Heartworm

This is the common name for Dirofilaria immitis, a roundworm that attacks dogs, and occasionally cats as well as ferrets and sea lions.  Which is a little odd, since it's carried by mosquitoes--I never imagined sea lions being attacked by mosquitoes, but I imagine if they swim up into an estuary area where there's freshwater near by it could happen.

What happens is that the mosquetoe bites the mammal and deposit the larvae, which then mature in the muscles of the body, and then in the lumen of the right heart and pulmonary arteries.  Which, really, is all you need to have it be a disaster, because they can cause a pulmonary embolism (or more, they are the emboli) and if they're big enough, they can keep the tricuspid valve from closing. 

These adults are the reproductive stage, and they release microfilariae.  Unlike some pathogenic worm infections, such as Onchocera volvulus which causes blindness, the inflammatory effect of the dying microfilariae does not seem to be the major issue, though it is unclear to me if that is because they are actually less pathogenic, or just because having worms growing from your heart into your lungs is a more pressing concern.