Heartworm is a serious and potentially fatal disease in cats. It is caused by foot-long worms (heartworms) that live in the heart, lungs and associated blood vessels of affected cats, causing severe lung disease, heart failure and damage to other organs in the body.
Disease Information Sites
The American Heartworm Society has great information about heartworm disease in cats.
The only support groups I was able to find for heartworm are groups for dog owners. Heartworm presents very differently in cats and cats do not currently have any treatment options available other than prevention. You could search for and join a group for this, but it’s unlikely to provide any real support for a cat owner.
You could join the Cat Health and Behavior support group for general support. There may be members of that group who have experience with heartworm.