What Can Happen to a Dog With Heartworms Who Is Given a Heartworm Preventative?

Dirofilaria immitis, the heartworm, can grow over a foot long at maturity. The heartworm larvae spreads by mosquito bites, when the insect bites an infected canine and spreads the larvae to another dog. In infected dogs, the heartworm invades the heart and pulmonary arteries. It may take five to seven months from the time the dog is bitten to the time a blood test detects the presence of heartworms, so veterinarians use another type of test.

Your vet conducts antigen testing on your dog to ensure he is heartworm-free before prescribing medication. Dogs given the medication who have heartworms may experience severe reactions. According to the American Heartworm Society, millions of "baby" heartworms circulate in the bloodstream of affected dogs. Giving medication to a dog with these microfilaria, as they're technically called, can cause a shock-like reaction in the dog as the microfilaria die off.

If your dog tests positive for heartworms, treatment is available. According to the Merck Veterinary Manual, the only available heartworm medication for treating adult worms is melarsomine dihydrochloride. Two injections are needed, one 24 hours after the first. If the dog has large numbers of worms, he's at risk for pulmonary thromboembolism for the next six weeks as the worms die off. He must be kept very quiet during the recuperation period, which can be hard with an active dog. The MVM states that other treatment protocols recommend the administration of prophylactic doses of ivermectin for one to six months prior to administration of melarsomine, if the worm burden is less heavy. This reduces or eliminates some of the circulating first stage larvae.

If your dog test negative for heartworm, your vet prescribes a monthly or daily preventative, either in pill or chewable tablet form or applied topically. Depending on your location, your vet will advise you whether to give the medication year-round or only during the seasons in which mosquitoes are active. Each year, when you take your dog for his annual check-up, your vet conducts another test to make sure he is still heartworm negative before prescribing medication.