Can you restart heartworm prevention without a heartworm test?

It is not advised to restart heartworm medication without a heartworm test.

One of the reasons is that if the dog is heartworm positive, the adults are in the heart and causing pathology to the heart, blood vessels, and lungs, with many of these changes causing severe, irreversible damage. The preventative does not KILL the adults so this pathology will continue.

The second major reason is that if the dog does have heartworm disease and the adult worms are producing microfilariae, when the preventative is given it will kill the microfilariae. The sudden death of the microfilariae can cause a severe anaphylactic (allergic) reaction which could be life-threatening for the pet.

If you have regularly been giving the heartworm preventative and miss a month or two, there is a risk that your dog could have been bit by a heartworm infected mosquito during that period and is now developing heartworm disease. However, the heartworm test detects the presence of adult worms and it takes a few months until the worms have reached the adult state.The adults do not start producing the microfilariae until 5-7 months post infection. So if only one or two doses were missed it would be reasonably safe to start the preventative, but be certain to check a heartworm test in 6 months or so.

The most important consideration is to stay on the preventative year round and not miss a dose. It is an easily preventable disease.

If you do miss a dose or two, check with your veterinarian and follow their recommendations for follow up heartworm testing.