The phrase
"annual heartworm" refers to a type of infection that affects dogs and sometimes cats. Heartworms are parasites that live in a dog or cat's heart and blood vessels. The term "annual" means that it is something that happens once a year. So,
"annual heartworm" means that it is a disease that can occur in pets every year if they are not protected. It is important to provide preventative medication to keep pets safe from heartworm infection.
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