Prevention is the key, even though it is a treatable disease. It is easy to prevent with a monthly treatment, but the medicine needs to be given every month. Mosquitoes are out when the temperature is 50 degrees, so this year is a good example of why to treat in the cold months.
Cats are also susceptible to heartworm disease, but it is more often fatal to them because they are not a natural host. Any outside cat should be on heartworm prevention. Since mosquitoes get in the house, treating indoor cats is a good idea.
Most of the preventatives also treat intestinal parasites, so your pet is also protected against many potential chronic problems.