Canine Heartworm
Heartworm and you dog.....

Canine Heartworm Disease

What is it? Canine heartworm disease is a serious and potentially fatal disease of dogs. Long white worms, technically known as Dirofilaria Immitis, are the cause. Adult worms, which reach a length of 6-14 inches, live in the right side of the heart and in the adjacent large blood vessels (heartworm). A dog may have several hundred of them in its system, although the number is usually much less.

Occasionally canine heartworms are found in other animals such as foxes, wolves, cats, and ferrets. In a few instances they have been reported in people, as small nodules in the lungs. Such human cases are considered biological oddities.

What do they do?

Adult heartworms in the heart and adjacent large blood vessels cause extensive damage to some of the dog’s vital organs. The lungs are the first organs affected, and just a few worms can cause a considerable amount of damage. As the disease progresses, the heart becomes enlarged and does not function normally. Later, the liver and kidneys may also be damaged. The important thing for dog owners to remember is that a great deal of damage can occur before any outward signs of heartworm disease are noticed.

Until recently canine heartworm disease was considered to be a problem only in warm climates, but in the past few years it has been found in almost all areas of the United States and Canada. Since dogs travel widely with their owners, and infected dogs can carry heartworms for several years, heartworm disease may be a problem anywhere in the nation.

Heartworm infection is transmitted by mosquitoes. When a mosquito bites an infected dog, it takes up blood which may contain microfilarae. These incubate in the mosquito for about two weeks, during which they become infective larva. Then, when the mosquito bites another dog, the infective larvae are passed into the second dog, infecting it. The infective larvae migrate through the tissues of the body for 2-3 months. They develop into several stages called L1, L2, and L3 stages. The L1 stage only lasts for 1-2 days. The L2 and L3 stages last for approximately two months. They then enter the heart where they reach adult size approximately 3 months after infecting your pet.

The mosquito is the only natural vector of transmission for canine heartworms, and about 70 species are capable of carrying the disease. As you might expect, heartworm infection is more common in areas where mosquitoes are numerous, and outdoor dogs constantly exposed to mosquitoes are the most frequent victims.

How is it detected?

Heartworm infection may be detected by examining the dog’s blood for the presence of circulating microfilarae. (microscopic baby forms) or by examining the blood for antigens that are produce by the adult worms. Some dogs have adult worms in the heart but have no circulating microfilariae. To diagnose the disease in these dogs antigen tests are used. Chest x-rays are also helpful in making a diagnosis and may also give some indication of the amount of damage caused by the infection. Ultrasound is also very useful in determining the severity of infection.

Signs of heartworm disease that dog owners are most likely to notice include frequent coughing, sluggishness, rapid tiring, and labored breathing. These signs are most easily noticed in hunting and working dogs. Ascites (accumulation of fluid in the abdomen) may occur in advanced heartworm infection. When the disease reaches its critical stages, the victim is usually weak, has difficulty breathing, and may faint. At this stage, damage to vital organs is so severe that treatment is much more difficult and the possibility of complete recovery much lower.

We also know that the worm as well as the microfilarae can cause a severe vasculitis (an inflammation to the lining of the arteries and veins) adding to the sickness of an infected dog.

Can infected dogs be treated?

Most dogs can be successfully treated for heartworms if the disease is detected early. Two commonly accepted treatment regimens are available to treat the disease. An older treatment involves the use of an organic arsenical drug. This drug is carefully administered intravenously through a series of injections. This treatment does potentially have many side effects.

A newer medication used in the treatment of adult heartworm is Melarsomine dihydrochloried, "Immiticide", which is given intramuscularly into the lumbar muscles. This appears to be a much safer approach to treatment of the disease and has fewer side effects.

A few days after treatment, the worms die and are carried by the bloodstream to the lungs where they lodge in small blood vessels. There they decompose and are absorbed by the body over a period of several months. Fatalities resulting from treatment are rare among dogs that are otherwise in good health. The patient should be given a thorough physical and laboratory exam prior to treatment. These tests should include a blood count, blood chemistry analysis, and a chest x-ray as a minimum data base. Any other problems that might cause complications should be corrected before heartworm treatment begins.

Following treatment, complete rest is required to prevent lung damage from the dead and decomposing worms. Excitement and exercise should be avoided for at least a month, followed by gradual return to normal activity.

Microfilarial treatment

After all adult heartworms are eliminated, another drug must be given to rid the bloodstream of microfilarae which are not affected by the drug to kill adult heartworms. These medications are administered 3-4 week after the adulticide treatment has ended. Medications use to eliminate the microfilarae include the use of ivermectin and milbemycin oxime. Careful observation is required after an initial dose in order to treat potential systemic side effects. There have been some neurological side effects in rough coated collies and some other pure breeds at high doses of ivermectin.


Prevention has proven to be highly successful in controlling heartworm infection. Several different types of  prophylactic heartworm medication are available:

The most commonly used method of prevention is the monthly administration of products made for this purpose. These are available in tablet and chewable forms (Heartguard, Interceptor) administered monthly.  A  topical product (Revolution) also applied monthly.  These products come in several sizes given according to the weight of the dog. This medication eliminates either the L2 and L3 stages of the microfilarae. That is why it is effective as a monthly medication, due to the length of time the larvae is in the dogs body. The medication is begun with puppies and is given throughout the life of your dog. The medication is started within a month after mosquito exposure and is continued  throughout exposure and the last dose is given within one month after the end of the mosquito season. In areas where mosquitoes occur all year, the medication is given throughout the year. It is advisable to test all dogs over 7-9 months of age and to have a blood test performed every year or two depending on the incidence of the disease in your area.

A new preventative (Proheart 6- has recently by made available.  This product is an injectable administered every 6 mos. and takes away the burden of remembering to give the monthly tablets to your pet.  It is only approved for dogs only and not for heartworm prevention in cats or ferrets.

Another method of prevention is administered daily. Diethylcarbamazine (D.E.C) has been use successfully as a heartworm preventative for over twenty years. The small amount of the drug is given according to the weight of the dog. It must be started at the time of mosquito exposure, continued daily throughout exposure and for two months afterwards. Its effectiveness is severely reduced if there is any interruption in the daily dosing. This particular medication is effective in only eliminating the short lived L1 larvae and consequently has a narrow time of effectiveness. It is essential that the dog be free of heartworm microfilarae before starting this medication. Severe and even fatal reactions may occur if D.E.C. is given to a dog with microfilarae before starting the D.E.C. preventative.

Outdoor dogs are most susceptible to heartworm infection. Even dogs kept indoors seldom can be completely protected from mosquitoes. Remember that infected dogs which aren’t properly treated serve as a constant source of infection, making it essential to test dogs regularly for heartworm.