The incidence of heartworm disease has increased in the past few years.  The problem was once confided to large dogs, particularly in the southern areas of the country, who spent a great deal of time outdoors and were not on any form of heartworm prevention.  However, the disease now covers a wider area of the country and a wide range of dog breeds.  Smaller housedogs are at a lower risk, but if they contract the disease it will cause more severe health problems.

WHAT IS HEARTWORM DISEASE?

Heartworms are spread from dog to dog by mosquitoes. The mosquitoes deposit the larvae (baby worms) into the bloodstream. It takes 3-4 months for the larvae to develop or become an adult worm which lives in the chambers of the heart. Adult worms can measure up to 6-8 inches long and, depending on the amount of worms inside the heart, can cause damage to the cardiac muscle eventually causing congestive heart failure.

Heartworm preventions work by killing microfilaria (baby worms) before they have a chance to mature and move into the heart. Heartworm prevention has no effect on adult worms. Once a dog is bitten by mosquitoes it is theoretically possibly the dog can have a negative test result for up to 4 months because heartworm tests only check for adult worms.

Heartworm preventions out today are safe to give even if the dog tests positive for heartworms.

SIGNS OF HEARTWORM DISEASE

Dogs can go for months with heartworms disease and show no clinical signs. Once clinical signs develop (cough & exercise intolerance) there is more risk to treating the dog. In this area it is recommended to keep dogs on heartworm prevention all year round as there are many warm days in the winter. Studies show that even dogs that spend most of their time inside are still at risk.

HEARTWORM TREATMENT

When a dog is tested for the heartworms, the results will differentiate if the dog is high (heavily infected) or low positive. If a dog is low positive it is possible to treat with 2 injections of Immiticide given 24 hours apart. Whereas if a dog is high antigen positive it is recommended to give 1 injection then wait 1 month, then 2 injections 24 hours apart. The 2 step process is done for the safety of the dog by killing off a percentage (approximately 30%) with the first injection, then following with 2 injections a month later to kill the rest of the worms. There is less of a chance of thrombo embolism (a blood clot formed by the worms) which can be life threatening in some dogs. The veterinary heartworm association recommends that the safest way to treat heartworm positive dogs is to do the 2 stage process even in low antigen dogs because there is less risk.

Current recommendations also include putting the dog on heartworm prevention for 3 months prior to starting heartworm treatment. This allows any immature worms to finish development so they will be killed with the heartworm treatment. It is also recommended that dogs stay on a low dose of prednisone throughout the entire treatment period to help prevent thromboembolism.

In some dogs it is recommended to treat with doxycycline (antibiotics) for 21 days prior to heartworm treatment to weaken the worms. The medication used in heartworm treatment (Immiticide) is given into the muscle along the back and often can cause pain and inflammation at the injection site. All dogs are treated with pain medication prior to the treatment and also sent home with pain medicine to deal with any discomfort that may occur. It is imperative that dogs have limited exercise post heartworm treatment to help prevent thrombo embolism.

ALTERNATIVE METHODS OF TREATING HEARTWORM DISEASE

The only FDA approved drug to treat and kill adult heartworms is Immiticide.  This medication has been unavailable from a U.S. source for 2 years.  However, Immiticide is now available from Europe through the company Merial.  Veterinarians are required to contact Merial on an individual case by case basis in order to obtain Immiticide for  individual case treatment.

Should Immiticide be unavailable, there are now other choices of treatment (listed below) which may be appropriate for your pet depending upon the case.  You should contact your veterinarian to discuss these choices in order to make the correct decision for your pet.

Due to the high cost and unavailability of Immitacide, Veterinarians and animal shelters have had to look for other options to help infected dogs.  Many recent studies have shown that a combination of Doxycycline, an antibiotic, and low dose ivermectin can kill adult worms at a faster rate than ivermectin alone.

The research was done on a small test population so it is not clear how effective it will be in the general population, but at this time with Immitacide unavailable it is a reasonable treatment option.  The American Heartworm Society (AHS) still recommends that once Immitacide is available again that it should be used, but we may find that this protocol works as well.  Any dogs diagnosed with heartworms will be evaluated and a treatment plan will be made on an individual basis depending on the severity of the infection.

Dogs will be put on doxycycline for 30 days once every 3 months.  They will also take low dose ivermectin every 2 weeks.  Many dogs on this protocol have been found to be cleared of adult worms within 36 weeks.  The AHS recommends that all dogs with Heartworms should have reduced exercise until the infection is cleared to reduce damage to the pulmonary vessels and heart.

Please contact us if your dog has tested positive for heartworms so we can discuss a treatment plan with you.

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