BOSTON (AP) – Environmental officials closed two areas off Martha’s Vineyard to shellfishing on Monday, as the largest red tide outbreak in decades continued its spread through New England waters.

The toxic algae bloom already has forced officials to shut down shellfish beds from Maine to Cape Cod so that people do not eat infected clams, mussels and oysters.

The closures off Edgartown meant the outbreak, possibly caused by an unusually cold and wet winter and spring, was continuing south, scientists said.

“It’s an unprecedented closure,” said Shelley Dawicki, spokeswoman at the Woods Hole Oceanographic Institution. “We don’t know how far it’s going to spread.”

Corbie Kump, a spokeswoman at the state Executive Office of Environmental Affairs, said state and federal officials were monitoring the bloom as it appeared to be heading toward federal waters.

Scientists say the algae contaminate only shellfish, making them unsafe for animals and humans to eat. Swimmers, fish, and popular sea foods such as lobster or shrimp are unaffected.

This year, strong east and northeast winds, including two May nor’easters, blew in a particularly heavy algae bloom that flourished in warmer coastal waters. The outbreak is the worst to hit Massachusetts since 1972, when the state enacted a blanket closure of all shellfish beds.