Heartworms are preventable…It is much smarter to prevent then to have to treat!
Dogs are the perfect hosts for heartworm infections. In experimental infections of heartworms, the percentage of infective larvae developing to adult heartworms in dogs is 40% to 90%. On the contrary, 100% of the experimentally-infected dogs had adult worms. The worm burden in dogs is usually high ranging from 1 to 250 worms. The life span of the worms in dogs appears to be about 5 to 7 years. The average prepatent period (the time elapsed from when the larvae enter the host until the adult female worms begin to produce microfilaria) in dogs is about 6-7 months.
Incidence
Heartworms are a common parasite in many areas. Each of the 50 states has had confirmed cases of canine heartworm infection. All dogs regardless of their age, sex, or habitat are susceptible to heartworm infection. The highest infection rates are found along the Atlantic coast. This means that from Texas to New Jersey and along the Mississippi River and its tributaries is a “danger zone.”
Other areas of the United States have lower incidence rates (5% or less) of canine heartworm disease, while some regions have environmental, mosquito, and dog population factors that allow a higher incidence of heartworm infection. Endemic regions have heartworm infections diagnosed in dogs as young as 1 year, with most areas diagnosing infections primarily between the ages of 3 and 8 years of age.
The infection rate in male dogs is as high as 4 times then females. Outdoor dogs have a infection rate of 4-5 times that of an indoor dog. Nonetheless, for all dogs that live in areas where mosquitoes can survive, preventative programs need to be used.
Prevention
While treatment of canine heartworm disease is usually successful, it is very expensive. Prevention of heartworms is a much safer and more economical way of dealing with infection. Multiple approved products are available in the US for the prevention of canine heartworm infection.
The routine administration of oral, topical , or injectable heartworm prevention will prevent a dog from becoming infected. These drugs are highly effective when administered at the appropriate doses and intervals. All dogs (especially those that live in warmer climates) need to be kept on preventative year-round. A heartworm test needs to be given to all dogs that are five months of age or older before starting prevention. This will confirm that the dog is clear before their first dose of prevention is administered. There are multiple different brands and types of heartworm preventions. Discuss which heartworm prevention works best for you and your dog.
Common Myths
Myth: My dog tested a “light positive” so it is no big deal
The heartworm antigen test only detects female heartworms. One female does not mean one male. There could be only one adult female in the heart along with multiple males. The test will not detect the males. In turn, a light positive does not mean that there are only a few worms, it actually means there are only a few females. There is ONE positive to having a “light positive” result. That is, if there are not many females then there are only a few females producing microfilaria. In turn, if you start your dog on heartworm preventative, there is a higher chance of beating the heartworms without treatment.
Myth: My dog can outlive the heartworms, so I do not need to treat it.
Well this is somewhat true, but it is not as easy as it sounds. If the dog is young when diagnosed with heartworms, it is possible that he/she can outlive the disease since the lifespan of a heartworm is between 5 and 7 years. In turn, the younger the dog, the higher the chance he/she can outlive the infestation. If you put your dog on heartworm medication IMMEDIATELY once you find out he/she has heartworms.
You may wonder why this is a necessary measure. Why treat a dog with heartworm preventative when he/she already has heartworms? The monthly chewable heartworm prevention actually only stays in the dog’s system for 24 – 48 hours. Chewable prevention is designed to enter the bloodstream, kill the microfilaria, and then leave the dog’s system. If you decide to not to fully treat your dog but decide to put him/her on heartworm prevention, then you will at least make sure that the heartworm offspring are not making it to maturity and causing a more severe infestation.
One circumstance you must consider is that the longer the heartworms are in the dog’s heart, the more damage they can do to the heart. In turn, there is a good chance your dog will end up with heart issues if he/she has to live with them in his/her heart for 5 – 7 years. If you choose not to treat your dog in any way, the infestation will kill him/her.
Myth: My dog does not need to be on heartworm prevention because he/she is an inside dog.
This is absurd! First, most dogs go outside to go to relieve themselves. Even if they do not, if you have a door to get into your house, you have a chance of having a mosquito in your house. All it takes is ONE mosquito that has previously bitten a dog with heartworms. This nasty little blood sucker then bites your dog and 6 months later your dog has heartworms. Believing that “indoor” dogs do not need prevention is ridiculous.
A Hypothetical:
Suppose that within a square mile there are 4 dogs (in addition to yours) and 20 mosquitoes. Now let’s say that 10 of the mosquitoes feed on humans and 10 feed on dogs. Lastly, let’s say that 2 dogs are on heartworm prevention and 2 are not. Out of the 20 mosquitoes, there is a 25% chance that a mosquito will bite one of the dogs that is not on prevention.
To make this easy, we will say 5 out of the 10 dog biting mosquitoes bit the dogs that were not on prevention. Imagine that one mosquito bit a human, two got stuck in a bug light, and three were eaten by predators. This leaves us with 14 mosquitoes that make it to your house. We have no idea which mosquitoes bit dogs and which ones bit humans.
We also do not know which mosquito’s bit the dogs that were not on heartworm prevention. There is no way to know if one of the remaining insects will end up biting your dog. Do you want to just cross your fingers and believe there is no chance an infected mosquito could get into your house? Would it not be smarter to just pay the small monthly fee to PREVENT your dog from getting this disease? In my opinion, this is an easy answer.
A Positive Antigen Test
What are Microfilaria
Microfilaria (The larval form of a heartworm) are relatively common in dogs. Microfilaria are the larval stage of heartworms. Not all heartworm infections result in the circulating Microfilaria that can be found upon examination of the dog’s blood. When there are no Microfilaria in the bloodstream, the infection is known as an occult heartworm infection and can be the result of a number of factors.
These factors include:
- single sex heartworm infections
- host immune responses affecting the presence of circulating microfilaria
- the administration of heartworm preventives
Severity
The onset and severity of disease is usually a reflection of the number of adult heartworms present. Dogs with higher worm counts show increased levels of severe cardiac and pulmonary disease.
Until the number of heartworms exceeds 50 per 25-kg (55 lbs), nearly all of the heartworms reside in the caudal pulmonary arteries. Higher numbers of heartworms will result in their presence in the right ventricle, and possibly the right atrium, of the dog’s heart. The most common pathological changes created by heartworms are due to inflammatory processes that occur in and around the caudal pulmonary arteries in response to the presence of heartworms.
Clinical Signs
Heartworm infection may cause multiple system dysfunction affecting the pulmonary circulation, heart, liver and kidneys. Heartworm disease may have an acute onset, but it is usually insidious, resulting from a chronic infection with a combination of underlying pathophysiologic responses (The functional changes associated with or resulting from disease or injury).
Dogs with low worm burdens that receive little cardiopulmonary exercise may never have overt signs of heartworm disease. The major organs affected by heartworms in dogs are the heart and lungs. Infected dogs can have no clinical signs, or can present with mild, moderate, or severe clinical signs.
| Acute = No abnormal, clinical signs |
| Mild = Possible cough |
| Moderate = cough. exercise intolerance, and abnormal lung sounds |
| Severe = Cough, exercise intolerance, dyspnea (difficulty breathing), abnormal lung sounds, hepatomegaly (enlargement of the liver), syncope (temporary loss of consciousness due to poor blood flow to the brain), ascites (fluid accumulation in the abdominal cavity), abnormal heart sounds, death |
Diagnosis
The diagnosis of canine heartworm disease depends upon an accurate patient history, the recognition of varied clinical signs, and the use of several diagnostic procedures that may include radiology, angiography, and ultrasonography, serologic testing, Microfilaria detection/differentiation, and clinical laboratory tests.
To catch an infestation quickly, many vets ask that a heartworm test is given every year. This is an easy and quick test to make sure your dog is clear.
A diagnostic test is necessary because the physical examination may be perfectly normal in heartworm infected dogs with mild disease.
The identification of microfilaria from a blood sample is indicative of infection with adult heartworms in over 99% of infested dogs. Veterinarians will often do a quick examination of a blood smear under a microscope to look for the presence of Microfilaria, but this procedure is not sensitive enough to serve as a final rule out for the absence of microfilaria in a sample.
Treatment
Of the dogs infected with heartworms, the majority of them will have successful treatment. The goal of treatment is to kill all adult worms with an adulticide. The microfilaria are then killed using a microfilaricide. It is important to accomplish this goal with minimal drug toxicity and nominal complications created by the dying heartworms.
Heartworm infected dogs which are normal or have mild disease have a high treatment success rate. Patients with evidence of more severe heartworm disease can be successfully treated, but the incidence of post-adulticide complications and mortality are greater. Certainly, there are patients whose severity of heartworm disease, or presence of other concomitant life-threatening diseases, prevent treatment of the heartworm infection.
Post Adulticide Complications
The primary post-adulticide complication is the development of severe pulmonary thromboembolism. Pulmonary thromboembolism results from the obstruction of blood flow through pulmonary arteries due to the presence of dead heartworms. If heartworm adulticide treatment is effective, some degree of pulmonary thromboembolism may occur. This means that some dogs may have so many worms in their heart that the dead worms running through the circulatory system create a blockage. When dead worms are numerous, widespread obstruction of arteries can occur. Most commonly observed clinical signs include fever, cough, hemoptysis (expectoration of blood). These patients require strict reduction in exercise and anti-inflammatory doses of corticosteroids.
Completion of Adulticide
Microfilaricide treatment is typically administered 3 to 6 weeks following completion of the adulticide treatment. This allows an adequate amount of time to pass for the adult worms to die and to have ceased production of additional microfilaria. The most effective drugs for this purpose are the macrolide anthelmintics – ivermectin and milbemycin. These drugs are the active ingredients in the commonly use heartworm preventives.
A positive dog needs to be hospitalized for 8 hours following treatment with anthelmintics. Observation is necessary for monitoring of possible adverse reactions resulting from rapid death of the microfilaria.
Confirmation of Adulticide Efficacy
Three weeks after microfilaricide treatment, a microfilaria concentration test is performed. Pets that test microfilaria negative have completed the treatment for canine heartworm infection. The dog can then be started on a heartworm preventive program.
The goal of adulticide treatment is the elimination of all adult heartworms. However, clinical improvement in dogs treated for heartworm infection is possible without completely eliminating the adult heartworms. During and after the heartworm treatment the dog MUST be kept on preventative. This can help eliminate the worms that the body is fighting.
Three to five months after the completion of treatment, another heartworm test is given. Heartworm antigen testing is the most reliable method of confirming the efficacy of adulticide therapy. The heartworm antigen will be undetectable by 16 weeks post-adulticide, if the adult worms have been, or are nearly destroyed. Dogs that remain antigen positive after treatment should only be considered as candidates for repeat treatment after a full case review.
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About the Author
I have been working with animals for as long as I can remember. Throughout my teens, undergrad, and graduate school, I worked as a veterinary technician. From 2007 – 2011, I owned and ran a registered dog rescue where we worked with the dogs that needed rehabilitation before placement in a forever home. In addition, I have been breeding dogs since 1996. Through all of this, I have consistently researched to keep myself apprised of the latest information concerning canine health and care. As a college professor, I feel knowledge is beneficial and I try to share it when I can. I put my heart and soul into my small breeding program. God, my family, my pups, and my extended fur-family are what brings me happiness.