The prime minister yesterday branded an extortionist’s claim to have infected a New Zealand island with a livestock disease as “bioterrorism,” while police revealed that the extortionist threatened to release the virus on the mainland later this week.

Officials have not confirmed that any livestock have been infected with the deadly foot-and-mouth disease as alleged and stress that they believe the claims made in a letter Tuesday to Prime Minister Helen Clark are a hoax.

But they consider it an emergency that potentially could devastate the country’s agriculture-dependent economy. The EU and Japan already have asked that livestock from the island be blocked from their markets.

“I regard this as a bioterrorism threat and very severe penalties are in place for such crimes,” Clark said in her first public comments about the disease scare. “The implications for damage to our economy of a genuine scare would be catastrophic.”

The letter warned that foot-and-mouth virus had been released Monday among stock on Waiheke Island near Auckland. It demanded ransom and changes in government tax policy. Police yesterday revealed that the letter writer also threatened to release the disease in New Zealand tomorrow.

“Friday is the next date for further threats,” police assistant commissioner Peter Marshall said.

“We have to take this very seriously even though we are reasonably confident it is a hoax,” Clark told a Canterbury student radio station.

Under the nation’s Biosecurity Act, anyone convicted of spreading foot-and-mouth faces up to five years in prison. Under other laws, threatening to seriously damage the economy carries a seven-year sentence, and causing disease in animals carries a maximum 10-year sentence. Animals on Waiheke — home to 18,000 sheep and 2,500 cattle — have been quarantined and are being inspected every 48 hours for symptoms.

The letter claimed animals had been fed hay infected with foot-and-mouth virus on Monday. Incubation of the disease normally takes 4-7 days. Foot-and-mouth is a highly contagious virus that affects cloven-hoofed animals such as cattle, sheep and pigs, causing sores, blisters and fever. It is deadly for livestock but harmless to people.

About half the economy in New Zealand, which has never confirmed a foot-and-mouth case, is based on agriculture and analysts believe an outbreak could affect a broad range of its annual US$22 billion in exports. Checkpoints have been set up at the island’s two ports and airport, and controls imposed on the movement of all animals and animal products to and from the island.

Roger Morris, a leading foot-and-mouth specialist, said “it’s most likely to be a hoax … but one can never trust that it is a hoax. You must take all necessary measures, then confirm it was a hoax.”

After New Zealand trade officials briefed 40 nations and the 15-member EU on the scare, the EU and Japan requested that no exports from the island be sent to their markets. Malaysia, Brazil and some small Middle East countries had still to give a formal response, and Mexico indicated it would likely inspect shipments from New Zealand before approving their import, said New Zealand Food Safety Authority executive director Andrew McKenzie. Australia did not ban imports.