There are fears in Japan that a potentially deadly Australian invader is beginning to close in on some of the country's most crowded urban areas.

It is believed redback spiders originally hitched a ride in a container ship, possibly one carrying woodchips.

A few months ago a six-year-old boy from Osaka was bitten by a redback in the first case where anti-venom was used in Japan.

His case is just one of a dozen this year in which Japanese have been nipped by this Australian arachnid, which is spinning its silk web from Nagoya to Fukuoka, to Osaka.

In fact the Environment Ministry says redbacks have now been reported in 16 of Japan's 47 prefectures. And in Osaka prefecture alone this year, 12 people have been bitten.

The managing director of the Aichi Prefecture Pest Control Association, Takesada Ohashi, is concerned by the trend.

"If redback numbers keep increasing and they spread throughout Osaka city, then I'll be very worried that more people could be bitten," he said.

He says the big fear is they may even be bitten without knowing.

Serious risk

If people are bitten they will need anti-venom, but it has not yet been officially approved for use in Japan.

Only a handful of hospitals in western Japan stock the anti-venom and luckily for the six-year-old Osaka boy, the anti-venom was in stock.

A former researcher at the Osaka Institute of Public Health, Masahiro Yoshida, says the Japanese government has done nothing.

"They only deal with the redback problem after someone is bitten," he said.

"Accidents must be prevented, so I think homes should be checked."

Dr Yoshida warns that someone will die if no steps are taken to halt the spider's spread.

Chopstick defence

Mr Ohashi has a uniquely Japanese approach to dealing with the deadly pests.

"In the case of big factories, we must move in and exterminate at once," he said.

"But in the case of households, we can use pesticides, although I'd rather pick up the redbacks with chopsticks and then kill them.

"Pesticides can flow into water supplies."

What is alarming many in Japan is that redbacks can survive the biting Japanese winter, which officially begins next week, mostly by nesting near heating systems.

And with electrically warmed toilet seats in almost every home, it may be best for the Japanese to check carefully before sitting.