Dog Heartworm Symptoms, 
Prevention & Disease

Dog heartworm symptoms alert the dog's owner to a health issue. This canine heart is infested with heartworms. The poor dog likely exhibited severe symptoms before dying.

Heartworm disease is caused by a parasitic worm that is spread through the bites of mosquitoes. While the dog is the most common host, other animals can also develop heartworm including cats, wolves, coyotes, foxes and ferrets. On extremely rare occasions, a human will also contract the parasite.

The heartworm lives in the dog's heart and can grow to be 12 inches,

Dog Heartworm 101

Young, immature heartworms are called microfilaria. They circulate through a dog's bloodstream. If a mosquito bites an infected dog, it can ingest microfilaria. When it later bites another dog, the mosquito deposits these immature heartworms into the new host, infecting the dog. The microfilaria eventually enter the dog's bloodstream and travel to the heart. It is there that the microfilaria become adults and have offspring. At this point, it is about six months after the initial mosquito bite.

Dog Heartworm Symptoms

Dogs with a mild case of heartworm infestation who are inactive couch potatoes may never show signs of the disease. But active dogs or those with a heavy infestation will show dog heartworm symptoms. Even then, symptoms usually do not emerge until the mature heartworm migrates to the dog's heart. At that point the following dog heartworm symptoms may emerge:

  • Coughing(Occurs early in the disease, especially following exercise)
  • Extreme weight loss, fainting, trouble breathing, weakness, coughing up blood and congestive heart failure (Occur in dogs with a heavy load of worms.)


It is important for your dog to be tested annually even if he does not display dog heartworm symptoms. It usually takes about three months (but sometimes as long as six months) after being bitten by an infected mosquito for a dog to test positive. And testing positive does not mean he will be showing any dog heartworm symptoms.

A diagnostic tool called SNAP 4Dx Test enables your vet to test for heartworm disease (even if there are no dog heartworm symptoms)and three dog tick-born diseases: Lyme disease, canine anaplasmosis and canine ehrlichiosis. The test can be run by your vet using a blood sample drawn from your dog. Test results are available in about 10 minutes. There is also a test for heartworm alone, if you and your vet feel your dog is not at risk for the other diseases and therefore does not need the additional tests, which will cost more than a single test.

It is possible for a dog with adult heartworms to test negative for microfilaria (immature heartworms). There are several reasons for this:

  • Young female heartworms can sometimes resist heartworm medicines. So after treatment some young female worms will remain unharmed. This is why it is important to test on a regular basis.
  • The dog is taking a heartworm preventive.

Conventional Treatment

A heartworm microfilaria taken through a microscope at 400x. ©Copyright Joel Mills

Immiticide®, an arsenic compound, is needed in order to kill adult heartworms. This treatment is very hard on the patient. For a dog with additional health issues, the treatment can be as bad as the disease. You and your vet will need to evaluate your dog's health and chance of recovery.

Before the adult worms can be killed, the microfilaria and the worm larvae that are living in the dog's skin must be destroyed. If not killed, they will eventually travel to the heart and reproduce. This will increase the number of adult heartworms that need to be killed and also increase the risk to the dog. So it is best to combat the immature worms first. Heartworm preventives that contain ivermectin, such as Heartgard®, will kill the larvae and immature worms. Most vets will recommend that the ivermectin-based medication be given for one to three months before treatment begins for the adult worms.

Immiticide is the only treatment available for adult heartworms. Treatment can be done in two doses or three doses depending on the severity of worm infestation. Many vets choose to use three treatments because it kills worms gradually and is less likely to create shock or embolism (internal blood clot), which are serious concerns.

An infected dog is hospitalized and the drug is given intravenously in the back. This is a painful procedure and the dog will probably need pain medication when he goes home. In the two-dose protocol, the patient receives treatment on consecutive days, 24 hours apart. In the three-dose procedure, the dog receives one treatment, then goes home. He returns to the vet in one month and receives two treatments 24 hours apart.

When the patient goes home, regardless of which treatment procedure was used, he/she must remain quiet for a month. The dog is taken outside on a leash to go to the bathroom, then immediately returns inside. There must be no walks or exercise of any kind. The heart needs to rest, and that means the dog needs to rest! Otherwise, the development of a blood clot is a very real threat.

It's important to contact your vet immediately if your dog is coughing or has a fever or nose bleeds. This can happen anytime during the month following treatment but it most likely to occur in the first week.

Natural Prevention

You will never have to worry about identifying dog heartworm symptoms if you are vigilant about prevention. Many holistic veterinarians use nosodes to combat or prevent a variety of illnesses in their patients, including heartworm. When using a nosode, the veterinarian takes a part of the illness or disease and creates a nosode to treat that particular illness. A heartworm nosode contains part of a heartworm. By administering this nosode over a period of time, the dog will build a natural immunity to heartworm disease.

A preventive tool that is even more important than a preventive nosode or heartworm drug is the dog's own natural defenses. A parasite wants a weakened host. Your dog is less likely to attract heartworms or other parasites if he is eating quality food, receives a minimum of vaccinations and his immune system is not overwhelmed by toxins in his environment.

According to Dr. Richard Pitcairn, DVM, a well-known and respected proponent of natural veterinary care, there has been a significant spread of heartworm in dogs throughout the United States. He writes, "So it is likely the combination of environmental upset, coupled with a deteriorating level of health through several dozen generations of dogs fed on commercial foods and poisoned with insecticides that has created this unnatural explosion of parasitism."

In addition to a quality diet that includes some fresh foods, Dr. Pitcairn advocates using garlic and yeast supplements, which help repel mosquitoes as well as fleas and ticks.

In a study at Auburn University several decades ago, it was discovered that stray dogs from Mississippi had a natural resistance to heartworm infestation; they showed no dog heartworm symptoms and tested negative. Dr. Don Hamilton, DVM, author of Homeopathic Care for Cats & Dogs, believes that most adult dogs have developed some amount of heartworm immmunity due to their exposure to the disease, even on preventive medicine. Because of this, he theorizes that most dogs over five years of age are less likely to be infected by heartworm.

Whether you choose a homeopathic nosode or conventional heartworm preventive drug, you still need to use a second or third line of defense as well. Your goal should be to never be in the position of having to recognize dog heartworm symptoms in your furry family member. That means taking preventive measure on several fronts. For instance, do not allow your dog outside, except for a few minutes to relieve himself, during dusk and evening hours. This is when mosquitoes are the most active. Also, use a repellent on your dog, preferably a natural one, to keep the mosquitoes at bay.

Never give a heartworm nosode or heartworm preventive until first testing to be sure your dog is negative and heartworm-free.

Conventional Prevention

It is important to realize that although drugs can offer benefits, they are not harmless. Potential side effects of heartworm preventive drugs are nausea, diarrhea, liver disease and autoimmune disease. As your dog's best friend and guardian, it is your responsibility to weigh the pluses and minuses--the good and bad influences on your dog's health--and decide which is the best alternative. Heartworm in dogs is a serious and potentially lethal disease. And by the time dog heartworm symptoms are noticeable, the disease is fairly progressed. So it is imperative that you take a pro-active approach to heartworm prevention. Doing nothing is not an option. Heartworm preventives work retroactively. In other words, you are killing any "baby" heartworms from the previous month; it does not protect your dog from future exposure.

It should be noted that one preventive, Interceptor, has 5 to 10 times the dosage of other preventive medicines because it is marketed as also preventing intestinal worms. Unless your dog has a severe worm problem, you should rethink using this product. Why would you want to load your dog with such a large level of drugs when a small dosage will still protect your dog from heartworm?

New Approach to Conventional Prevention

Once a dog is bitten by an infected mosquito, there is a window of 6-8 weeks when the heartworms are susceptible to conventional heartworm preventives. Given this fact, you may be wondering why heartworm preventives have traditionally been given to dogs once every four weeks.

In Homeopathic Care for Cats & Dogs, Dr. Don Hamilton, DVM, theorizes that the "once a month" approach was adopted long ago because the heartworm preventives manufacturers (and vets) believed it would be too difficult for people to remember to administer the pill every seven weeks. Now knowing this, are you comfortable with a continued regimen of monthly dosing? Probably not. Why would you want to give your dog more drugs than he needs?

Here is what Dr. Hamilton suggests as an alternative. On the first day you see mosquitoes, mark it on your calendar. Wait four to five weeks and give your dog a dose of heartworm preventive. After that, give the preventive every six to seven weeks throughout the mosquito season. After you've seen the last mosquito, which usually occurs with the first hard frost, wait six to seven weeks, then give the final preventive of the season. If you live in an area that has mosquitoes year-round, give a dose every six to seven weeks throughout the year.

You have a number of choices for preventive medicines, both conventional and natural. Only you can decide what is best for you dog. But please do not delay taking a proactive approach to prevention. If you do, your canine best friend may eventually display dog heartworm symptoms. Think "prevention" and your dog will thank you for it!

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