What Should I do if my Pet’s Heartworm Preventative is Late?


Oops! It seems my pups heartworm medication has lapsed and they’ve been off the meds for over a month now. Do I need to have new blood work done or can I just go into my vet and request refills of the medication?


Heartworms are blood parasites of dogs and cats. Heartworms are spread by mosquitoes. The parasites can cause heart failure and other serious pathology in infested pets. Heartworm disease is easily preventable with medications (called heartworm preventatives) that are administered monthly.

Most heartworm preventatives do not block heartworms from entering pets’ bodies. When an infected mosquito bites a pet, microscopic larvae are injected into the blood stream. These larvae undergo several progressive changes, ultimately becoming full-grown adults after 4 – 6 months. Most heartworm preventatives work by killing all of the microscopic larvae in the body once each month (the most commonly used preventatives are cleared by the body within a day or two). This prevents any larvae from maturing and causing disease.

It turns out that heartworm larvae are susceptible to heartworm preventatives for a period of six weeks. Heartworm preventatives generally are administered every four weeks. This means that heartworm preventatives can be administered up to two weeks late without your pet suffering any increased risk of disease.

However, if you are a full month late, that means your pet has been open to heartworm infection for eight weeks. If you give the pill today, you will kill any larvae that infested your pet during the last six weeks. However, you can’t count on the preventative to kill larvae that may have infested your pet during the first two weeks.

The good news is that it’s not common for pets outside of the Gulf Coast and Mississippi River basin to become heartworm positive in a two week window.

I recommend that you administer a heartworm preventative to your dogs immediately. Follow up with your veterinarian in 3 – 5 months for a heartworm test (the test only detects adult worms, so you’ll have to wait the length of a heartworm life cycle). Hopefully the tests will be negative.

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