ANNE ARUNDEL COUNTY, MD -- It happened on the Magothy River in 2004, and again in 2014.

Now the alarm is being raised over a population explosion of tiny shellfish which some fear have been linked to deaths in dogs that play in the water and eat them.

"There is some relationship with the dogs where it becomes toxic for them," say Paul Spadaro,

President of the Magothy River Association in Severna Park which has posted an advisory for pet owners on its Facebook page.

Spadaro began to notice an explosion in the population of shellfish called False Dark Mussels on the Cattail Creek tributary of the Magothy River a few weeks ago. The mussels cling to everything in the water, forming huge underwater clumps on pilings, boat hulls, and swim ladders as the summer progresses.

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The outbreak may be linked to heavy rains earlier in the year, Spadaro said.

During a similar explosion in 2004, at least 20 dogs got sick with symptoms of suspected liver poisoning. Half of them died. Another episode of dog illnesses occurred during a mussel boom in 2014.

Spadaro said dogs that love water play can ingest mussels by chewing waterlogged sticks and other items they find in the river.

Maryland environmental and natural resources officials who have studied the mussels have never identified a toxin that might be harmful to dogs.

While they have not been able to rule out a relationship between the mysterious dog illnesses and outbreaks of mussels, they are not able to conclude the tiny shellfish are the cause.

"We can't say there's a linkage," said Bruce Michael of the Maryland Department of Natural Resources.

Ironically, the mussel outbreaks have been linked to temporary dramatic improvements in water quality because the little shellfish filter large volumes of water as they feed on plankton and other microscopic food sources.

Billions of mussels have turned the usually murky waters of the Magothy river nearly crystal clear in previous outbreaks.

"The mussels are giving us hope that we can filter the water and make the water healthy again," said Spadaro who advocates the restoration of oysters because they are filter feeders like the mussels. "But at the same time the mussels provide a danger to pet owners."