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Bug Catchers at Forefront of Disease Prevention

While epidemiologists, virologists and microbiologists have been busy looking for ways to fight the deadly H5N1 avian influenza virus, Liang Guodong has been working in the wilderness catching mosquitoes.

Over the past two years, a team of scientists led by Liang, deputy director of the Virological Institute, have travelled across the country to learn more about arboviruses, or viruses spread by insects. The Virological Institute is affiliated with the Chinese Centre for Disease Control and Prevention.

The team has already identified 123 strains of arboviruses, 51 of which scientists did not previously know about, says Liang, adding that the discoveries have raised as many questions as they have answered.

Some of the unknown viruses could evolve or mutate to cause large-scale epidemics, such as severe acute respiratory syndrome (SARS), which spread quickly throughout the world three years ago.

"We can't rule out that possibility," says Liang.

Arboviruses are viruses carried by bloodsucking insects such as mosquitoes, midges, fleas and bedbugs, and these viruses are able to replicate in their hosts. These diseases do not affect the host insects, but they can be spread to other animals and human beings. Hosts can spread illnesses such as yellow fever, Japanese Encephalitis Virus (JEV), dengue fever, West Nile virus disease, and rift valley fever.

Some of these diseases have spread in the past throughout Europe, the Americas, Asia and Africa, killing tens of thousands of people in each outbreak, says Liang.

Yet he says a more serious issue is widespread public ignorance of the problem.

"You can't predict when or whether you will be bitten by mosquitoes, and many arbovirus diseases have symptoms quite similar to colds and common influenza," Liang says.

Over the past 30 years, Liang has witnessed a number of serious arbovirus disease outbreaks.

One hot summer in a village in east China's Zhejiang Province, two people were suddenly infected by dengue fever. Symptoms can include fever for three to five days, intense headaches, gastrointestinal problems and skin rashes. Relatives of the two infected villagers did not take them to hospital because they thought they were simply suffering from common colds. Half of the village fell ill within a matter of days however.

"The spread of dengue could have been avoided if the farmers had put those patients under quarantine and made sure that mosquitoes didn't reach them," says Liang.

But he admits it is quite difficult to do this, because of a general lack of knowledge and the prevalence of disease-carrying insects in rural China, especially throughout hot and wet areas in the south.

Although they have caused millions of deaths, arboviruses have been poorly studied. Only 35 types had been identified throughout the world by the 1950s. Military programmes to prevent germ warfare and huge advances in microbiology had led to the discovery of 535 arboviruses by 1992. These viruses have since been categorized into 14 virological classes. They have already identified more than 100 types that cause diseases in human beings.

"Arboviruses can spread across every continent in the world, especially in tropical and subtropical regions, where insects are more active," says Liang.

He adds that China could be home to a wide range of arboviruses because stretches from the tropics to northern temperate zones.

"In China's rural areas, ponds, sties and living areas are close to each other, providing ideal conditions for mosquitoes to spread arboviruses between livestock, poultry and human beings," Liang says.

Since the 1950s, Chinese scientists have mainly focused on four arboviruses, including JEV, Spring-Summer Encephalitis Virus, Xinjiang Hemorrhagic Fever Virus, and Dengue Fever Virus. Of these four, JEV, or type-B Encephalitis, has been the biggest problem, causing between 20,000 and 40,000 human infections each year. Large-scale vaccinations have prevented the disease from spreading further.

Nationwide survey

"Although there have been relatively few outbreaks of arboviruses in China, this is not because we are luckier than other countries," says Liang. "Ironically, this is because we haven't conducted enough research. We may not know the real situation."

He says that remote and poor regions are most often affected by arboviruses. China's epidemic control teams are based in urban centres and are thus unable to visit these places frequently.

Scientists cannot leave these viruses unidentified, however, because they pose potential public health threats, Liang adds. This was particularly true after the sudden SARS outbreak, which claimed 813 lives worldwide and infected more than 8,000 people in 2003.

The Ministry of Science and Technology decided in December 2003 that a nationwide survey on arboviruses would be included under the national 973 Programme research project. With assistance and funding from the ministry and regional branches of the Chinese Centre for Disease Control and Prevention, Liang, his colleagues, and scientists from the Military Medical Science Academy began to look at 16 different regions. The project covered southwest China's Yunnan and Guizhou provinces, as well as northeast China's Heilongjiang Province and northwest China's Xinjiang Uygur Autonomous Region.

"We select sample sites in accordance with previous epidemics or based on past evidence. In each place we tried to collect enough live mosquitoes or midges, but it wasn't easy," says Liang. Mosquito traps help, but they have to be closely monitored to ensure the right types of insects are caught.

Preventing the further spread of disease among researchers at these sites is a serious issue.

"We can't use too much mosquito repellent because this could keep the insect away. Protective clothing is also inconvenient because of the often extremely hot and wet weather in the south," Liang says.

He admits that exposure during research carries its share of risks.

"But it is a process that shows whether a particular arbovirus is powerful," he says.

After researchers trap the sample insects, they immediately put them into liquid nitrogen kettles to keep the virus alive. This means researchers always must carry heavy nitrogen kettles, regardless of how remote, rough or difficult the study sites are.

Over the past two years of fieldwork, scientists from the two institutes have collected 110,000 insect samples, rats and bats, as well as samples from sick livestock and human beings.

Unfinished work

Liang and his colleagues have isolated and identified viruses in the samples that have been collected. Of the 123 varieties of arboviruses, a number of them belong to different variations of the Japanese Encephalitis Virus.

"The survey indicates the arbovirus situation in China is more serious than we expected," says Liang. "But I can't say how serious it is, partly because our work has not been finished, and partly because our basic research hasn't been comprehensive enough."

The so-called national survey has only covered a few points in each of the 16 provinces investigated so far. The results do not yet cover a wide enough range of geographical regions and climate zones.

Liang says vaccination is the only effective way to prevent most arboviruses. Vaccines based on modern genetics are generally the most effective.

"More investment in epidemic research and close cooperation between epidemiologists and molecular biologists are necessary to reach this goal," says Liang.

(China Daily February 7, 2006)

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