Horacio asked a good question in the comments section of my recent post on heartworm.

Horacio posted a comment on July 26th, 2010 at 10:24 am

If I have been giving my Dog heartworm prevention every month, but for whatever reason has missed a few appointments. Is there any risk of restarting with the prevention medicines?

Horacio, you most certainly are not the first person on earth to overlook your dog’s heartworm preventative. Your situation is a common one.

Any time a dog goes more than six weeks without heartworm prevention the dog is at risk of infection. Dogs with mature infections (in which hearworms are mating and producing larvae in the bloodstream) may experience a shock-like adverse reaction to heartworm prevention. Mature infections take four to six months to develop.

In other words, four to six months after a six week lapse, heartworm medication might pose a risk to a dog.

However, I am happy to say that the adverse reaction described above is very uncommon.

Horacio, I recommend that you have your vet run a heartworm test on your dog. Repeat the test in four to six months (the test can only detect mature heartworm infections). Re-start your dog’s heartworm preventative after you receive a negative result on the first test. The benefits of re-starting the medication outweigh the risks in almost every case.