Should dog owners in the Pacific Northwest worry about Heartworm disease?

(recently published in GH Home and Family magazine)

Heartworm disease can be found in all 50 states.  It is potentially life threatening and treatment costs can be more than 10 times that of prevention, not to mention the potential side effects of the treatment itself.  While it is true that our Pacific Northwest climate puts our dogs at less risk, the number of cases in our area have been increasing over recent years and experts believe the number of cases will continue to rise.  In addition, many of us travel with our dogs outside of this area, where the risk of infection is higher.  For these reasons and more, it is important to become acquainted with heartworm disease.

Heartworm disease is spread by mosquitoes.  After a mosquito takes a blood meal from an infected animal, the baby heartworms, also known as microfilariae, are transferred to the mosquito and over several weeks develop into the infective larval stage.  When the infective mosquito bites a new animal, it leaves a small amount of mosquito saliva near the entry site.  In this saliva are heartworm larvae, which over the next few hours travel to the tiny opening in the skin and make their way inside the victim’s body.  Over several months, the larvae mature and eventually make their way to the heart and major vessels that lead to the lungs.  Here, the now-mature heartworms mate, and the 10 to 12 inch long female releases microfilariae into the bloodstream where they can be picked up by the next mosquito and then spread to the next victim.

Although many dogs infected with heartworms may show no signs at all, depending on the severity of their infection and how advanced the infection is, infected dogs can suffer from coughing, lethargy, difficulty breathing, heart failure and sudden death.  This is a terrible disease.  Treatment is expensive and complications of treatment can be life threatening.  Luckily, prevention is easy.

There are several heartworm preventatives on the market.  The most popular ones come as chewable tablets and are given on a monthly basis.  In addition to preventing heartworm disease, most of these tablets also kill intestinal worms, which is a tremendous added benefit.  Not only are intestinal worms bad for dogs, several types can spread from dog to human.  Roundworms in particular can be especially harmful when contracted by humans.  In some instances roundworm infections in humans can result in blindness, as the worms may migrate to the eye and cause severe inflammation.  This is called ocular larva migrans.  Unfortunately, roundworms are rather common, especially in puppies and kittens, and young children are especially susceptible to these parasites.  For this reason alone, monthly heartworm prevention is a good idea, especially in those families with young children.

If you are interested in starting your dog on heartworm preventative, a heartworm test will likely be recommended.  Dogs greater than 7 months should be tested to determine their heartworm status.  Testing is not recommended for dogs less than 6-7 months because as a result of the heartworm’s maturation process, it takes at least 6-7 months for an infected dog to test positive.  It is important to remember that heartworm prevention is not the same as heartworm treatment.  Preventatives work by killing immature larval stages.  They do not kill adult heartworms.  The sooner adult heartworm infections can be detected, the sooner treatment can be initiated, thereby reducing the long lasting and possibly irreversible consequences of the disease.  In addition, in rare instances, giving certain types of prevention to a dog who is positive for heartworm disease can result in serious negative side effects.

What about the side effects of the preventative?  The two most common ingredients used to prevent heartworm disease are ivermectin and milbemycin.  At the very low dose that is used to prevent heartworm disease and to deworm for intestinal parasites, side effects are very rare for either medication.  Of course with any oral medication, vomiting may occur. Some breeds, like Collies, can be extra sensitive to ivermectin and milbemycin.  However, the low doses used in the preventatives are considered safe, even in these breeds.

A common misconception about these medications is that they are present in the dog for an entire month at a time. This is incorrect.  They work by killing immature larvae at the time that the medication is given.  This needs to happen within 30 days from the time the dog is first bitten by an infected mosquito.  Giving the medication before this time will do nothing to prevent possible infection from future bites, only from bites taken before the medication was given.  If the larvae mature past 30 days, the heartworm prevention is ineffective as some of the larvae may continue to mature and progress to full-blown heartworm disease.  This is why prevention is given every 30 days.
In conclusion, heartworm disease can be life threatening and treatment is expensive and may result in serious complications.  In addition to preventing heartworm disease, most preventatives also eliminate intestinal worms and therefore decrease the chance of human infection.  Although the risk is relatively low in our area, it is helpful to understand the disease when deciding if heartworm preventative is right for you and your dog.

Josh Wohlstadter, DVM is owner of Purdy Veterinary Hospital, in Gig Harbor, WA.  He and his wife Jessica live in Gig Harbor, WA with their three dogs and one bird: Murphy a Mastiff, Charlie a Standard Poodle, Tuck a Miniature Poodle, and Sally, a Green Wing Macaw.