Tibetan Mastiffs

Gold Line

Timing Heartworm Preventative

Several people have requested that I put this information on my site so that everyone can benefit from it. I have finally taken the time to do it! This is not an in depth explanation of why it is best to avoid giving heartworm pills during times when they are not necessary although it is common sense that you would not want to give medication to your dog if they do not require it. Many other sites discuss the side effects that can be associated with these types of preventatives. This page will provide you with the information you need so that you can estimate, based on your geographical area, which month you should begin/stop giving heartworm preventative. Even if you do not live in the United States you can still figure it out simply by using the temperature recommendations.

In a study entitled “Seasonal Timing of Heartworm Chemoprophylaxis (Heartworm Meds) in the United States”, Dr. David Knight and James Lok of the American Heartworm Society have taken the guesswork out of when to start and stop heartworm prevention. It was found that specific conditions must exist in order for the larvae in the mosquito to move on to stage L3, allowing the mosquito to pass on heartworm through its mouthparts. The larvae require approximately 30 consecutive days of 60-degree weather where the temperature does not dip below 57F (14C) in order to reach this condition.

Simply put, it must stay above 60 degrees for 30 consecutive days AND nights for the larvae to progress to stage L3 and be passed through the mouthparts of a mosquito to a host animal. If the temperature dips below 57F the maturation is retarded and cannot continue. This process would become accelerated if there were two weeks of temperature at or above 80F (27C), days AND nights. As a result, heartworm disease is not only geographically limited, but also seasonally limited. For many of us this means that year round heartworm prevention is totally unnecessary. In fact, in my area of the country, I would not even need to begin giving heartworm meds until the first of June and would give their last dose of the year on the first of November. I feel very strongly that the dogs should only have to take heartworm meds for the months that they are truly at risk. Below I have provided the maps from the study that give you a guideline of when to begin and stop heartworm medication. Click on the graphics for a larger view.

I would like to acknowledge the efforts of Steven Turitz who took the initiative to acquire better copies of the maps indicated in this study.  FINALLY the numbers are clear and easy to read! Thank you Steven!
Heartworm Map A
Heartworm Map B


Because of the recent reports that heartworm disease may be developing a resistance to heartworm preventatives it is no longer advisable to give every 45 days which has been safe to do in the past. I would give preventatives every 30 days as directed and would suggest testing bloodwork for heartworm twice (every 6 months) instead of just once yearly. If you live in a warm, humid climate you may even want to consider testing 3 times yearly.

Milk Thistle

Because of the powerful antioxidant properties of the silymarin compounds found in the herb Milk Thistle, it is an ideal herb for detoxification.

Many drugs used today have significant liver toxicity, Milk Thistle is great for combating the toxic side effects of drugs like:

  • Chemotherapy drugs
  • Chemical heartworm medications
  • Antibiotics
  • Corticosteroids

Milk Thistle when used properly has an excellent saftey record. It is recommended that you give 1 capsule of a standardized (70 or 80% silymarin) Milk Thistle product twice daily for 7 days after giving heartworm medication.

Dr. Jean Dodds of Hemopet recommends using milk thistle in the doses listed below:

Dog's size :: Dose as % of adult human dose

  • 5 lbs :: 10%
  • 5-10 lbs :: 15%
  • 11-20 lbs :: 20%
  • 21-40 lbs :: 30%
  • 41-70 lbs :: 50%
  • 71-100 lbs :: 75%
  • 100 lbs :: 100%


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