A review of heartworm treatment guidelines

Words by:

Kelly Dunham, DVM — Area Medical Director for Greater New York

Let’s talk heartworm! The American Heartworm Society has great, detailed resources for diagnosis and treatment of heartworm disease. Below, I review keys points on susceptibility, transmission, prevention, diagnosis and treatment. You can view their guidelines in full here.

Heartworm susceptibility

Heartworm infection has been reported around the world and is present in all states across America. While heartworm transmission does go down during the winter, it never reaches zero.

The range and number of heartworm cases continues to increase year after year. Heartworm has the highest morbidity and mortality of any other vector borne disease in the US. It’s not just the weather, but relocation of heartworm positive dogs and urban growth, which play significant roles in the continued spread of heartworm infection in our communities.

For all those people who think, I live in the city, I don’t need to worry — There can be an increased risk even in the colder months as the increased surface area of pavement and buildings provides a warmer environment for mosquitoes to reproduce.

Heartworm transmission

So how do we protect our patients? Through vector control and year-round heartworm preventative. Treating the area (the yard, the home) for mosquitoes can help decrease the risk of transmission by limiting the number of mosquitoes the patient could come into contact with.

This is particularly important in shelter or hospital boarding/ kennel situations, as having a heartworm positive dog present can increase the risk of transmission from 2-20% to as high as 74%!

Mosquitoes become infected as they take a blood meal from an infected host. The microfilaria then matures from larval stage 1 to stage 3, at which point the mosquito can infect new hosts when taking a blood meal.

Once infected it only takes 3-12 days for the microfilaria to molt into stage 4. The microfilaria does not reach the juvenile stage until 50-70 days, at which point they enter the circulatory system, arriving in the pulmonary vasculature by 90-120 days. As early as 6 months from infection, sexual maturity of the heartworm is complete.

The location of the adult heartworm depends largely on the size of the patient and the worm burned. Locations range from the lobar arteries to the right ventricle, atrium and vena cava. Ultimately this can lead to heart failure, liver and kidney disfunction and hemolysis.

Heartworm prevention

When discussing prevention with clients I always stress the importance of year-round use and go over the risks to the heart, lungs, kidney, and liver which likely will never heal, at least not fully, even with successful treatment.

So much more is known, and so much has changed in heartworm treatment and prevention that it becomes even more important to educate our clients. I find a lot of clients know about heartworm in general, but have outdated information, or don’t understand the severity. Education is key!

The current recommendation from the American Heartworm Society is to start all puppies on heartworm prevention by 8 weeks of age. Any puppy starting on prevention later than 8 weeks should be tested at 6 months of age.

Any dog being started on prevention at 7 months or older should be tested prior to starting prevention. All dogs should then be tested annually. Following a period or non-compliance the dog should be tested every 6 months for a year, then annually.

Preventatives can be given orally, topically, and subcutaneously. While heartworm infection on dogs who have been on year-round prevention is rare it can happen. The most common causes are human error (not giving enough, or regularly), canine error (vomiting up dose) or inappropriate metabolism and immune response in the patient (extremely rare).

Macrocyclic lactones, which includes ivermectin, milbemycin oxime, moxidectin and selamectin, are the most used preventatives. Not only are they highly effective at targeting microfilaria, stage 3 and stage 4 larval, but they are also extremely safe to use when given per label, including in dogs with MDR 1 mutation.

Heartworm diagnosis

The American Heartworm Society recommends microfilaria and antigen testing annually for all dogs over 7 months old. Infection cannot be detected prior to 5-6 months from the time of infection.

Echo, radiographs, heat treated serum and modified knots tests can also be used for additional testing, particularly in suspected heartworm positive cases. Heat treating the serum can help to activate the antigen in pets who have tested microfilaria positive and antigen negative.

This test should not be done in hospital as it is not compatible with most ELISA snap tests and should be run through the referral lab. The modified knots test can be performed in a hospital, and is done by mixing 1ml of EDTA blood with 9ml of 2% formalin. The mixture should be inverted several times to lyse the red blood cells, then spun at 1100-1500 rpm for 5-8 minutes.

Once spun, you can pour off the liquid and add a drop of methylene blue to the sediment. This can then be examined on a slide under the microscope at 100x for the presence of microfilaria.

Antigen testing is the most reliable method to confirm efficacy of treatment.

There is now a strain of resistant heartworm. Unfortunately, resistance is still being researched, and no test currently exists to evaluate possible resistance.

Heartworm treatment

Treatment can be challenging as the more adult heartworms killed the greater the risk for obstruction and/or inflammation and anaphylaxis. Because there is no definitive way to determine worm burden prior to starting treatment, you should always proceed with caution and assume the worst.

Restricting activity, and reducing overheating are two of the most important factors during treatment and the 6-8 weeks following treatment in reducing adverse effects.

Any dog being started on heartworm treatment should be stabilized first. Once the pet is stabilized, treatment can begin. Deep IM injection of Melarsomine is the only FDA approved adulticide and is typically given in the epaxial muscle between L3 and L5 in a three-dose protocol. The three-dose protocol involves a single dose of 2.5mg/kg, followed by an additional two doses 1 month later, 24 hours apart.

The American Heartworm Society recommends giving a Macrocyclic lactone for the 2 months prior to starting Melarsomine. This will help to reduce new infections as well as clear existing susceptible larvae.

Doxycycline is also recommended for 4 weeks prior to starting Melarsomine and can be given concurrently with the Macrocyclic lactone. Concurrent use has been shown to decrease adverse effects of treatment.

Doxycycline helps to prevent bacteria which lives inside of heartworm from being released and contributing to inflammatory and anaphylactic type reactions during treatment. Doxycycline has also been shown to help disrupt further transmission of heartworm disease from an infected dog.

NSAIDS are not recommended for concurrent use during treatment and show no clinical benefit. Steroids and antihistamines, however, can help to reduce inflammation and control clinical signs of pulmonary thromboembolism during treatment.

In dogs diagnosed with heartworm, be on the lookout for sudden onset of severe lethargy, weakness accompanied by hemoglobinemia and hemoglobinuria as these are sings of Caval Syndrome. Dogs displaying signs of caval syndrome require surgical removal of the heartworms, as it is otherwise fatal, typically within 48 hours.

In conclusion

Remember, when treating heartworm disease, you are not alone. The American Heartworm Society has amazing resources for reference, including the detailed treatment chart available here.

Dr. Kelly Dunham is area medical director for IndeVets.