Heartworm Disease: An Ounce of Prevention is Worth a Pound of Cure

By:  Loren W. Noblitt, DVM, MS

     Heartworm disease is a serious and potentially fatal condition caused by parasitic worms called Dirofilaria immitis.  These worms-heartworms-live in the arteries of the lungs and occasionally in the right side of the heart of dogs, cats and other species of mammals including wolves, foxes, ferrets, sea lions and, in rare instances, human beings!  Heartworms are classified as nematodes (roundworms) and are filarids, one of many species of roundworms. Dogs and cats of any age or breed are susceptible to infection.
    
     Naturally acquired heartworm infection in dogs and cats is identified as a worldwide clinical problem in veterinary medicine today. Despite improved diagnostic methods, effective preventives, and increasing awareness among veterinary professionals and pet owners, cases of heartworm infection continue to appear in pets around the world.  It occurs wherever the following four factors are found: i) a susceptible host population, ii) a stable reservoir of the disease, iii) a stable population of vector species, iv) a climate that supports the parasite's life cycle.
    
     Dogs are considered to be the definitive and "susceptible host" for the heartworm parasite, meaning that even though the disease is not transmitted directly from one dog to another, untreated dogs provide a "stable reservoir" for the disease for mosquitoes. Mosquitoes of several different species are the "stable vectors" and are considered to be intermediate hosts for the developing microfilariae (baby heartworms).  Development of the microfilariae in the mosquito requires a temperature at or above 80 degrees F for about two weeks.  Interestingly, no larval development takes place in the mosquito below 57 degrees F.
    
     In the United States, heartworm disease continues to be a problem.  It has been reported in all 50 states, all countries, and all continents except Antarctica. The map below shows particularly endemic areas in the United States based on the number of cases reported by clinics.  Note that Indiana is in the "26-50 cases" of Heartworm disease reported per Veterinary clinic, and that on average, the Zionsville Animal Hospital treats 10-15 cases of heartworm disease per year.


The Life Cycle

     Adult female heartworms bear live young - thousands of microfilariae in a day-released into an animal's bloodstream. Then, while taking a blood meal from the infected animal, mosquitoes become infected with these microfilariae.  During the next 10 to 14 days, the microfilariae mature to the infective larval stage within the mosquito. After that, the mosquito bites another dog, cat or other susceptible animal (see above), and the infective larvae actually burrow into the animal through the mosquito's bite wound and undergo several changes to reach adult form.  They then travel to the right side of the heart through a vein and await the opportunity to reproduce.
    

Heartworms in the Pulmonary Artery of a dog

    
    
     Microfilariae cannot mature into adult heartworms WITHOUT first passing through a mosquito.  It then takes a little over six months for the infective larvae to mature into adult worms. In dogs, the worms may live for up to seven years if there is no treatment given.  Adult heartworms can reach twelve inches in length and can remain in the dog's heart for years.  In cats, by contrast, heartworms live only several years.  In both cases, there can be tremendous, long-term damage to the lungs as a result of heartworms.

What Are the Signs of Heartworm Disease?

     For both dogs and cats, clinical signs of heartworm disease may not be recognized in the early stages, as the number of heartworms in an animal tends to accumulate gradually over a period of months and sometimes years and after repeated mosquito bites.  The first sign of heartworm infestation is often a soft cough that increases with exercise, and commonly may be dismissed as being trivial or unimportant by the owner.  But often the cough worsens and the dog may actually faint from exertion, tires easily, becomes weak and listless, loses weight, and may cough up blood.  Breathing becomes more difficult as the disease progresses because the worms are multiplying in the lungs and heart. The progression is traumatic: the dog's quality of life diminishes drastically and he/she can no longer retrieve a ball or Frisbee; can no longer take a nice walk in the park without respiratory distress.  Congestive heart failure ensues, and the once-active family pet is in grave danger.
    
     Cats may exhibit clinical signs that are very non-specific, mimicking many other feline diseases.  Chronic clinical signs include vomiting, gagging, difficulty or rapid breathing, lethargy and weight loss.  Signs associated with the first stage of heartworm disease, when the heartworms enter a blood vessel and are carried to the pulmonary arteries, are often mistaken for feline asthma or allergic bronchitis, when in fact they are actually due to a syndrome defined as Heartworm Associated Respiratory Disease (HARD).  Chest x-rays and objectively measuring the cat's heart, in conjunction with heartworm blood testing, should be done to look for heartworm disease.  These tests are recommended even if a cat has been on heartworm prevention reliably.
Detecting Heartworm Disease
    
     Heartworm infection is usually detected with blood tests for a heartworm substance called an "antigen" (found only on the adult female's body) or actually seeing the microfilariae under the microscope on a routine blood smear.  Neither test is consistently positive until about seven months after infection has occurred.  An antibody test is the preferred blood test for cats because the worm burden is usually smaller in cats versus dogs and there may not be significant antigen (e.g. significant number of female worms) in an affected animal's blood to make the test positive. Heartworm infection may also occasionally be detected through ultrasound and/or x-ray images of the heart and lungs, although these tests are usually used in animals already known to be infected.
Prevention
    
     Because heartworm disease is preventable, the American Heartworm Society (AHS) recommends that pet owners take steps now to talk to their veterinarian about how to best protect their pets from this dangerous disease.  Heartworm prevention is safe, easy, and inexpensive; however, treatment for heartworm disease is a complicated and expensive process, taking weeks for infected animals to recover and, during the recovery period, STRICT exercise restriction is mandatory. Recently the drug used to treat heartworms in dogs, Immiticide (melarsomine), has become increasingly hard to obtain, resulting in some animals with heartworm disease having to wait for treatment to be available. Thus, the expression, "an ounce of prevention is worth a pound of cure" is truly apropos. There is currently no effective treatment for heartworm disease in cats, so it is imperative that disease prevention measures be taken for cats, especially for at-risk, outdoor cats in particular.  For more information on this, please ask your veterinarian.
    
     There are a variety of options for preventing heartworm infection in both dogs and cats, including monthly tablets (Interceptor, Sentinel, and Trifexis) and chewables (Heartgard Plus), monthly topicals (Revolution and Advantage Multi) and a recently re-released, six-month injectable product called ProHeart available only for dogs.  Some of these preventives also kill gastrointestinal parasitic worms and fleas, as well.  All of these methods are extremely effective, and when administered properly and on a timely schedule, heartworm infection can be completely prevented. These medications interrupt heartworm development before adult worms reach the lungs and cause disease.  Veterinarians recommend a year-round heartworm prevention program to guard against the occasional mosquito flying about in areas with mild winters, such as Indiana.

Treatment

     The first step is to evaluate the dog and treat any secondary problems of heart failure, liver or kidney insufficiency, so that he/she can withstand the treatment. The next step is to kill the adult worms with Immiticide (an arsenic compound).  This compound is carefully injected deep into the muscles adjacent to the lower spine through a series of treatments. Treatment may be administered on an outpatient basis, but hospitalization is sometimes recommended, depending on a number of variables such as energy level, attitude, home environment, other pets or not, etc.  When the dog returns home, strict exercise restriction must be enforced to decrease the risk of partial or complete blockage of blood flow through the lungs by dead worms ("worm plugs" or "worm mats").  Importantly, re-infection during treatment is prevented by administration of a heartworm preventive, as described above.  Usually, all but the most advanced cases of heartworm disease can be successfully treated in dogs.  Again, there are no products in the United States currently approved for the treatment of heartworm infection in cats-the disease just has to run its course.  Veterinarians will often attempt to treat an infected cat with supportive therapy measures to minimize immune reaction; however it is always best to prevent the disease.

     For more information on heartworm disease or if you have specific questions about this disease or specific preventives for your dog or cat, please contact the veterinarians or staff at the Zionsville Animal Hospital.

Photos Courtesy of:  The American Heartworm Society
http://www.heartwormsociety.org/pet-owner-resources/heartworm.html