Jonathan Smith, VMD

Heartworm is a word that we hear all the time at Lucky Dog. For our own dogs we give a pill or flavored tablet every month as preventative, and some of us have even taken "heartworm positive" dogs into our homes. Monthly preventative can often seem like a challenge to give on a regular basis, and it is common for give a dose late or miss it completely. While many of us are guilty of this at some point, it is important to remember why we give this pill every month, and how easy it is to prevent this life threatening disease.

Heartworm has been diagnosed in all 50 states, and any area with mosquitoes has the potential to infect our dogs. Well, then why do we see it so commonly with our dogs from South Carolina, while it appears to be rare in our dogs in the DC area? Many of the dogs we transport up from the southern states spend most of their life outdoors, and are exposed to a higher number of mosquitoes for longer periods of time. Also, there are many more infected dogs and wild canids living outdoors, enabling the parasite to multiply and causing more of the mosquitoes to potentially carry heartworm disease.

You might ask yourself, why not just test for the parasite and deworm like we do for intestinal parasites? Unfortunately, unlike most intestinal parasites, adult heartworm infection is a serious and potentially life-threatening condition. Prevention is a much cheaper, easier, and safer method of treatment then waiting for infection and killing the adult worms. It is very important to understand how this little worm is transmitted to our dogs, and how testing and prevention works.

Adult heartworms live within the pulmonary artery, which is the large vessel leaving the right side of the heart, connecting to the lungs. If left untreated, severely affected dogs can develop right sided heart failure and/or other changes within the heart, lungs, liver or abdomen. The adult female worms produce very small versions of themselves, called microfilaria. When a mosquito bites an infected dog, it collects these microfilaria into its own body. Once inside the mosquito, they develop into larvae, an infectious version of the heartworm parasite. After a mosquito feeds on an unsuspecting victim, the larvae are released back into the dog, where they travel to the heart and mature into an adult heartworm over a 6 month period.

Your veterinarian tests for adult heartworms by collecting a small blood sample and checking it for very small pieces of the female adult worms, called antigen. A positive result indicates an active heartworm infection, and a negative indicates that there are no adult worms. However, there is a 6 month period where dogs can be exposed to heartworms and will not come up positive on our tests! That is the stage between when the mosquito bites and deposits larvae into the dog and when the larvae mature into adult heartworms. This leaves a grey area in our diagnostics, where even if a dog tests negative they could still have immature larvae which will go on to develop into an adult heartworm despite starting preventatives.

So why not simply give heartworm preventative every 6 months?  The fact is most of our oral preventatives are only effective at killing off that heartworm larvae which our dogs were exposed to within the last 4 weeks. After that, the migrating larvae cannot be treated with our preventatives OR the treatments for adult heartworms. So, if you don’t treat every 4 weeks (1 month), you risk having larvae grow unhindered into adult worms. There are many preventatives available from your veterinarian, all of which are effective at prevention. However, some preventatives are potentially harmful if your dog has an adult heartworm infection. Therefore it is always important to make sure to let your veterinarian know if you have missed any doses; they will likely switch you to a preventative that is safe for potentially heartworm positive dogs.

Once infected with adult heartworms, there are a number of treatment options. The most common and most successful treatment is immiticide (melarsomine) injections. These are usually administered via injections into their back muscles, using either 2 or 3 injections depending on the protocol. The injection sites are often very painful, but the main concern is that, once the heartworms are killed, they have the potential to suddenly dislodge and travel to the lungs. This could potentially lead to hospitalization or even death. This is why it is so critical to restrict our recently treated dogs from any high impact activities such as running, jumping or playing for at least 4-6 weeks after treatment. Ideally, they should be restricted to a crate when not under supervision, and only allowed short leash walks during the day. In order to ensure that the heartworms have adequately treated, it is recommended to retest for adult heartworms 6-7 months after treatment.

As with any infectious disease, prevention is key. Therefore always keep your dogs on monthly heartworm prevention 12 months out of the year and have them tested at their annual exam. If you have missed one or more doses of your heatworm preventative, please let your veterinarian know before renewing your prescription. For more information, please visit the website for the American Heartworm Society (http://www.heartwormsociety.org/) or visit your local veterinarian.