Commentary: An argument for year-round heartworm prevention in dogs
Now let's do some math. The risk of a dog's being infected with heartworm disease each year is 250,000 out of 50,000,000; this translates to one in 200 dogs becoming infected each year. The chance that you will be diagnosed with cancer this year is about one in 200—the same odds as a dog's acquiring heartworm disease.2 Yet heartworm disease in dogs is virtually 100% preventable. Would you take medication once a month to prevent being diagnosed with cancer this year? But there is no good reason for dogs to receive preventives all year; it is just not needed.THE TREATMENT
Now let's look at the heartworm treatment options. Melarsomine dihydrochloride, an arsenical, is the treatment for heartworm infections in dogs; nothing is approved to treat cats. Melarsomine is given at a dose of 2.5 mg/kg. The LD50 for organic arsenic in mongrel dogs is 14 mg/kg.3 A three-fold overdose of melarsomine can be lethal.4 To put this in perspective, the epa has set the limit for arsenic in drinking water at 10 parts per billion5 —that is 10 µg/L—based on a no-effect risk of 1 to 10,000 to 1 to 1,000,000. So if you drink 10 L of water each day containing arsenic at the maximum allowable level, you would consume 100 µg of arsenic a day, or 36.5 mg of arsenic a year. A 50-kg dog receives a total dose of 250 mg of melarsomine, which contains about 37.5 mg of arsenic, in two injections. It would take one year of drinking high levels of arsenic in your water to get the same dose of arsenic given to a dog to treat heartworm disease. Thus, the dose for killing heartworms in dogs is far from a negligible amount. But there is no good reason for dogs to receive preventives all year; it is just not needed.
Of course, treatment with arsenicals is far better than the long-term effects of large worms living in the pulmonary arteries of their hosts. The lungs do get better after treatment.6 The adult heartworms are long: males are 12 to 20 cm long, females are 25 to 31 cm long, and both are about 1 mm in diameter.7 If a dog has a relatively light burden of 12 worms—six males and six females—that is still a large mass of worms in the pulmonary arteries.
The disease is one of chronicity caused by the worms interacting with the surface of the pulmonary vessels.8 The presence of the worms in the bloodstream also leads to the physical rupture of red blood cells and the deposition of hemoglobin within fixed macrophages of the lungs, making the lungs of dogs with chronic heartworm infections appear brown. Villous proliferations on the vessels also lead to the formation of small thrombi that are carried deeper into the lungs and ultimately induce chronic lung disease.
But keep in mind that even with therapy, the worms are in the pulmonary arteries, not the intestinal tract. All that can happen after arsenical therapy is that the worms are driven deep into the lungs where they die in tightly coiled bundles, decay, and are ultimately cleared by the host's cellular response. However, this takes a long time and is not without effects; remember, each dead worm is about 20 cm long. But there is no good reason for dogs to receive preventives all year; it is just not needed.