Understanding Heartworm Disease

What is heartworm?

• Heartworm disease is caused by an infection with the parasitic worm Diroflilaria immitis.
• No symptoms of the disease are apparent until the disease is very advanced. The symptoms of heartworm disease develop gradually over many months, and closely resemble those of congestive heart failure.
• In severe cases, the dog will have difficulty breathing and sometimes coughing, is very intolerant of exercise, and will become lethargic and have a decreased appetite.

How is it transmitted and what is the life cycle of the parasite?

• Dirofilaria immitis is transmitted by mosquitoes. The cycle begins when a mosquito bites a heartworm-infected dog and picks up the microfilariae (immature heartworms) found in the infected dog’s bloodstream.
• During the next 2-3 weeks the microfilariae develop into infective larvae within the mosquito.
• When the mosquito feeds again, it can now transmit the infective larvae into a healthy dog when it bites them. The larvae penetrate the dog’s skin and migrate through the tissues, developing over the next few months. Eventually they reach the dog’s heart.
• Once inside the dog’s heart, the worms can grow up to 14 inches long and can cause significant damage to the heart and lungs. Hundreds of worms can be found in an infected dog’s heart and the surrounding major blood vessels. If left untreated the worms can block the blood flow and ultimately cause death.
• The entire life cycle of the heartworm is between 18 to 24 months.

Are all pets at risk of being infected with heartworm disease?

• All dogs are at risk and even cats now have been reported to get heartworm infections
• Even if pets are mostly indoors they still can get the infection, because it is transmitted by a simple mosquito bite.

What should be done to prevent heartworm disease?

• An annual blood test is recommended to ensure that your dog is negative for the infection from mosquito exposure the previous year.
• Dogs that test negative for heartworm should be started on preventative medication.

How long would my dog take medication?

• The duration that preventative medication is given is directly proportional to the length of the mosquito season. The medication works backwards by a month, and catches any larval forms of the worm that a mosquito might have deposited the month earlier.
• If you are traveling with your pet to the sunny southern USA in the fall, winter or early spring, your dog should be on heartworm prevention year round to keep them protected.

What kind of medications are available?

• There are many different types of heartworm preventatives, with the most popular and recommended ones being those that combine heartworm prevention, with flea prevention and some kind of intestinal de worming protocol. Your veterinarian will be able to discuss the best choice for your pet. Some of the preventatives are:
  • Sentinel (milbemycin oxime plus lufenuron) is an oral tablet given once monthly which prevents heartworm disease, fleas and some intestinal parasites. Cost is about $8.00 per month.
  • Revolution (Selamectin) is a topical spot on product which is applied to your dog’s skin once monthly. This product prevents heartworm, fleas and some intestinal parasites. Cost is about $8.00 per month.
  • Interceptor (milbemycin oxime) is an oral tablet given once monthly that prevents heartworm and some intestinal parasites, but not fleas. Cost is about $8.00 per month.
  • Heartguard Plus (ivermectin) is an oral tablet given once monthly that prevents heartworm and other intestinal parasites, but not fleas. Cost runs about $6.00 per month.
  • Proheart 6 (moxidectin) is an injectable form of sustained release heartworm prevention given at the beginning of heartworm season, and lasts for 6 months. It doesn’t protect against fleas or as many of the intestinal parasites as the other preventatives do. Cost is about $50.00 every 6 months.

Is heartworm disease treatable?

There is medical treatment available, but it is quite expensive and not all dogs survive the treatment. The medication used to kill the adult worms is very toxic and must be administered under very close veterinary supervision. Since all of these adult worms are being killed at once, there is a risk for an anaphylactic or shock reaction, and a very high risk of the dead worms lodging in the blood vessels if the pet is active, so the dog must be restricted to cage rest until the danger from the treatment is past. This treatment is given in a 4-step program:

1. Two injections in rear quarters of the dog administered 24 hours apart. The theory is that 1st shot kills 50% and the 2nd shot kills the remaining 50%. This kills the adult heartworm and as it breaks up it is absorbed back into the body. During this treatment the dog must be kept extremely quite and cannot be allowed to exercise. Outside breaks should be on leash and the dog should be crated. I have found the real danger period to be 10 to 14 days after the 2nd shot. I have seen dogs become lifeless and they must be taken back to the vet and put on medications for them to survive. I have seen several dogs die and it is not a pleasant thing to witness. I recommend this treatment only for dogs up to age 8 that are showing physical signs of heartworm disease.
2. After 4 weeks the dog is taken to the veterinarian for the oral does of ivermectin with kills the microfilariae and the veterinarian monitors the dog for 8 hours in case there would be an adverse reaction.
3. Two weeks later the dog is taken to the veterinarian, where a smear test is given and the dog is put on a month preventative.
4. Six months after the initial treatment the dog is taken to the veterinarian and given the occult heartworm test to assure that it is free of adult heartworms

The advantage of this treatment it kills the adult worm without any further damage to the dog. The disadvantages are:

1. It is cost prohibitive for some people. The cost is from $400.00 to $900.00 depending on your area of the country.
2. It is hard on the dog and probably should not be given to a dog over 8 years of age or showing any physical signs of the disease.

 

Although this is the only form of treatment approved by the American Heartwearm Association there are some alternative treatments being used widely across our nation. Because of legal paranoia we will not discuss those treatments in this open forum. If you would email us privately, we would be glad to discuss other options with you.