Pet Vet: Heartworm disease is far easier to prevent than to treat

May 18, 2015

Today we are going to return to a subject we have discussed here in the past, spawned by a question from Kevin. Even though the question is the same as one answered before, perhaps, that answer might have changed.

Kevin lives in Modesto, Calif., and wants to know whether or not he should put/ keep his dog Rondo on heart worm preventative. Some of his friends have their dogs on the preventative, but he has heard that it is not really necessary here in our geographic area.

To answer this, first it is best to understand a bit about heartworm disease, its manifestations and how it is transmitted. Heartworms live in the heart of infected dogs, hence the name. They actually can occur in cats and even in humans though they do not seem to be able to reproduce in cats or people. To be more precise, these worms are located in the right ventricle section of the heart, the lower pressure side. They can also move out from the right ventricle into the pulmonary artery.

Male and female worms do what male and female anythings do — they breed. Offspring are very tiny larval forms which develop in the blood stream of the infected dog to a stage called an L4 larvae. Once to this point development stops and these L4s circulate in the blood until a mosquito comes along and sticks its tubular proboscis into the heartworm infected dog and sucks out some blood. Along with the blood comes the L4 heartworm larvae which then set up camp inside the mosquito to continue further development.

The fully developed larvae inside the mosquito migrate up to the mouth area and when the mosquito lands on a new unsuspecting dog and inserts it proboscis, the larvae enter the dog’s blood stream and begin a new case of heartworm disease in a new victim. These mature larvae then journey to the right side of the dog’s heart and build a new home. Interestingly, the aforementioned mosquito can reinfect the same dog over and over again thus ever increasing the population of adult worms in the infected dog’s heart. The life cycle takes six months to complete one generation of worms.

As one might imagine, the heart is not designed to accommodate a bunch of six inch long worms and over time, their presence there becomes quite debilitating to the dog and can result in death.

Heartworm disease is treatable but, as is the case with many diseases, it is far easier to prevent than to treat. In order to do so, dogs are first tested, a simple blood test taking just a few minutes. It tests the presence of adult heartworms and if the test is negative, they can be placed on a preventative, usually given orally once a month (there are other methods, too) to prevent the disease. When a mosquito passes on the heartworm larvae to a protected dog, the larvae simply die without incident.

Historically, heartworm disease was most prevalent where their intermediate host, our friend the mosquito, was most prevalent. This was the eastern part of the U.S. and the south. With the movement of dogs infected with the heartworms to other parts of the country the disease has spread to the west, including California. We have mosquitoes, too. Over the past decade, the incidence of heartworm disease in California has increased in some areas, though specifically in Modesto it has been extremely low. The cases I have personally treated were all in dogs that had come from out of the area, but recent conversations with a few of my local colleagues have revealed that there have now been several cases of heartworm disease in dogs that have spent their whole lives in Modesto. This means that there is a reservoir population of heartworms in our town and we have the mosquitoes to transmit it between dogs.

The incidence of heartworm disease is still characterized as low in our area, however I do suspect it to rise some in the future. So what does this all mean in reference to Kevin’s question on whether or not to put his dog on heartworm preventative medication? The answer comes down to a risk benefit analysis and your willingness to gamble that your dog will not get heartworm disease without using the preventative. That risk is definitely higher than in the past. For you gamblers, that means the odds are shifting away from your favor.

My recommendation to my clients historically was to, at least, have their canine companions tested for heartworm disease on a regular basis. Annually is the usual choice. With the most recent development of cases of heartworm in only locally living dogs, I am going to add heartworm prevention to that recommendation. I also should mention another excellent benefit of heartworm preventative medication: It also prevents multiple species of intestinal parasites which are relatively prevalent in our canine pals.

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(Jeff Kahler is a veterinarian in Modesto, Calif. Questions can be submitted to Your Pet in care of LifeStyles, The Modesto Bee, P.O. Box 5256, Modesto, CA 95352.)



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