The Vancouver Aquarium says a wild porpoise, which was rescued from a Salt Spring Island beach three weeks ago, was infected with a parasite normally found on land mammals, when it died on Sunday.

The porpoise, which was named Siyay by the Vancouver Aquarium staff, was watched by a medical team 24 hours a day in an attempt to nurse it back to health.

But the porpoise stopped breathing on Sunday around 5 a.m. PT and couldn't be revived with resuscitation attempts.

Doctor Martin Haulena, the staff veterinarian for the Vancouver Aquarium said the animal had many ailments but eventually died from fluid on the brain, a condition that may have been caused by an unexpected parasite called protozoan endephalitus.

Haulena says the parasite is normally found only in animals like possums, and its detection in a porpoise is a mystery.

"Why are marine mammals getting infected by a terrestrial parasite? That is a huge issue," he said on Monday morning.

"Now we have a live animal, we have lots of samples over three weeks, we have fresh tissues, which are incredibly rare for us to have, so even in death he is going to be a very important animal," he said.

But Haulena stresses the parasite is just a part of the puzzle of what actually caused the death of this animal.

When it was first rescued, the staff at the aquarium's Marine Mammal Rescue Centre noted the creature's chance of survival was about 10 per cent. The animal was suffering pneumonia, muscle damage, stomach ulcers and parasites in its lungs.

At least four other porpoises have been found on beaches on the South Coast of B.C. this summer, but this was only the second time in five years the aquarium has rescued a live porpoise.