TOKYO - Three deaths from a viral disease transmitted by wild ticks have been confirmed in the nation, prompting health authorities to urge the public to stay calm.
Five more suspected cases in which people died or became seriously ill after being infected with the virus that caused Severe Fever with Thrombocytopenia Syndrome (SFTS) have also been reported.
However, an official of the Health, Labor and Welfare Ministry said a series of such reports does not necessarily mean the risk of the infectious disease has increased.
In late January, a woman in Yamaguchi Prefecture was confirmed to have died from SFTS, while earlier this month, two men in Ehime and Miyazaki prefectures were found to have died from the infection. They died last autumn.
The cause of their deaths was determined after part of the SFTS virus' DNA was detected in blood and other samples examined by the National Institute of Infectious Diseases (NIID).
The examinations showed the DNA of the SFTS virus found in the woman's blood samples was slightly different from that of the virus found in patients in China, where SFTS viruses have reportedly spread. The health ministry therefore suspects the viruses detected in Japan and China have different origins.
In an effort to ascertain the situation in Japan, the ministry intends to require doctors to report to health authorities patients infected with SFTS under the law concerning the prevention of infectious diseases and medical care for patients with infections.
Despite the reported deaths, a ministry official played down the risk of an epidemic.
"The SFTS virus appears to have existed in Japan for a long time. Cases of the infection have been reported lately perhaps because it has become possible to identify the source of the infection. So we don't necessarily face a higher risk of infection," the official said.
House dust mites are different
Ixodid ticks are different from house dust mites and other species of ticks normally seen indoors, in terms of ecological forms and habitats. A species of ixodid tick--believed to be a host of the SFTS virus in Japan--inhabits forests and mountains across the nation.
The species grows in stages by sucking the blood of animals such as mice and boars and shedding its skin. An adult of the species is normally three to four millimeters long, but expands to more than one centimeter after feeding. Ixodid ticks feed once in each growing stage.
Meanwhile, house dust mites are about 0.5 millimeter to one millimeter long and parasitize animals such as mice. The mites become active usually at night and feed on people in bed, for example. Dust mite bites cause intense itching, but the mites are not believed to carry the SFTS virus.
Dr. Masaru Natsuaki of the Hyogo College of Medicine Hospital's dermatology department said: "Mites seen at home and ixodid ticks are significantly different in appearance and characteristics. It's important not to mix them up."
Wear clothes that cover skin
The ixodid tick population is active from spring to autumn. The tick's thin, pointed jaw is narrower than a hypodermic needle, so its bites are painless, even afterwards. As a result, people often do not realise they were bitten by a tick.
Wearing clothes that do not expose the skin during activities in the mountains or in grass is an important safety precaution for tick bites.
Insect-repellent sprays and other medicines are said to be effective in reducing tick bites, though there is no solution that can completely prevent them.
It also is important to be able to recognise what an ixodid tick looks like.
Ixodid ticks can also live in the grass of parks, urban areas or humid yards inhabited by animals.
However, Yasuhiro Yano, associate professor of Fukui University and an expert on ixodid ticks, said the ticks may have difficulties living in urban areas as they are sensitive to dry air.
Not all ixodid ticks carry the SFTS virus, and the infection is directly transmissible only through blood-to-blood contact. The virus cannot be transmitted through the air or tiny water droplets, such as those expelled during coughing.
According to research conducted in China, where numerous people have become infected with the virus through the ixodid ticks, the virus-carrying rate was 5.4 per cent, sources said.
"An ixodid tick bite does not necessarily mean you will get the disease, so there's no need to be too afraid of ixodid ticks," Yano said. "But its jaw is sometimes lodged into the skin, so if you notice it, it's best not to try to take it out yourself. Go to a hospital's dermatology department instead."
Monitoring the symptoms
The incubation period for the SFTS virus ranges from six days to two weeks. A person infected with the virus may experience a fever above 38 C, nausea and diarrhoea.
Masayuki Saijo, director of the NIID's Virology Department, said SFTS virus symptoms are very similar to cold symptoms. "It's impossible for the lay public to determine whether the symptoms are caused by the SFTS virus," he said.
If symptoms increase in severity, platelets or white blood cells can decrease or bleeding can even occur below the skin.
In the worst-case scenario, patients could die of multiple organ failure.
Ixodid tick bites may not leave visible marks on the skin. Therefore, a person who suffers a fever for several days after returning from the mountains is advised to see a doctor.
Doctors have been asked to verify whether patients with such symptoms have been infected with the virus.
"If it is suspected [that a patient has been infected with the virus], doctors can send a blood sample to the NIID to ensure accurate diagnosis. However, there is no need to panic simply because you have a fever," he said.
There is no remedy or vaccine for the virus, but the symptoms can be alleviated through treatments such as a blood transfusion to make up for blood loss or intravenous drips to prevent a drop in blood pressure.
In China, large-scale outbreaks of infections carried by ixodid ticks have been reported recently. Several cases of infection have been detected in Japan, but the disease has not spread since.
It is highly suspected that the virus did not become acute all of a sudden, but that past cases were overlooked.
In China, 97 per cent of those infected with the virus were farmers living near mountains or hillside, with many of them over 40 years old.