Oregon officials closed the state’s entire coast to shellfish harvesting on Friday after an unprecedented outbreak of paralytic shellfish poisoning, which appears to also be affecting parts of Washington and California.

During a press conference Friday morning, officials said Oregon had never experienced such a large outbreak before, in terms of both the toxic levels being detected in shellfish and the number of people becoming ill.

At least 20 people who harvested shellfish have become ill. An Oregon Health Authority spokesperson said most illnesses have been mild, although some people required hospitalization.

“These are unprecedented levels that we’re seeing, and especially in bay clams and oysters,” Matthew Hunter, shellfish program manager with the Oregon Department of Fish and Wildlife, said.

Paralytic shellfish poisoning is the most severe type of shellfish poisoning. It is caused by eating shellfish contaminated with saxitoxins, a naturally occurring toxin. Many kinds of shellfish can be contaminated by this toxin, but it most often affects mussels and clams. Saxitoxins and similar biotoxins affecting shellfish are tied to algal blooms in the ocean. These blooms are colloquially called “red tides” or “brown tides,” though they don’t always color the water.

Symptoms usually appear within an hour after a person eats toxic shellfish. They include numbness and tingling of the face, lips, tongue, arms and legs, as well as digestive issues like diarrhea, vomiting and nausea. Extreme cases could include neurological symptoms, like paralysis, or respiratory failure.

Cooking shellfish will not destroy these poisonous biotoxins, and there is no antidote for biotoxin poisoning. People who become ill need to wait for the toxins to naturally flush from their bodies.

Hunter said officials need to do more testing to see if these toxin levels have hit their peak, or if they are starting to decline.

“I doubt we’re at the decline because we’re seeing Washington have some closures,” Hunter said.

California closed part of its northern coast to razor clam harvesting in early May due to domoic acid, which is tied to a different type of biotoxin. That state has yet to close parts of its coast to shellfish harvesting because of paralytic shellfish poisoning, but Hunter said he “would be very surprised if this isn’t an event that’s covering all three states.”

Oregon first detected high levels of toxins among shellfish along the Central Oregon Coast on May 17. Since then, high levels of toxins have been found in several different types of shellfish across the Oregon coast, including bay clams, razor clams, oysters and mussels.

Paralytic shellfish poisoning has a long history of sickening people and animals along the West Coast, but Hunter said it and other shellfish poisons appear to be getting worse along with climate change.

“We’ve shown that the changing of the oceans has definitely increased the frequency, the duration and the toxicity of all of our biotoxins,” Hunter said. “That’s definitely something that we’re going to have to do adaptive management for.”