A thorough understanding of heartworm disease will help you protect your dog and understand the signs, symptoms, and treatment options.

If you have a dog, you already know something about heartworm disease. But there are several misunderstandings surrounding this disease that need clarification. In the first of this two-part article, we’ll look at the difference between heartworm infection and disease, risk factors and symptoms, and how heartworm is diagnosed.

Infection or Disease?

To start with, it’s important to know the difference between heartworm infection and disease.

Heartworm infection occurs when an infected mosquito deposits immature larval heartworms into the dog while feeding on him. These dogs are not clinically ill, do not act sick, and appear totally normal. Usually, a diagnosis of heartworm infection is made through the dog’s annual blood work. People are usually shocked to discover that their healthy dogs are infected with heartworms and are at risk of developing heartworm disease. Most dogs I see with “heartworm” have heartworm infection, not disease.

Heartworm disease results when a dog infected with heartworms develops clinical signs (see next section). Fortunately, I very rarely see dogs with heartworm disease. Treatment requires much more aggressive and expensive therapies; dogs with heartworm disease are more likely to suffer side effects from the traditional medication used in treatment than those with heartworm infection.

Signs and Symptoms

Clinical signs of heartworm disease usually represent the pathology of the heart and lungs. In fact, “heartworms” are really “lungworms”. In most cases, the worms are found in the large blood vessels of the lungs, not the heart. The worms only “back up” from the lung blood vessels in severe infections, in which case they end up in the chambers of the heart (right side) and caudal vena cava blood vessel. When the disease was first discovered, so many worms were found in animals that they had in fact backed up into the heart, so the disease was named heartworm when it would be more accurate to call it lungworm.

Clinical signs, especially early in the disease, involve coughing, most often when the dog is active. The coughing is due to the presence of worms in the pulmonary blood vessels, and the associated inflammation the worms produce. With time, the disease may progress and cause more severe signs of lung and/ or heart infection and inflammation, such as weakness, more serious and constant coughing, weight loss, decreased appetite, and fluid accumulation in the chest and abdomen (signs that predispose the dog to sudden death). In some cases, however, the only sign of heartworm disease is sudden death.

Diagnostics Have Improved

Heartworm infection is usually easily diagnosed with a simple blood test. However, there is still some confusion among veterinarians and their clients on what the “best” test is.

Three types of blood tests can be used to diagnose the presence of heartworms in a dog’s body.

  • When heartworms were originally discovered in animals in the 1970s, the blood test for it involved simply examining a drop of blood under the microscope for the presence of heartworm microfilariae (baby heartworms produced by female worms in the dog’s body). While this test would often reveal the presence of microfilariae, there were two limitations. First, another species of microfilariae (most commonly one that does not cause disease in dogs) was often found. Second, it was discovered that many heartworm infections (in some cases up to 70%) are occult infections, meaning that adult worms are present but microfilariae are not. Using this test meant that many heartworm infections were missed or misdiagnosed if another type of microfilariae was detected in the blood sample, or if no microfilariae were seen.
  • The second type of heartworm test to be developed was the filter test. In this instance, a 1 ml to 3 ml sample of the animal’s blood is pushed through a filter apparatus, and then a second sample of tap water is used to flush the filter. The filter is removed, sometimes with a microbiological stain, and is examined microscopically. While this test was better than simply examining a drop of blood under the microscope, it too had the same limitations. Because of the limitations associated with these two tests, they are not recommended as the sole way to diagnose heartworm infection. However, they should be run if there are positive results on the occult test, to determine if additional treatment is needed to kill the microfilariae.
  • The most accurate and commonly run test is called an occult heartworm test. It is an antigen-antibody test that determines the presence of antigen from female heartworms in the dog’s body. Unlike the other two tests, it is very accurate and can pick up infections when only a few worms are present, even if no microfilariae are seen. This test should be done annually on healthy dogs six months of age and older (the test is inaccurate in younger dogs, as it takes a minimum of six months from the time of a mosquito bite before the dog would test positive). This test should also be done on any dog suspected of having heartworm infection/disease. It is also done following heartworm treatment to ensure success, usually, six months after the dog finishes the treatment.

If heartworm disease is suspected, or if infection has been proven via blood testing, other tests should be done before proceeding with treatment. This testing is vital to determine the stage of the disease/infection, and to uncover any problems that might complicate or postpone treatment.

Heartworm disease is diagnosed when clinical signs of the disease are present after a diagnosis of infection via the testing just discussed. Laboratory tests needed to confirm and stage the disease include chest radiographs (X-rays), a blood chemistry profile, CBC, urinalysis, and an EKG. Dogs with heartworm infection will be normal on all of these tests, whereas dogs with heartworm disease will show some abnormalities. Even if clinical signs of heartworm disease are not present when infection is diagnosed, it is imperative to run these additional tests prior to beginning treatment. Normal test results mean that treatment can begin, severe side effects are not likely to occur, and a baseline value for each test is established in the event complications occur during therapy.

Preventing Heartworm

If you live in a region where heartworm is prevalent, you need to be proactive about prevention. Start by taking steps to minimize your dog’s risk of being bitten by mosquitoes.

Your dog may also need to receive a conventional heartworm preventive. Many people shy away from these medications, but it’s important to realize that depending on where you live, they are an effective way to protect your dog, and are part of an integrative prevention approach. There are several choices – oral, topical, or injectable.

  • Oral monthly medications use medicine such as ivermectin or milbemycin. In general, these drugs are nearly 100% effective when given following label instructions, and are actually among the safest medications used in clinical practice. In general, most holistic veterinarians prefer oral medications for conventional heartworm prevention because they don’t last as long in the body as topical and injectable medications (only 24 to 48 hours as opposed to months). While some people have expressed concerns about ivermectin, because certain breeds are more sensitive to these chemicals, the sometimes-fatal reactions found in some lines of collies and collie crosses involve much higher levels of this chemical than what is found in heartworm preventive.
  • Topical medicines are applied monthly to the dog’s skin. After application, they enter the bloodstream through the skin; concentrations of the medicine in the blood and tissues prevent heartworm disease. Because they persist in the dog’s body for several months, however, they aren’t the first choice with most holistic veterinarians.
  • There is currently one injectable medication for heartworm prevention. It was recently returned to the market after being removed several years ago due to severe side effects in a number of dogs. There are still significant warnings and restrictions on the label, and veterinarians must be certified in order to administer this product. Because the medication lasts in the dog’s body for at least six months, most holistic doctors don’t prefer it.

Using Oral Preventatives Properly

It’s important that oral preventive medication be given per the manufacturer’s recommendations. I’m often asked if it’s okay to give the medication every 45 to 60 days rather than every 30 days. The people who ask this tend to be those who really resist giving their animals any medication. While I would like to find (or maybe even invent!) a natural heartworm preventive that’s as effective as the monthly oral medications currently prescribed, the low dose of medication contained in these products is way below the toxic or lethal dose.

In general, most holistic veterinarians prefer oral medications for conventional heartworm prevention because they don’t last as long in the body as topical and injectable medications.

When you administer the medication, it is quickly absorbed into the body. If an infected mosquito bites your dog that day, the medication will kill any immature heartworm larvae injected into him by the mosquito. If the mosquito bites the dog a few days later, the medication has already been expelled by the dog’s body and the injected heartworm larvae will begin developing. However, if you give the next dose of medication 30 days later, it will kill the developing larvae (despite what some people think, the medication does not work “retroactively” but actually kills developing larvae at the time of administration). If you wait longer than 30 days to give the next dose, then the drug is less effective due to the maturation of the heartworm larvae. For this reason, it’s really important to give the drug as close to every 30 days as possible.

Treatment — Conventional Medications

Prevention is always preferable to treatment, since the latter is costly, involved, and requires strong drugs. Prior to treatment, it is imperative to confirm the diagnosis. I see a number of animals that test positive for heartworms (usually on the less accurate filter test) but are found to be negative when re-tested using the occult test. These dogs should be tested at least twice more in the next one to two months to make sure they are still negative and don’t end up being treated for a disease they don’t have.

For dogs that are positive on re-testing, a pre-treatment evaluation is necessary to stage the disease. This includes chest radiographs (X-rays), a blood chemistry profile, CBC, urinalysis, and an EKG. Dogs with heartworm infection will be normal on all these tests, whereas dogs with heartworm disease will show some abnormalities. Even if clinical signs of disease are not present once infection is diagnosed, it is imperative to run these additional tests prior to starting treatment. Normal test results mean treatment can begin, severe side effects are unlikely, and a baseline value for each test is established in the event complications occur during therapy. In the event any of the tests are positive, further evaluation and treatment are needed before beginning heartworm therapy. This prior treatment is needed to stabilize the dog and make the actual heartworm treatment safer.

Currently, the conventional treatment of choice for heartworm involves a drug called immiticide. It is given via deep intramuscular injection into the lumbar muscles. Treatment is given as a two- or three-dose series of injections. The two-injection protocol (given 24 hours apart) kills about 90% of the adult worms, whereas the three-dose protocol (one injection followed at least one month later by two injections 24 hours apart) kills 98% of the worms. Keep in mind that this standard protocol may be modified depending on the health of the dog, stage of the disease, and any side effects.

Following treatment, the dog should be retested in four to six months to ensure the infection is successfully cured.

What are the Side Effects?

Minor side effects seen with treatment include pain and inflammation at the site of the injections. This can be managed with conventional analgesic medications. Additionally, I have often seen panting lasting up to eight to 12 hours following the injection. There is no specific therapy for this and it has always resolved.

The most serious side effects involve blood clotting (thromboembolism) and inflammation of the blood vessels of the lungs. Several factors influence the development of these severe side effects, including pre-treatment disease, the activity level of the dog, the extent of blood vessel disease, and the severity of infection (high versus low worm burdens).

Dead and dying heartworms cause significant damage to the blood vessels. As worms die from natural causes or as a result of treatment, they lodge in the blood vessels of the lungs, causing inflammation and blocking blood flow. During periods of increased activity or exercise, the increased blood flow to these blocked vessels can cause blood vessel rupture, and possibly heart failure. There is a direct correlation between the activity level of the dog and the chance of these side effects occurring.

The greater the number of heartworms killed during treatment, the higher the potential for side effects. Unfortunately, there is no way to accurately determine the number of heartworms present, although some occult tests can indicate the potential for low versus high worm burdens. The best course of action is to assume that post-treatment complications could occur and that every infected dog must be managed with strict rest for four to six weeks following treatment. Signs of post-treatment complications can include coughing, weakness, passing out, and sudden death. If coughing or weakness is seen, aggressive hospitalization and treatment with supportive care, oxygen, IV fluids, and corticosteroids are needed.

Natural Support

Unfortunately, no natural treatments have been proven effective as the sole means to kill adult heartworms. Nevertheless, holistic veterinarians often employ natural medicines at the time of conventional treatment to support the animal during therapy. In my own practice, the following natural medicines have been used safely and effectively.

  • Milk thistle helps support the liver as it detoxifies the heartworm medication and dying adult heartworms. Its antioxidant effects may also reduce blood vessel damage caused by the significant oxidation following treatment.
  • Mild diuretics such as dandelion help encourage urinary elimination of toxins and any fluid that might accumulate in the lungs.
  • Coenzyme Q10 and hawthorn can be used to maintain proper heart health and encourage normal blood flow through the pulmonary vessels.
  • Lung and cardiac glandulars help support the lungs and heart.
  • Immune support using astragalus or Echinacea can support whole-body health.
  • Homeopathic/homotoxicologic support of the immune system, and general body detoxification using remedies such as Berberis and Nux Vomica, are also helpful.
  • Symptomatic therapy with herbs and homeopathics can be used for dogs experiencing side effects. For example, muscle soreness after injection with immiticide can be addressed with Arnica or Hypericum.
  • Antioxidants and fish oil help reduce the inflammatory reaction that occurs from heartworm infection and treatment.

Heartworm infection and disease are easily prevented with an integrative approach. If treatment is needed, staging the disease and using carefully selected natural therapies will make it more effective and safer.

Is Your Dog at Risk?

All dogs (and other animals) are at risk of developing heartworm infection. Since exposure to infected mosquitoes is necessary for infection to occur, dogs that spend more time outdoors are at increased risk. Also, because mosquito activity occurs as the temperature increases, there is often a seasonal trend to exposure and infection. As an example, it’s warm all year here in Texas (with occasional daily exceptions), so year-round heartworm prevention is important. In other areas of North America, the warm season is shorter and transmission is less likely to occur year-round. Often, heartworm prevention is only needed during the warmer months or not at all, depending on heartworm populations in your area, and your veterinarian’s advice.

Heartworm has been found in all 50 states, and across Canada, but some areas are much more prone to it than others. In the US, the prevalence of heartworm is highest in the southeast (including Texas east to Florida), and lowest in the Midwest. In Canada, heartworm is most prevalent in southern Ontario and the Winnipeg region of Manitoba. Wherever you live, it’s important to find out what the heartworm numbers are like in your region, so you and your veterinarian can decide if and/or when your dog needs preventative medication.