Heartworms, called Dirofilaria immitis, are very thin, thread-worms. You won’t see these worms yourself though, as they live within the right side of the heart, and the main artery leading from the heart to the lungs.
Heartworms are nasty, and thankfully are not found in the UK. They are transmitted by mosquitos, and are therefore found in warmer countries, such as those in southern Europe.
Heartworms reproduce by producing tiny offspring, called microfilaria. In infected dogs these can often be picked up by testing, but cats are not classic hosts for heartworm so worms find it much more difficult to reproduce. This means cats are much less likely to have these microfilaria in their blood which can make a diagnosis of heartworm more difficult to reach.
Despite struggling to reproduce in cats, heartworms can still have serious health impacts for cats.