What Is Heartworm Disease?
Heartworm (Dirofilaria immitis) is a parasitic disease that has been documented in our companion canines for over 100 years and affects over 30 species of animals, (including humans!)
How Is Heartworm Transmitted?
Heartworms are transmitted by mosquitos that harbor heartworm larvae (immature stages) that are capable of infecting our pet. When the mosquito bites your dog (which can happen even through thick fur) the mosquito transmits the infecting heartworm larvae into the blood stream. Within your pet’s body the larvae grow, develop, and migrate becoming sexually mature male and female heartworms. Adult heartworm will eventually reside within the heart, lungs, and those blood vessels closely associated with these major organ systems. When mature, the female and male heartworms will mate and produce offspring called microfilariae that will enter into your pet’s blood stream. Detection of the microfilariae (offspring of the adults) within your pet’s circulating blood can occur as soon as 6 months after your pet was bitten by a mosquito carrying heartworm larvae. Male and female heartworms become mature in approximately one year (reaching up to 12 inches in length!) and can live for 5-7 years. Usually, the number of adult worms in an infected dog is very large ranging from just a few to 250 in number.
What Does Heartworm Infection Mean?
Microfilaremia (presence of heartworm offspring in the blood) is common. However, not every dog bitten by an infected mosquito results in an infection with heartworm offspring (known as occult heartworm disease). This may be due to infection with only a single sex of heartworm resulting in a lack of production of offspring, the dog’s natural immune response, and most significantly by heartworm preventative medications.
Where is Heartworm Disease Found?
Heartworm disease has been found in all 50 states. All dogs, regardless of their age, sex, breed, fur length, and living situations (indoor only) are susceptible. In the northeast, mosquito’s have been found within households ALL YEAR round and outside for the majority of the year.
What Clinical Symptoms Does Heartworm Disease Produce?
Heartworm disease has been known to cause a variety of clinical symptoms that involve the heart, lungs, and kidneys primarily. The onset of symptoms and severity of the disease is primarily due to the number of worms, how long the infection has been present, and the overall health/activity level of the dog. Dogs with a higher number of worms have more severe disease symptoms. Medically, we know that until the number of worms reaches a threshold level (approximately 50 for a 55# dog) almost all of the worms like to stay within the arteries of the lower lung lobes. When the numbers become higher we begin to see heartworms within the right side of the heart. Most of the initial symptoms seen are a result of a local inflammatory response to the presence of the worms in the lower part of the lungs. Later on in the progression of the disease signs of heart failure may be seen as the heart enlarges/weakens to try and compensate for the extra work created by the presence of the worms. Logically, a very active dog whose heart and lungs have to work hard may have more severe disease with less worms present than a sedentary dog with a large worm burden. In very severe cases, there can be so many worms present they obstruct the primary vein (vena cava) between the heart and the liver which can result collapse/death if not manually removed in surgery. Usually symptoms begin slowly (some dogs that are not active may never have symptoms) and usually involve signs associated with heart and lung disease. Look for the following
Early infection: (from time of mosquito bite to 6-7 months): no symptoms
Early Disease: Mild cough
Moderate Disease: Cough, exercise intolerance, harsh lung sounds
Severe Disease: Cough, exercise intolerance, harsh lung sounds, trouble breathing, collapse, enlarged liver, abnormal heart sounds (murmur), fluid in the belly
How Is Canine Heartworm Disease Diagnosed?
Diagnosis of canine heartworm disease is often multi-faceted. At Glove Cities Veterinary Hospital we recommend (supported by the American Heartworm Association) annual screening of canine heartworm disease. This test, performed within the clinic in 10 minutes using only three drops of blood, is a very sensitive test for heartworm disease. The test that we use also tests for three tick borne diseases (lyme, erhlichia, and anaplasmosis). We recommend annual screening for a number of reasons including that no preventative is 100% effective, many owners forget or do not treat year round or at all for heartworm prevention, and parasites become resistant to our preventatives. This quick in hospital test performed once a year is the best screening method available to initially diagnose heartworm disease as early as 5-6 months after initial infection and with as few as 1-2 adult female worms present in the pet’s body.
A physical examination in addition to radiographs (x-rays) may be needed if heartworm disease is suspected. A physical examination is important for any animal thought to be ill, and should be performed once a year on a wellness basis. If lung abnormalities are heard on auscultation of the chest, x-rays may be recommended. Radiographic (x-ray) abnormalities can be seen early on in the course of disease and the findings can be used to evaluate the severity of the disease and discuss the prognosis. Changes to the lung, heart, and surrounding vasculature are often seen.
Advanced diagnostics such as an ultrasound of the heart may be recommended in certain situations. An ultrasound of the heart can evaluate the size, structure, and function of the heart, the connecting lung structures, and the actual heartworms themselves can sometimes be visualized.
How Is Heartworm Disease Treated?
Most canines diagnosed with heartworm disease can be successfully treated. However treatment is time consuming, expensive, and not all dogs fully recover. Treatment seeks to kill the adult worms with one medication and kill the circulating microfilariae with another medication. The goal is to try and minimize harmful side effects of the medications and dying worms on the pet’s body.
Adult heartworm therapy utilizes a singular FDA approved drug for the elimination of adult heartworms. The compound is an organic arsenical called Melarsomine (brand name Immiticide). Most canines need more than one treatment and close observation during treatment is recommended. The most dangerous aspect of adult worm treatment is the presence of the dead worms within the body. Some degree of pulmonary thromboembolism (blockage of the lung vessels with dead worms) and long term damage is expected to occur with successful treatment. It is critical when being treated that the pet is kept quiet to minimize post-treatment complications.
Microfilariae treatment is most effectively performed with anthelmintics (ivermectin, selamectin, milbemycin oxime, and moxidectin). These drugs are known as the majority of the active ingredients in prescription heartworm preventatives. When a heartworm positive dog is treated with these medications they should be hospitalized for at least 6-8 hours after they are given as the rapid death of the microfilariae can cause an anaphylactic (allergic) response. This is another reason why we advocate yearly screening of heartworm disease BEFORE giving out heartworm preventative. If a heartworm positive dog is given the heartworm preventative medications it is possible for severe side effects to occur. The microfilariae will be slowly eliminated over the course of a few months.
With any treatment confirmation of the elimination of all the adult worms is necessary. The best way to do that is to re-screen for heartworm antigen with in house testing. As this test shows only female adult heartworms, it is possible to still have male heartworms present. Depending on the patient, even if the repeat antigen test is negative, long term monthly treatment with another product or repeat treatment with Immiticide might be recommended.
Canine Heartworm Preventatives:
While heartworm disease can be treated, use of a veterinary recommended preventative is logically safer and more cost effective. There are a variety of products available from chewable tablets to topical products applied to the skin that are all very effective when used on a year round monthly basis to prevent heartworm disease. The American Heartworm Association and Glove Cities Veterinary Hospital recommend year round prevention. This adds to increased overall compliance, better year round protection, and better control and prevention of intestinal parasites (as most of the products also treat and prevent intestinal parasites that are also transmitted to people).
At Glove Cities Veterinary Hospital we carry two specific products that are very safe and effective at heartworm prevention.
Revolution (by Pfizer) is a once a month topical preparation (applied to the skin) that is approved for heartworm prevention that also is used for flea and tick control. It has an alcohol base which quickly evaporates leaving very little residue and has the added benefit of providing flea, tick, and heartworm all in one. It is also safe to use with canines with food allergies that cannot have chewable tablets due to flavoring components.
Iverhart MAX is a once a month chewable tablet given orally that is approved for heartworm prevention. It has the added benefit of being approved to treat and control a number intestinal parasites that can be transferred to humans including hookworms, roundworms, and tapeworms.
We hope this information has been helpful and informative!
Dr. Santspree
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