Heartworm Disease In Dogs

R. S. Hines DVM PhD 4/28/06

Canine heartworms affect dogs throughout the world. These worm's scientific name is Dirofilaria immitus. They are common in the hearts and major heart blood vesicles in pet and stray dogs in all 50 American States. The male worms are a few inches in length. The female worms are much larger in size and cause most of the damage The worms are transferred from dog to dog through the bite of an infected mosquito. After a mosquito sucks larval heartworms with blood from an infected dog; it must rest for a period of time before these larva become infective. If that mosquito then bites another dog or the same dog, it transfers these microscopic larva as it bites. Then, during the next few months, these larva migrate through the dogs body arriving at the heart several months later. There they mature into adult worms. Damage to the dog’s heart is due to adult worms and depends on the number of worms present, the worm’s sexes, and the area of the heart in which the worm lodges as well as the length of time the worms are present. Genetic factors present in the dog probably also govern the severity of the disease.
Heartworm disease is commonest in warmer areas of the world where mosquitoes are active throughout the year. Statistically, male dogs are slightly more prone to the disease than females - probably due to life style. . Large-breed dogs also suffer more from the disease as do short hair dogs and dogs kept out-of-doors.
The first sign of heartworm disease is often premature aging. Dogs gray prematurely about the muzzle and forelegs. Their activity levels decrease and their coats loose luster. They loose the "bouce in their step". As the disease progresses, a chronic dry cough often begins. This cough is most noticeable at night when the dog is resting or in a sitting position. This is due to the bronchitis that pieces of dying worms cause in the lungs as well as due to fluid accumulation in the lungs as the heart fails. Later, the dog’s tummy (abdomen) assumes a pear-shaped look as the dog’s liver enlarges and fluid accumulates in the abdomen. While these events are occurring, the dog’s heart and pulmonary arteries enlarge due to mechanical obstruction of the worms, inflammation and valvular damage to the heart.

Drugs called macrocyclic lactones are extremely effective in preventing heart worm infection. The oldest and best known of these drugs is ivermectin. The original brand was Heartgard. However the drug is off- patent and many effective brands are now sold. Other drugs in this class that seem equally effective are milbemicin oxime (Interceptor), moxidectin (Pro-Heart 6, Advantage Multi) and selemectin (Revolution). They appear to be able to "reach back" and kill migrating hearworm larva that were transferred by a mosquito bite up to two month earlier. Ivermectin should not be used in Collies, Australian shepherd Shetland sheepdogs, Old English sheepdogs and English sheepdogs.

Once your pet has been diagnosed with heartworms treatment of the disease is entirely different. This is because the drugs that prevent infection do not cure infection. Unfortunately, only one drug is currently on the market that will do that. It is called melarsomine and it is marketed as Immiticide by Merial Pharmaceutical Company. It contains arsenic. The drug can result in numerous side-effects, even an occasional death in the later stages of the disease.

Immiticide is given in a deep intramuscular injection into lumbar or loin muscles. If the dog is already clinically ill, I try to stabilize it with medications before begining the injections. The danger occurs soon after injection when the adult heart worms die and begin to disintigrate. Portions of the worms are swept into the lungs where they have the potential to cause fatal blockages (emboli) of circulation. To minimize this exercise must be strictly restriction during the recovery period. Two of these injections are usually given with a 24-hour interval between them. Some institutions use this 3-dose protocol in all cases.

We know that giving heartworm preventative, such as ivermectin, weakens and shrinks the size of adult worms and may decrease their lifespan. Because of this, some veterinarians put infected dogs on 2-6 months of preventative before administering melarsomine. Some give it twice a month. If financial or other considerations prevent Imiticide treatment, putting posative dogs on heartworm preventative may hasten the death of the adult worms, improve health and prevent the spread of the disease.

Unfortunately, adult heartworm treatment does not always kill all the worms that are present. It is a dificult decision whether to repeat the melarsomine in dogs that still come up posative on an occult heartworm blood test six month after the treatment. Since the majority of the worms have been eliminated, I rarely suggest repeat treatment.

Heartworms only rarely affect human beings. When they do, they form nodules in the lungs which can be mistaken for tumors.

Because of the expense of melarsomine treatment as well as the risks involved in severe cases, some owners and humane groups elect to simply place posative dogs on ivermectin, hoping that weakening and shriveling the heartworms will improve the dogs general condition while one waits for the heartworms to die naturally. The normal lifespan of a heartworm is approximately 2 years possibly shorter when the dog receives monthly or bi-monthly ivermectin.

Wolbachia is an organism that lives inside of heartworms. Some veterinarians believe that Wolbachia is responsible for some of the embolisms and malase that occur during treatment. Because of this, some veterinarians are pre-treating dogs with doxycycline to destroy Wolbachia prior to administering melasromine

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