(First published September 2005 in The Mayor’s Alliance for Mayor’s Alliance for NYC’s Animals newsletter, Out of the Cage!)
The heartworm parasite is transmitted to dogs and cats by mosquitoes.
According to the Heartworm Society, “an ounce of prevention beats a pound of cure.” I encourage my clients to take this literally. Occasionally, I encounter a naturalist who is wary of overmedicating his or her pet. A very small dose of a very safe medication is all that is required to prevent heartworm disease. The heartworm (Dirofilaria immitis) is a blood-borne parasite, which is transmitted by mosquitoes. The disease, though easily preventable, is now found in all 50 states. Areas with large mosquito populations have the highest infection rates.
You may have seen a picture at your veterinarian’s office of a heart filled with spaghetti-like worms. Apart from making you feel squeamish, the parasites are actually quite dangerous. Untreated dogs can die, and those who do undergo treatment have a very difficult recovery. Harsh medications are required to kill and expel the worms.
Prevention, on the other hand, is quite safe and simple. There is no reason we should even have cases of heartworm disease in the United States! All adult dogs should have a blood test taken prior to starting prevention (to be sure he or she isn’t already infected). If the test is positive, your veterinarian can guide you through the treatment. Incidence of heartworm disease is much lower in cats (especially indoor cats who live in cooler, dryer climates). The disease develops differently in cats and can be trickier to diagnose than a simple blood test. Your veterinarian will let you know if there is any reason to suspect heartworm disease in your cat.
Symptoms of heartworm disease in the dog include progressive cough, exercise intolerance, lethargy, weight loss, or poor body condition. Ultimately the disease can lead to congestive heart failure. Untreated dogs act as a reservoir for the disease, even though it’s not transmitted directly from dog to dog.
If your dog is diagnosed with heartworm disease, other blood tests and x-rays will be needed to determine the full extent of the infection. Any secondary problems will need to be addressed prior to starting treatment.
Harsh medications are required to kill and expel heartworms once your dog is infected with this parasite. (Image courtesy of Heartworm Society)
The treatment itself involves use of an arsenic compound (as in Arsenic and Old Lace!). Fortunately we have newer drugs that are safer and have fewer side effects, but it remains a fairly harsh process with some risks. Based on the staging of your dog’s disease, your veterinarian will choose an appropriate course of treatment. It’s important to keep the dog quiet after the treatment so that the dead worms don’t dislodge and travel to the lungs. In rare, severe cases, a surgical procedure might be needed to physically remove the worms. Your veterinarian will advise you on any follow-up screening.
The Heartworm Society recommends annual blood testing, even for dogs on year-round treatment, because owners might occasionally forget a dose. All of this information (and more!) is available on the website of the American Heartworm Society.