Heartworm PreventionThere are a few key facts about the transmission of Heartworm disease that provide practical insight into the timing and use of preventative medication against this problem in our region of Vermont and New Hampshire.  These are fun parasitology facts that every veterinary student memorized in school, and that in my opinion, are often glassed over by pharmaceutical companies whose goal is to sell more preventative pills each month.

First, Heartworm infection is a mosquito-borne disease, so no mosquitoes, then no disease transmission.  Second, in order for the infective larvae to become dangerous inside the mosquito, the mosquito’s body temperature needs to stay in the 55 to 57 degree Fahrenheit range for 14 days.   Dr. David Knight, a cardiologist who performed much research on Heartworm disease when he was Chief of Cardiology at UPenn veterinary school, used sophisticated weather maps and meteorological research of degree days to demonstrate the areas of the United States where Heartworm disease could be transmitted.  Though global warming is changing our weather patterns, it is still difficult for a mosquito to stay warm for 14 days in our region for much of the year.

So, there are only three or four months of the year when Heartworm disease can be transmitted in our area.  As a result, at Veremedy we recommend six doses of the preventative each year to insure against the transmission of this fairly rare, though potentially fatal, problem.  Is there any harm to giving the preventative medication 12 months out of the year?  No, and in fact for some people, it is easier to remember the monthly medication if it is a habit.  In addition, many of the monthly Heartworm preventative pills also contain a dewormer against intestinal parasites.  For dogs who frequent dog parks, or who have a high exposure risk to intestinal parasites, this monthly dewormer can have some medical benefits.  If you are not sure how often you should administer anti-parasite medications, please feel free to give us a call or send us an email.


  1. Julie Kenison's Gravatar Julie Kenison
    July 25, 2012 at 2:34 PM

    What six months do you recommend the heartworm disease medication be given? Thanks!

  2. July 25, 2012 at 9:29 PM


    We recommend using the monthly preventative medication from June 1st to November 1st. It is important to remember that the medication works backwards, that is, the pill given on June 1st protects your pet from heartworm for the month of May. Technically, the pill reaches back and kills infective larvae that may have infected your dog 42 days or so prior so this gives protection back into late April. Yes, we will see mosquitoes by this time, but they will not be warm enough. On the other end of the summer, we will occasionally get a warm spell in late September or early October, so the November pill works backwards and takes care of that time period. Hope that helps.

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