Redback spider bites increased to 13 in Osaka Prefecture this year, the most since 1995 when the poisonous arachnid was first thought to have migrated from Australia on cargo vessels.

The number of reported cases increased from nine last year, according to the prefectural government's Web site. In June, a 6-year-old boy in Osaka was given the antivenom after being bitten, the first case of the treatment being used in Japan.

"We have low reserves of the antivenom," Takashi Kuramochi, a spokesman for the prefecture's Environmental Hygiene Department, said Wednesday. "I don't know why the number of redback bites is up."

The redback, common in Australia and related to the black widow, delivers a bite that can cause severe pain and possibly death, according to the Australia Museum's Web site. No deaths from the spider's bite have been reported after Australia introduced antivenom, the Web site says.

Called Seakagoke-gumo in Japanese, the spider was first found in 1995 in Takaishi in southern Osaka. It has since been sighted in 16 prefectures.

No deaths from bites by redbacks, so named because of the red stripe on their abdomen, have been reported in Japan. Females of the species have bodies up to 1 cm in length, while males are usually 3 or 4 mm long.