BTRNT - Emergency & Health

Canine Heartworm Disease

by Frances Smith, DVM *

Velvet
Velvet did not survive her heartworm treatment. Velvet had an embolism from the treatment and died due to the neglect of her previous owner. She is a huge example of why you must administer heartworm preventative to all of your canine children.

Heartworm disease in dogs is caused by a parasite called Dirofilaria immitis. This disease can be found all over the United States, and in many other places around the world. Infections have been documented in other species as well, including the cat, wolf, coyote, bobcat, jaguar, tiger, muskrat, raccoon, ferret, otter, bear, horse, orangutan, gibbon, sea lion, and even man.

The life cycle of the heartworm includes the mosquito. When an infected animal that has circulating baby heartworms (microfilaria) is bitten by a mosquito, the blood that the mosquito gets out of that animal contains the baby heartworms. These baby heartworms then develop inside the mosquito for at least 14 days to become infective larvae. When the mosquito containing these infective larvae bite an uninfected dog, it injects those infective larvae into the dog, thus infecting it. The time it takes from this mosquito bite to the time when the dog has adult heartworms that are able to produce microfilaria is about 6-7 months. The adult heartworms can live in the dog for about 5-7 years.

The adult heartworms live in the heart and large blood vessels around the heart. These worms can be 12-31 cm (4 ½ to 12 inches) long and 0.7 to 1.3 mm wide. Dogs can carry 1 – 250 of these worms at one time. They cause an inflammatory response that over time can result in heart failure and lung problems. The symptoms vary from no symptoms at all to coughing, exercise intolerance, difficulty breathing, fainting, coughing up blood, jaundice, fluid in the abdomen (ascites), and death. The severity of the symptoms is related to how many worms the dog carries, the amount of time infected, and the severity of damage caused by the worms. Heartworms can infect other areas of the body such as the eye, abdominal cavity, blood vessels of the brain, and spinal cord, but this is not common.

Heartworm disease is diagnosed most commonly by blood tests done in your veterinarian’s office. One type of test looks for the presence of the microfilaria in the blood. Another is what is commonly called the occult heartworm test. This is another blood test that checks for the antigens produced by the heartworm. It is generally more accurate that looking for microfilaria because the number of microfilaria can change depending on the time of day and time of year. Other tests such as radiographs (x-rays) of the chest, blood work, and ultrasounds of the chest can help support a diagnosis of heartworm disease and determine the severity of the changes that the heartworms have caused.

The treatment for heartworm disease involves killing the adult worms with one drug and the microfilaria with a second drug. Most dogs with no symptoms or mild symptoms are treated with a high rate of success and few complications. Dogs who are showing more symptoms can still be treated, but the whole process may take longer and the incidence of complications is higher. A few dogs are so ill or have other diseases present so that treatment is extremely difficult and sometimes not recommended.

There are two drugs approved for the adulticide phase of heartworm treatment. The older drug, Thiacetarsamide sodium, is rarely used today. The newer drug, Melarsomine dihydrochloride, or Immiticide ® (produced by Merial) is safer and more efficacious than the older drug. It is given by deep injections in the lumbar muscles in the dog’s back. After this phase of treatment it is very important to keep the dog quiet for the next few weeks to prevent severe pulmonary thromboembolism. If this occurs the dog will have fever, cough, and may cough up blood. These dogs need additional treatment of VERY strict rest and corticosteroids to reduce the inflammatory response. Three to six weeks after the Immiticide is given, a microfilaricide is given. This time allows the adult heartworms to die and the body to clean them away, and for no new microfilaria to be produced. The drugs used most commonly are ivermectin or milbemycin. The dog is usually kept in the hospital or clinic for at least 8 hours after the drug is administered to watch for adverse effects from the rapid kill of the microfilaria. Three weeks after this, the dog is tested for the presence of microfilaria, and if none are seen, the treatment is complete and the dog begins preventative treatment. By 16 weeks post treatment, an antigen test should also be negative. It is possible to greatly improve clinical symptoms of heartworm disease without completely killing all of the adult worms and repeat treatment in dogs that show a persistence of microfilaria or the antigen should be considered on a case-by-case basis.

Preventing heartworm disease is much safer and more economical than treating the disease. There are several drugs approved for prevention of heartworm disease in dogs. Diethylcarbamazine and the macrolide anthelmintics (ivermectin, milbemycin, moxidectin, and selamectin) are the drugs in use today. The biggest advantage of the macrolide anthelmintics is that they can be given monthly. For most of the United States, especially here in Texas, it is recommended that these drugs be given year round. If possible, dogs should be started on these preventatives as young puppies and they should be continued throughout the dog’s life. If you have a dog that is older than 6 months old, it must be tested BEFORE you begin giving the preventative. If you get a dog that already has heartworms, it needs to go through the treatment process before preventative medication can be given.

-Diethylcarbmazine is the oldest preventative drug and was our mainstay for many years. It is reliable, safe, and works very well, but it must be given daily to be effective. It is available in many formulations including chewable tablets with an intestinal worm medication added (Filaribits Plus ® made by Pfizer).

-Ivermectin, more commonly known as Heartgard ® or Heartgard Plus ® (made by Merial), was the first of the macrolide anthelmintics to be approved for use as a heartworm preventative. It is effective in blocking development of larvae that infected the dog as long as two month prior to treatment. The “Plus” version also contains an ingredient that treats hookworm and roundworm intestinal parasites.

-Milbemycin, also known as Interceptor ® (made by Novartis) is similar to ivermectin, but it also controls roundworm, hookworm, and whipworm intestinal parasites without the addition of another ingredient. This one is also safe to use in collies that are sensitive to ivermectin.

-Moxidectin or ProHeart ® (made by Fort Dodge) is a very effective preventative but does not kill microfilaria at the preventative dose. It can be given as an oral pill monthly or in the newer formulation, ProHeart6 ® , as an injection every 6 months. The convenience of every 6 month dosing is very appealing to many people, but the first dose must be given after the dog is 6 months old and at a fairly stable weight.

-Selamectin, or Revolution ® (made by Pfizer) is the newest drug to be approved for heartworm prevention in dogs. It is administered topically and also is effective against fleas, ear mites, and sarcoptic mange. It also helps control the American dog tick.

The best way to choose a heartworm preventative medication for your dog is to talk to your veterinarian. He or she knows your dog and the conditions of your area and can make the best recommendations. These medications are purchased by prescription only. A yearly negative heartworm test is usually required to refill the prescription and will be necessary wherever you choose to have the prescription filled.

The old saying that an ounce of prevention is worth a pound of cure is very, very true with heartworm disease in dogs. It is much easier and economical to prevent this disease than to treat it. The drugs available today for prevention are safe and easily administered. Keep your dog healthy by preventing this life threatening disease!

Frances Smith, DVM

BTRNT - Emergency & Health

Canine Heartworm Disease

by Frances Smith, DVM *

* Dr. Frances Smith is a member of NTBTC and is a licensed vet in Texas