SALEM, Ore. (AP) — Oregon authorities have expanded shellfish harvesting closures along the state’s entire coastline to include razor clams and bay clams, as already high levels of toxins that have contributed to a shellfish poisoning outbreak continue to rise.

The Oregon Department of Fish and Wildlife said the new closures were due to “historic high levels” of a marine biotoxin known as paralytic shellfish poisoning. The move, announced by the department in a news release on Thursday, came after state officials similarly closed the whole coast to mussel harvesting last week.

Agriculture officials have also closed an additional bay on the state’s southern coast to commercial oyster harvesting, bringing the total of such closures to three.

Elevated levels of toxins were first detected in shellfish on the state’s central and north coasts on May 17, fish and wildlife officials said.

The shellfish poisoning outbreak has sickened at least 31 people, Jonathan Modie, spokesperson for the Oregon Health Authority, said in an email. The agency has asked people who have harvested or eaten Oregon shellfish since May 13 to fill out a survey that’s meant to help investigators identify the cause of the outbreak and the number of people sickened.

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Officials in neighboring Washington have also closed the state’s Pacific coastline to the harvesting of shellfish, including mussels, clams, scallops and oysters, a shellfish safety map produced by the Washington State Department of Health showed.

Paralytic shellfish poisoning, or PSP, is caused by saxitoxin, a naturally occurring toxin produced by algae, according to the Oregon Health Authority. People who eat shellfish contaminated with high levels of saxitoxins usually start feeling ill within 30 to 60 minutes, the agency said. Symptoms include numbness of the mouth and lips, vomiting, diarrhea, and shortness of breath and irregular heartbeat in severe cases.

There is no antidote to PSP, according to the health agency. Treatment for severe cases may require mechanical ventilators to help with breathing.

Authorities warn that cooking or freezing contaminated shellfish doesn’t kill the toxins and doesn’t make it safe to eat.

Officials say the Oregon Department of Agriculture will continue testing for shellfish toxins at least twice a month as tides and weather permit. Reopening an area closed for biotoxins requires two consecutive tests that show toxin levels are below a certain threshold.