torn off and remain in the skin.

If the mouthparts remain in the skin, they become inflamed and swollen as the body's defense response to the foreign object. If you visit a dermatologist in that state, you may need local anesthesia and an incision to remove the foreign body.

Type of tick

In Japan, 2 families, 8 genera, and 47 species of ticks have been confirmed. I will introduce representative species of them.


It got its name because it has a thin body and looks slightly yellowish. It is collected even in the middle of winter in the warm plains west of the Kanto region. The body length is about 3 mm, and when it swells, it is about 8 mm.

Ixodidal tick

It has a unique shape and is about 2-3mm in size, and 5-20mm when swollen. Large numbers of large wild mammals such as deer and bears and grazing cattle are parasitized. In addition to wild animals, they sometimes attach themselves to dogs while they are walking, and are often detected indoors.

Yamato tick

It is said to be the most widely-distributed tick among the genus Ixodes, and is the most common type of tick that infests humans. It also parasitizes house mice, etc., and can be seen around houses.

What really scares me is the infectious disease SFTS

If only itching and excision are enough, it is not such a big problem, but in very rare cases, severe febrile thrombocytopenic syndrome (SFTS) develops due to ticks carrying an infectious disease virus.

Case reports increase every year from April to November, and 355 cases in 23 prefectures have been confirmed so far in Japan (including 63 deaths). Last month, a man in his 80s in Onomichi City, Hiroshima Prefecture, was infected with SFTS and died.

According to a survey by the National Institute of Infectious Diseases, the SFTS virus antibody prevalence rate among 370 wild boars and 502 deer captured in Yamaguchi Prefecture was found to be 8.6% for wild boars and 43.2% for deer. It was said that it was a positive rate.

Symptoms of SFTS

The main symptoms are fever and gastrointestinal symptoms. As mentioned earlier, it can lead to severe illness and even death. The fatality rate* of SFTS in Japan is said to be about 20%, and it is believed that the elderly are particularly susceptible to severe disease.

*The fatality rate is the percentage of reported patients diagnosed with a particular disease who die within a specified period of time.

After an incubation period of 6 days to 2 weeks after being bitten by a tick, fever and digestive symptoms (decreased appetite, nausea, vomiting, diarrhea, abdominal pain) appear. Occasionally, headache, muscle pain, neurological symptoms (disturbed consciousness, convulsion, coma), lymphadenopathy, respiratory symptoms (cough, etc.), and bleeding symptoms (purpura, melena) may occur. Early symptoms are similar to acute gastroenteritis and influenza, but there is no vaccine.

Measures to prevent infection

To prevent infection from ticks, the Ministry of Health, Labor and Welfare recommends the following preventive measures.

(1) Wear long sleeves, long pants, shoes that completely cover your legs, hats, gloves, and wrap a towel around your neck to reduce skin exposure.

(2) After outdoor activities, wipe your body and clothes to check for tick bites. Do not bring jackets or work clothes into the house.

(3) If you find a tick that is sucking blood, treat it at a medical institution as much as possible.

(4) If you have symptoms such as fever after being bitten by a tick, seek medical attention.


For hunters and people who spend a lot of time outdoors, talking about infectious diseases may be something they don't want to hear about, but it's important to be aware of the risks of infection and how to deal with them.

Many people have already taken the above preventive measures, but if you are bitten by a tick, pay attention to changes in your physical condition, and if you develop a fever or digestive symptoms, seek medical attention as soon as possible.

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