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How to choose a heartworm preventative for your cat
Have your cat tested for heartworm.
Even before you begin looking for a heartworm preventative, you should take your cat into your veterinarian's office and have the animal tested for heartworm. It would be deeply unfortunate to begin giving your cat preventative medication, only to find that it already has heartworm.
If the test results come back positive, you can speak with your vet about how to treat the heartworm. Talk to your vet about preventative heartworm medications.