Questions about heartworm disease in dogs?

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While most of the dog-owning general public is no aware that heartworms (HWs) do exist, I want to set clear a few myths or areas of confusion I frequently see clients unaware of.

Exactly how dogs get HWs: the only way dogs can get HWs is through the bite of an infected mosquito. That means that dogs can not get HWs through other dogs infected with HW disease.

Heartworms in dogs are parasites that live in the arteries of the lungs, the right side of the heart, and the vena cava. HW disease is the clinicopathologic manifestation of infestation with Dirofilariia immitis, the HW parasite. HW disease may result in pneumonitis, pulmonary endocarditis, hypertension, and thromboembolism, and death.

Where HW disease is found: HW disease has been reported in all 50 states–yes, even Alaska. All it takes is the bite of one mosquito infected with the heartworm larvae and your dog can contract HWs. With irrigation and building happening in many locations, mosquitoes have moved in. Unfortunately those dogs living in areas with typically no or few mosquitoes are now at risk. HW disease is found throughout Australia, Japan, and some Mediterranean countries.

What exactly a negative test at the vet means: If you get a negative test today at the vet, it means that about 6-7 months ago, your dog was HW negative. It takes almost 7 months once a dog is bitten by an infected mosquito for the larvae transferred to mature into adult HWs, which is what the test looks for. This means, if you skipped a month or two and you rush into the vet to get Fido tested, a negative test does not mean you are in the clear. A negative test in 7 months is what gets you off the hook and Fido out of the danger zone.

Who exactly can get HWs: Wild and domestic canines are the natural host, but other species (such as cats) can be affected. It was fairly recent that veterinary scientists discovered that cats could get HW disease too. Interestingly enough, there are a few reports of humans being infected with HW disease, though this is extremely rare. This does not mean that you can get heartworm disease from an infected dog; remember, HWs are only transmitted via infected mosquitoes, and even when people are infected, the HW cannot complete its life cycle.

Adopting shelter dogs with HW disease: HW disease is very common in shelters nowadays, and the majority of shelters in the US will pay for HW treatment (or least partially). One thing I do want to mention, is a reminder that a negative HW test just means that the dog was negative for HWs 6 months prior to the date of the test. Unfortunately, I have had many understandably upset clients who adopted a dog that tested negative for HW disease when he or she first arrived at the shelter, and then positive immediately after adopting. While this is frustrating, the shelters will often still help alleviate the financial burden of adopting a dog with HWs.

Symptoms of HW disease: While I mentioned several symptoms at the beginning of the article, there are initially no symptoms of HW disease. It is only as more and more worms crowd the heart and lungs of the dog that he or she will begin to cough. Symptoms are more prevalent at severe end stages of the disease.

Treating HW disease: It is not considered safe to just give your dog HW preventative. There are rumors that this is effective form of treatment, but it is very risky. Giving monthly preventative will slowly kill heartworms, but there are two major problems with this route of treatment. The first is the damage that the HWs will continue to do for the two years that it takes from the HWs to die. Their heart, arteries, and lungs will all be damaged irreversibly. Secondly, and more scary in my opinion, is that as the worms die, they are at risk for causing life-threatening blockages from the worms in their arteries or lungs.

The best treatment available involves an extensive work-up to see how advanced the disease is, and then a series of injections with immiticide. The cost can range from a few hundred to over $1000 depending upon the clinic and location.

Skipping HW preventation in the winter: Year-round protection is strongly recommended. While there may be few to no mosquitos in your area during a rough winter, HW prevention also protects against tapeworms, roundworms, and whipworms. Many clients of mine that have not given HW prevention in the winter have come back into my hospital infected with worms. It is far less expensive to give the meds year-round, not to mention, most of us are busy enough that we might forget to start up again at the appropriate time. Forgetting to give HW preventatives is a universal problem for people, and this is coming from a veterinarian.

Reinfection with HW disease: HW disaese is not like getting the chicken pox. If your dog is unlucky enough to get it once, he or she will have no lasting immunity, and are just as at risk to get them again. This is why prevention is so important.

Remember, HW disease is easily preventable. It is fatal left untreated, and very expensive to treat.