A tourist suffered heart inflammation after being bitten on the penis apparently by a katipo spider, following a skinny-dip in Northland.

The 22-year-old Canadian left his clothes in the sand dunes while he went for a nude swim and slept on his return, according to a report on the case in today's online NZ Medical Journal. "He woke to find his penis swollen and painful with a red mark on the shaft suggestive of a bite. He rapidly developed generalised muscle pains, fever, headache, photophobia [light sensitivity] and vomiting," writes Dr Nigel Harrison, of Whangarei Hospital, and colleagues.

"It was a rather nasty, ill-placed bite," Dr Harrison told the Herald yesterday.

Alleged spider bites were the cause of 20,000 claims to the Accident Compensation Corporation in 2005-06, a separate report in the journal says.

By the time the Canadian reached Dargaville Hospital, his penis was severely swollen, his blood pressure was up and his heart beat racing.

Chest pain and other symptoms developed the next morning and it was presumed he had been bitten by a katipo. He was treated with antivenom medicine sent from Whangarei Hospital and rapidly improved.

However heart problems persisted and he was treated at Whangarei Hospital and Auckland Hospital before returning to Canada.

"He was discharged after a total of 16 days in hospital. On review he was generally well," the journal report says.

It is the first known case of myocarditis - heart inflammation - following a katipo spider bite.