Heartworms: Truths and Myths

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Heartworms: Truths and Myths

Dr. Blagburn a distinguished university professor in the Department of Pathobiology in the College of Veterinary Medicine at Auburn University in Auburn, AL., graciously has allowed TVGRR to reprint this article that appeared in a Spring issue of HealthyPet. Dr. Blagburn is a board member and past president of the Companion Animal Parasite Council, and an executive board member of the American Heartworm Society.

Focus: Heartworms. Test your knowledge of how dangerous these parasites are to your pet.

Among the illnesses that affect our pets, few are better known than heartworm disease. And for good reason; it is potentially life threatening to our pets. But do you truly understand all aspects of this disease – and how to prevent it? To test your heartworm smarts, determine whether the following statements are true or false. I’ll bet many of you will be surprised by some of the answers.

Cats Don’t need heartworm prevention. –FALSE. Heartworms affect cats and dogs differently, but both species need to be on preventive medication. In fact, prevention is especially important in cats since feline heartworm disease can be difficult to diagnose. Why? Because heartworm-infected cats seldom harbor adult heartworms or their prelarval state, known as microfilariae. Detecting the adult worms or circulating microfilariae is the basis for diagnosing heartworm infections. Although an additional test for cats can detect heartworm-specific antibodies, this test only confirms exposure to heartworm-infected mosquitoes.

To understand how heartworms affect cats – and dogs – it’s important to know the heartworm lifecycle.

    1. First, an infected mosquito ingests microfilariae circulating in the blood by biting an already infected animal.


    1. The microfilariae develop into an infective larval state inside the mosquito.


    1. The infective larvae are transmitted to another animal via a mosquito bite.


    1. The larvae migrate to the animal’s heart and pulmonary arteries where they develop into adult heartworms over about six months.


    The adult female heartworms release microfilariae into the animal’s blood stream after mating, starting the cycle all over again.

In cats, many of the larvae never develop into adult heartworms but instead migrate to the lungs through the pulmonary arteries where they die and cause inflammation. This can cause cats to develop severe respiratory disease, even without worms in their hearts. Respiratory diseases, including coughing and labored breathing that we see in many heartworm-infected cats, is similar to other feline respiratory or heart diseases such as asthma, cardiomyopathy (disease of the heart muscle), or bacterial or viral infections that cause pneumonia.

Finally, and most importantly, we have effective treatments for canine heartworm disease – although they can be costly and painful, and if not begun soon enough, might not prevent heart and lung complications. These same treatments cannot be used to eliminate adult heartworms in cats.

All pets require heartworm prevention regardless of whether they reside indoors or outdoors. –TRUE. Even though truly indoor pets are less likely than outdoor pets to be bitten by heartworm-infected mosquitoes. Indoor pets still get infected with heartworms. In fact, North Carolina University’s recent survey of heartworm-infected cats revealed that 27% of the cats were characterized by their owners as residing entirely indoors. One reason this misperception continues is that some pet owners think that if their pets live inside, they just cannot be exposed to mosquitoes. However, many pets are allowed to sit or lie near open windows or accompany their owners out on a deck or patio. Also, most of us have seen mosquitoes inside our homes. Do not allow this myth to be perpetuated. Insist on year-round protection regardless of your pet’s indoor or outdoor habits.

Heartworm prevention should be discontinued during the colder months. –FALSE. Many pet owners in the northern United States practice heartworm prevention only during the warmer months. Historical climatologic data are often used to predict start and stop times for heartworm preventive medications. However, continued urbanization and commercial development of rural habitats have had a significant local effects on seasonal temperatures. Man-made water systems and urban development, including more concrete and asphalt surfaces, have affected our environment. These changes have increased average daily temperatures and the potential for heartworm transmission periods to change. therefore, the presumption that the heartworm prevention strategies that worked decades ago remain valid today is often erroneous.

Another point to consider is that many heartworm preventives also provide protection against other parasites such as roundworms, hookworms, whipworms, fleas, and even some mites. Failure to use parasite-prevention products year-round affects our capability to control these important parasites.

Finally, recent surveys indicate that heartworm disease is present in all parts of the United States. Also, dogs are often relocated from high heartworm prevalent areas, such as the southeastern United States, to areas where heartworm is less common. For example, dogs rescued in Louisiana after Hurricane Katrina were sent to Ontario, Canada. According to the Hamilton Academy of Veterinary Medicine, the years following the relocation of the infected dogs, the prevalence of heartworm in Ontario increased dramatically. the changing geographic prevalence of heartworm infection, due in part to pet location and travel to heartworm-endemic regions, should be reminders of the continuing risk of heartworm disease.

Dogs and cats receiving heartworm preventives do not require regular heartworm tests. –FALSE. Although the heartworm preventive medications are highly effective, pets do rarely acquire heartworm infections while receiving them. Most of these infections are due to pet owners forgetting to administer their preventives every month, pet owners administering them improperly, or pets not swallowing their tablets. Some cats and dogs are capable of hiding their pills, leaving the unsuspecting pet owner believing the pet was properly treated. Annual testing can detect these infections before they can cause severe damage to a pet’s heart or lungs. Also, the tests your veterinarian uses to detect heartworms are more accurate than they were previously. In many cases, a single heartworm can be detected in infected pets. Although feline heartworm disease is more difficult to diagnose than canine heartworm infection, many veterinarians still recommend regular testing in cats.

Know the Signs of Heartworm Disease

The following symptoms warn of heartworm disease in dogs and cats. However, they could also be signs of other medical conditions. Therefore, if your pet exhibits these symptoms, it’s important that your pet see the veterinarian to get a diagnosis. Only then will the veterinarian be able to recommend proper treatment.

Dogs – Coughing is the more frequent symptom of dogs suffering from heartworm infection. Affected dogs may also exhibit shortness of breath and exercise intolerance, which means they may be easily fatigued with normal physical exercise. The physical signs you see depend on the duration and severity of the infection as well as the individual dog’s reaction to the parasite. If only a few heartworms are present, you may observe no signs, which illustrates the importance of routine heartworm testing. But if the disease has already progressed, a dog may show weigh loss – even emaciation – and difficulty breathing, fever, fluid in the abdomen, and even sudden death.

Cats – Since heartworm disease affect cats’ lungs more often than their hearts, cats suffering from heartworms usually show respiratory signs such as coughing, rapid breathing, open-mouthed breathing, and gagging. Furthermore, vomiting that’s not associated with eating and fainting is more common in cats than in dogs. Other signs of heartworm disease to watch for in cats include diarrhea, anemia, weight loss, and lethargy.