How to Treat Heartworm

This month’s blog post is an expansion to our Facebook post concerning heartworm and the risks it poses to infected dogs. Recently there has been a huge movement in bringing shelter dogs from the southern USA (where high kill shelters are abundant) to adopt in more northern regions. While this is wonderful in that more dogs are finding homes, we are seeing that many well-intentioned adopters are not fully aware of the potential risks that accompany a heartworm positive dog. This is not surprising given that this is a disease that is so rarely seen in our part of the world because of our climate and therefore has very low awareness from the general public. Here are some must-knows about heartworm:

1 – Heartworm is a parasitic worm that lives in the heart and lung blood vessels of dogs and cats. If left untreated it will cause heart failure and eventually death. Transmission is through infected mosquitos, which are the largest contributing factor to why we do not see heartworm in more northern climates (yet!) – our winter temperatures are an extremely effective method of breaking the life cycle. This is also why the disease is so much more prevalent in southern locations as mosquitos live all year round in warmer temperatures. For a detailed diagram of the heartworm lifecycle, please click here.

2 – Heartworm tests can take up to 6 months post infection to turn up positive on a blood test (for example, if a dog contracts heartworm in April, a test may not be positive until September). This is to say that if a dog tests negative at the time of adoption, it does not mean it will still be negative six months from then. This is why whenever we see a dog for the first time post adoption from the south we recommend repeating the test 6 months after leaving the heartworm endemic region.

Monthly administration of a heartworm preventative for a “slow kill” treatment approach is not the best option for treatment. The theory behind this method of treatment is that giving the dog monthly heartworm preventative will prevent re-infection while allowing the established adult heartworms to die off when they have lived out the span of their life which can take anywhere from 1-3 years. There are a few reasons veterinarians do not recommend this treatment as a first option:

1- The older the worms are at the start of “slow kill” therapy, the less efficacious the therapy is in the patient. Slow kill has been scientifically shown to be less effective in eliminating adult worms than fast kill (melarsomine injections).

2- Slow kill requires up to a year—and sometimes much longer—to achieve results, during which time the adult worms remain in the blood vessels causing physical irreparable damage and scar tissue which will cause heart and lung issues even after worms are dead and gone.

3- Heartworm treatment of any kind requires strict exercise restriction throughout treatment. This is because worm die-off predisposes dogs to thromboembolism (worm clots), even more so in an active dog due to increased blood pressure and heart rate. Because there is no way of knowing when worms are dying within the body with this method, keeping dogs quiet throughout their years of treatment is important, but often unrealistic and therefore dogs are at higher risk of potentially fatal clots with this approach. With melarsomine injections, worm death occurs in a predictable time period and focusing heavy exercise restriction is much more attainable as it is so much shorter in length (2 months).

4- Immune complex formation in dogs infected with heartworm and managed with slow kill can cause false negative test results, misleading veterinarians and owners about the efficacy of this approach. We have seen dogs treated with a slow kill, test negative, stop administering preventative (because it’s New Brunswick, why would you need to?), and return symptomatic for heart failure with a positive heartworm test 1-3 years later.

The recommended treatment method is fast kill: a series of drug injections (melarsomine) over 2 months time that could also kill the dog if the worm burden is heavy enough and costs a significant amount of money (~$1000CA). This is the gold standard of therapy that veterinarians recommend due to the above shortcomings of slow kill and the excellent efficacy of melarsomine: 99% kill within 2 months (versus slow kill which can vary anywhere from 56% to 90% within 8 months of administration). Although slow kill treatment is still used, it is often because financial limitations are at play and are better than doing nothing at all. From a rescue standpoint, however, it is our belief that treating fewer dogs appropriately is better than treating multiple dogs potentially ineffectively. We would never recommend a treatment strategy that is less efficacious, more dangerous, and without a method for reliable testing after treatment for a patient as a first option.

4 – Heartworm positive dogs coming into our province act as reservoirs for infection of our healthy dogs at home – most of which are not on a monthly preventative as it is not considered necessary here (yet). I include the “yet” because our climate is changing and I fully expect heartworm to be a prevalent enough disease here within the next 10 years to require monthly prevention as is common in the USA and Ontario.

This blog is by no means meant to discourage adoption but to bring awareness of heartworm to our clients and friends so that all the resources are available to them when considering bringing home a potentially heartworm positive dog. This issue is close to home for me – I treated my own dog for heartworm after I adopted her from Georgia during vet school. For more information on heartworm, please visit the American Heartworm Society website.

Written by: Dr. Katie O’Hanley, Veterinarian