I'd never heard of this before ... something else to be aware of, especially with water loving labradors...

---Fwd'ed
I lost a 9 month old Border collie last week. The
story was almost identical to the one making the rounds, except
substitute "playing with the sprinkler" (so no algae exposure) for
swimming.

Ingestion of massive amounts of water- usually through play, as the
thirst regulatory centres would stop a normal healthy dog from
consuming excessive amounts of water, is associated with profound
electrolyte disturbances; the body accumulates free water faster
than it can process it (usually by peeing it out), and this dilutes
the concentration of many substances below critical levels.

Electrolytes, simple molecules that carry an electrical charge, are
critically important in the movemment of substances across cell
membranes. If the electrolytes are seriously out of whack, the cells-
espcailly muscle and nerve cells, do not function well. Water
intoxication typically causes a very significant reduction in
sodium, potassium and chloride. Death is not an unlikely outcome!

Treatment of water intoxication can also be very problematic, as too-
rapid replacement of potassium can cause cardiac muscle issues, and
too-rapid replenishment of sodium can lead to brain swelling and
brain herniation.

So, while access to water is critical to hydration and
thermoregulation, too much can be a real problem. water intoxication
is rare, but potentially so serious as to be life-threatening, so we
need to be aware of the problem. Swimming is fine, but maybe not for
hours at a time. Playing with a hose, in a wading pool or tub, ditto.

Claire Duder DVM