Yes, cats should be on a heartworm preventive medication. Heartworm disease in cats is very dangerous, as the death of only a single worm when it reaches the end of its life span in two to three years can lead to a fatal immune reaction in the pulmonary blood vessels supplying the lung tissues of an infected cat.
Heartworm associated respiratory disease (HARD) can be easily confused with feline asthma or bronchitis, and diagnosis of heartworm disease in cats is tricky due to testing being less reliable in cats compared to dogs.
Even indoor cats should be on heartworm prevention, as a bite from a single infected mosquito can cause disease and no home can be fully proofed against mosquitoes.
Although the manufacturer of Heartgard provides guidelines for seasonal use, the perception of mosquito “season” may be different to different people. Many may misjudge how early mosquitoes may appear in the spring and how long they may persist in the fall. The “season” of mosquito presence may also vary greatly depending on the region of the country, with some regions having mosquito presence year-round.
Because of the insidious nature of heartworm disease in cats, its potential for causing an acutely fatal reaction in a cat otherwise appearing healthy, the poor predictability of transmission seasons and the fact that there is no treatment for cats with active heartworm disease, many veterinarians recommend use of a heartworm preventive year-round.