Toxoplasma gondii is a protozoan that is fatal to otters if it’s ingested. They may be getting in it from the shellfish they eat, or just drink it in sea water. The most likely explanation is housecats. Cats shed live toxoplasma eggs in their scat, which then washes downstream until it reaches the places where otters live. Not only is this bad for otters, it also shows us that pollution of the water is going on in ways we hadn’t anticipated, which means if we don’t clean up now, there may be some more surprises down the road–such as what effect these protozoans might have on humans, since they are harmful to us too.