As many of you are aware, spring is the time when your veterinarian recommends that your dog take a preventive medication to protect against heartworm diease. Below, you will find information that will help explain to you what heartworm disease is and why it is important to prevent the disease.
At the current time, we are not recommending heartworm prevention in cats in Quebec since the disease is still very rare in this species in this geographical area.
If you need additional information, do not hesitate to call us anytime to speak with one of our animal health technicians or veterinarians. We will be happy to answer your questions.
The Baker Animal Hospital Team
What is heartworm disease?
Heartworm disease is one of the major health problems of dogs in the United States and throughout the temperate and tropical areas of the world. As well as being found in dogs and other species, it is now being found in cats in ever increasing numbers. The disease develops when a pet becomes infected with parasites called Dirofilaria immitis that are transmitted through the bite of an infected mosquito. Dogs may be infected by a few or up to several hundred heartworms. Cats are similarly infected although usually by only a few worms. Heartworm infection often leads to severe lung disease and heart failure and can damage other organs in the body as well.
How is heartworm disease transmitted from one pet to another?
Adult female heartworms living in an infected dog or other host release their young, called microfilaria, into the bloodstream. Mosquitoes become infected by the microfilaria while taking a blood meal from these infected animals. During the next 10 to 14 days, microfilaria mature to the infective larval stage within the mosquito. When the mosquito then bites another dog, cat or susceptible animal, the infective larvae exit the mosquito’s mouth parts and are deposited onto the surface of the animal’s skin. The infective larvae can then actively enter the new host through the fresh bite wound.
Inside a new host, it takes a little more than six months for the infective larvae to mature into adult heartworms. Once mature, heartworms may live up to five to seven years, and because of their longevity, each mosquito season can lead to an increasing number of worms in our pets.
What physical signs could my dog have?
Heartworms may accumulate gradually over years, or quickly when conditions allow exposure to high numbers of mosquitoes carrying infective heartworm larvae. Clinical signs of disease may not be easily recognized in pets that have been recently infected or in those with low numbers of heartworms as they may not yet exhibit outward signs of disease. However, pets heavily infected with heartworms or those with chronic disease often show prominent clinical signs.
In dogs, signs of heartworm disease may include a mild persistent cough, reluctance to exercise, fatigue after moderate activity, decreased appetite and weight loss. As heartworm disease progresses, pets may develop heart failure commonly recognized by an accumulation of fluid in the abdomen giving the pet the appearance of a “swollen belly.” Dogs infected with large numbers of heartworms can develop a sudden blockage of blood flow within the heart leading to a life threatening form of cardiovascular collapse called “caval syndrome.” Signs of caval syndrome include a sudden onset of labored breathing, pale gums and dark bloody or “coffee-colored” urine. Without prompt surgical removal of the heartworm blockage, few pets survive.
Can you tell me more about heartworm testing in dogs?
Two common types of tests exist for diagnosing heartworm infection in dogs. Because adult heartworms release their young (microfilaria) directly into a dog’s bloodstream, a relatively simple blood filter test can identify them. A positive test tells us adult worms are present. Positive means positive. Unfortunately, 15 to 20 percent of heartworm-positive dogs will not have “microfilaria” circulating in their bloodstream and a negative test will sometimes be falsely negative. The most accurate test for detecting heartworm infection in dogs is the antigen test. This test looks for the presence of small proteins released by adult female heartworms into the dog’s bloodstream. A positive test tells us mature female worms are present. And, while false negative results are uncommon, they can occur if a pet has a “male-only” infection (since the test detects antigen from females), if only one or two worms are present, or if the female worms are immature.
Your veterinarian may have reason to suspect a negative test result to be inaccurate and might recommend re-testing using other methods. Chest X-rays and ultrasound evaluation can help identify heartworm disease and may be indicated.
How long does it take before heartworm infection can be detected by blood tests?
It takes five to seven months from the time a dog is bitten by infected mosquitoes until a blood test can accurately detect the presence of adult worms.
Do you need a prescription for heartworm preventive medication? If so, why?
Yes, heartworm preventives must be purchased from your veterinarian or with a prescription through a pet pharmacy. The U.S. Food and Drug Administration (FDA) labeling on heartworm preventives indicates that the medication is to be used by or on the order of a licensed veterinarian. This means a veterinarian must have a doctor-client-patient relationship in order to write a prescription. Typically, prior to prescribing a heartworm preventive, the veterinarian will perform a simple heartworm test to make sure your dog doesn’t already have adult heartworms. It is not necessary to test very young puppies prior to starting preventives since it takes approximately six months for adult heartworms to develop to adulthood in a dog. If the pet is free of heartworms, prevention is prescribed. Giving preventives to dogs infected with heartworms can lead to rare but possibly severe reactions that could be harmful or even fatal to the dog.
There are many types of medications available for heartworm prevention. What is the difference between the daily and monthly tablets?
Until the late 1980s, the only medication available for the prevention of heartworms had to be given daily. These products work by killing the microscopic heartworm larvae deposited by the mosquito, but must be given every single day to be most effective. This is because infective heartworm larvae quickly molt within two to three days into their fourth stage of development. This fourth stage can not be killed by the daily medication. Daily heartworm preventives have largely been replaced by monthly products and are no longer commercially available in the United States. Compounding pharmacies still formulate daily preventives on an “as needed” basis. An important note to keep in mind is that monthly medications are quickly eliminated from a pet’s system and do not continue to work for 30 days. Instead they work “backwards” to eliminate the larvae the pet acquired the previous 30 days, in essence, “de-heartworming” our pets each month in many cases, these monthly preventives control other parasites too.
How do monthly heartworm preventives work?
Fortunately, there are many very effective once-a-month heartworm preventives available today. Some are chewable tablets and others are topically-applied solutions. Monthly heartworm preventives, because of their ease of use and effectiveness, have become the popular choice for prevention of heartworm disease. Unlike the daily products of the past, these compounds are capable of killing developing heartworm larvae, and administering the preventive every month will effectively eliminate the chance of infection. Check with your veterinarian to see which product is right for your pet.
Why do dogs need to be blood tested before starting heartworm medication?
Before starting a preventive program, all dogs should be tested for heartworms. Giving preventives to dogs that have adult heartworm infection can be harmful or even fatal to the pet.
Adult heartworms produce millions of microscopic “baby” heartworms (called microfilaria) into the bloodstream. When you give a monthly heartworm preventive to a dog with circulating microfilaria, this can cause the sudden death of microfilaria, triggering a shock-type reaction. Even if your dog does not have this type of reaction, heartworm preventives do not kill the adult heartworms (although they may shorten the worms’ life expectancy). This means an infected dog will remain infected with adult heartworms.
Unfortunately, as long as a pet remains infected, heartworm disease will progress and damage the heart and lungs, which can lead to life threatening problems. Giving heartworm preventives to heartworm-positive dogs can mislead an owner into thinking everything is all right, while within a pet, heartworm disease is worsening.
I heard that certain heartworm prevention medications will also protect against intestinal parasites. Is this true?
Certain heartworm preventive products are also effective in removing specific intestinal (and external) parasites and are labeled for such uses. They either contain a single active ingredient that is effective against several parasites including developing heartworm or a combination of ingredients to achieve control of many different parasites. Such products have been tested and meet the same safety requirements as the heartworm-prevention-only products.
I heard that the heartworm prevention medication is toxic to certain breeds of dogs, particularly collies. Is this true?
When given as prescribed, all of these medications are safe. It has been found, however, that some dogs are genetically predisposed to be more sensitive when doses dramatically exceed the recommended amount. Problems can occur when products designed for large animals (horses, cattle, pigs) are inappropriately used in dogs, or when dogs are dosed incorrectly. Heartworm preventives are safe for all breeds of dogs when used as directed.
Is a puppy born with immunity to heartworm disease?
No, even nursing puppies are at risk for heartworm infection. Puppies of any age exposed to mosquitoes carrying infective larvae can become heartworm-infected, so it is important to begin prevention early. Puppies can be started on heartworm preventive as early as four to eight weeks of age, depending on the label recommendations of the preventive.
Is there an effective natural prevention for heartworm?
No, there is no natural prevention for heartworms.
What is the treatment for heartworm disease in dogs?
If a dog is infected with heartworms, the treatment needs to kill the adult and immature worms. Currently, only one product is approved by the FDA for this purpose (Immiticide®- melarsomine hydrochloride). It is given by deep injection into muscle. A series of injections are given, either over a 24-hour period or two treatment periods, one month apart. While treatment may be administered on an outpatient basis, hospitalization for the procedure is often recommended. Other medications may be given at the time of treatment depending on the stage of heartworm disease.
What causes the death of a dog due to heartworm disease?
Heartworm disease may cause a combination of medical problems within the same dog, including heart, lung, kidney and liver disease. The worms are found in the right side of the heart, and in the major vessels that bring blood to and from the right chambers of the heart. The worms cause inflammation of the blood vessels and can block blood flow leading to pulmonary thrombosis (clots in the lungs) and heart failure. Heartworm disease can also lead to liver or kidney failure, causing death by one or a combination of these problems.
Is heartworm contagious from dog to dog?
No. Heartworms can be transmitted from animal to animal only by the bite of a mosquito carrying the infective stage of the larvae. Heartworms release live young (microfilaria) directly into the bloodstream of a dog. When a mosquito takes a blood meal from an infected pet, it may become infected by several microfilariae. The larvae then develop into an infective stage within the mosquito. As the mosquito bites another susceptible dog or cat, the infective larvae can be left behind to cause infection. The life cycle of the heartworm requires the mosquito as an “intermediate host.” Without the mosquito, heartworms can not be transmitted.
Can children get heartworm disease by playing with and being licked by a dog with heartworm disease?
No. Heartworm disease is transmitted only by the bite of a mosquito that is carrying the infective stage of the parasite.
I have missed two months of heartworm prevention for my dog. Should I worry?
Yes, you should worry. You need to consult your veterinarian, and immediately start your dog back on monthly preventive and retest in seven months. The reason for testing seven months later is that heartworms must be approximately seven months old before the infection can be diagnosed.
The above questions and answers were provided courtesy of the American Heartworm Society. For more information, go to www.heartwormsociety.org