Do Dogs Need to Be Tested for Heartworm Even if They Are on Heartguard All Year Long?

Heartgard prescription renewal requires an annual heartworm test.
Heartgard prescription renewal requires an annual heartworm test. (Image: Jupiterimages/Brand X Pictures/Getty Images)

You know that canine heartworm disease can be deadly -- so you're always certain to give your dog his monthly heartworm preventative tablet. Since he's on the medication year-round, why does your vet insist on a heartworm test before she writes you an annual prescription? The answer lies both in sound veterinary practices and in law.

Heartgard for Dogs

Heartgard is a proprietary heartworm preventive medication that contains ivermectin, a broad-spectrum wormer that kills heartworms circulating in the bloodstream in the larval stage, preventing the larvae from growing into adult heartworms and invading the heart and lungs. Many veterinarians recommend Heartgard Plus, which also contains pyrantel. This wormer controls roundworms, hookworms and whipworms.

Legal Requirements

When you take a medication long-term, even if you experience no side effects, you still need regular visits to your doctor for a check-up before she will renew the prescription. The same holds true for veterinarians and their pet patients. Most heartworm preventatives require a prescription, and that means your vet must follow various state and federal regulations, including documenting an established veterinary and client relationship. For that purpose, your vet must conduct an annual physical examination, including testing for heartworm, before prescribing the medication.

If You Missed a Dose

Most dog owners conscientiously give their pets a monthly Heartgard tablet, marking the calendar for the next dose date with the little red reminder hearts provided in the packaging. However, life happens. It doesn't mean you're a bad dog owner if you find you have one tablet left at the end of a year's supply. Vacations, business trips, entertaining guests -- all sorts of events can disrupt your schedule, causing you to lapse in giving your dog a tablet one month. During the period corresponding to that missed dose, your dog could acquire heartworm. While Heartgard is palatable to dogs, it's possible that your dog might not swallow it or might cough up the chewable tablet shortly after administration without your knowledge.

Heartworm Transmission

Dogs can't pick up heartworm disease from other canines. Transmission occurs solely via mosquito bite. A female mosquito may carry baby heartworms or microfilariae, which she transfers to a host when she probes for blood. The heartworm larvae develop inside the host. When your vet tests your dog's blood sample for heartworm, she's looking for microfilariae or an antigen indicating the presence of heartworm. If your dog tests positive for heartworm, he can't simply take the preventive -- he needs special treatment that can require a period of inactivity. The latter requirement is especially difficult for young, energetic canines.

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