Last week we asked what (and where) this was in a dog. Fido has a friend very close to his heart, as realised by commenter Grae — the answer is Angiostrongylus vasorum (lungworm), in the pulmonary arteries which supply the lung.
You can see the damage caused to the lungs by the larvae of lungworm. There are immature lungworm larvae (black arrow) in the alveoli (lung air sacs; the white spaces), red blood cells where the lung is bleeding (red arrow), macrophage cells clearing up red blood cells by eating them (yellow arrow), thrombi (clots) in the blood vessels (white arrow), and multinucelated giant macrophages (blue arrow) trying to clear up all of the mess! All this leaves very little room for actual air.
The lungworm is a parasite which lives in slugs and snails for part of its life-cycle. Foxes are the natural host, but dogs can be infected too. When a dog eats the slug or even licks the slimy trail (yum!), the lungworm begins a new part of its life-cycle by transforming, boring through the gut wall and traveling in the bloodstream to the pulmonary arteries which leave the heart. The worms lay eggs into the lungs, and these are coughed up, swallowed, and passed with the faeces. Lungworm larvae hatch and infect new slugs and snails, and the cycle begins again!
Sometimes the larvae get lost in the bloodstream, and end up reaching other organs by accident (so commenter Reno Hates Me was almost right!). This is a dead-end for the larvae, and not great for the organs they’re trapped in either — a lose-lose situation for everyone involved! In this case lost larvae were found in the kidney, squashing the filtration units (glomeruli).
In dogs, the symptoms of lungworm infection are often subtle and chronic; usually a mild cough, some breathlessness, and weight loss. However, sometimes lungworm is associated with sudden and life-threatening illness. Lungworm can interfere with normal blood clotting, leading to widespread internal bleeding. Heart failure can be a risk if there are many worms clogging the arteries, and rarely they can cause paralysis and seizures.
So remember to worm your pets and poop-scoop! Avoid slug bait to get rid of slugs and snails, since the metaldehyde they contain is toxic to pets — stick to throwing them at the neighbour’s prize petunias instead.