heart worm prevention

When it comes to talking about parasites most people cringe – and for a good reason.

Parasites are freeloaders; they are like the guests that come to your house uninvited with no intentions of leaving.

But parasites are important from an evolutionary point of view and play a large role in the natural selection process. From a dog lover’s point of view however, parasites are nothing more than unwelcome invaders that threaten the wellbeing and sometimes the lives of our best friends.

Most people are generally afraid of parasites and the pharmaceutical industry has been making hefty profits cashing in on this fear and making things look worse than they really are. But our goal should be to minimize the use of drugs because no matter what kind they are, they are foreign to the body and often toxic. No one can be absolutely certain that down the road, the drugs will not cause side effects and affect your animal friend’s health in general.

From time to time, I like to check the facts surrounding various drugs and vaccines to see if they can be reduced and still achieve the objectives.

Today’s topic is heartworm.

History Of Heartworm Prevention Drugs

In the 90’s, when the manufacturers appeared to be taking North America by storm, drug reps used to visit the vet clinic I worked at on a regular basis.  They used to tell us that eventually heartworm would spread widely throughout Canada and heartworm preventives were an absolute must.

These visits were also accompanied by the side note that selling the heartworm tests and preventive drugs could bring a significant source of income for the practice.

As time progressed, the heartworm doom and gloom case scenario in Canada didn’t materialize and it was clear that the risks of heartworm infection in Western Canada were clearly exaggerated. However, this doesn’t mean that heartworm isn’t a serious problem on the East coast of North America and southern parts of the US and in Mexico.

In order to evaluate the risks of heartworm, you need to ask the following questions:

favicon What is the risk of heartworm disease in the area your dog lives or travels to?

favicon What do I need to know about the heartworm life cycle?

favicon What is the minimal drug frequency to prevent heartworm?

favicon Are there any heartworm drug alternatives?

(NOTE: YES, you can prevent heartworm without meds! Click Here and we’ll show you what simple test you can use to safely replace heartworm meds)

Let’s look at these points in better detail …

1.  Evaluate the risks of heartworm disease

The heartworm life cycle is very much dependent on temperature. The parasite’s life cycle will not be completed unless the region’s average temperature is  57 degrees F  (14 C ) or more for a period of 45 consecutive days and at least two weeks of temperatures over 80 F. If these conditions are not fulfilled, your dog is very likely safe without giving a heartworm preventive or doing any testing.

Based on the recommendations of Dr David Knight and Dr James Lok from the American Heartworm society, even with the most cautious conventional medical protocols, the year round heartworm preventive schedule is exaggerated with the exception of Florida and some parts of Texas and Hawaii.

2.  Heartworm life cycle

Heartworms go through several developmental stages before reaching maturity and it takes 2 ½ to 4 months before the tiny microfilaria in the first stage leave the muscles and start settling in the pulmonary artery. When heartworm reaches its final destination of the pulmonary artery near the heart, it then takes about 3 to 4 more months to reach maturity.

One doesn’t need to have a math degree to figure it takes a minimum of 5 ½ to 8 months for microfilaria to mature into an adult worm.  This would mean that your dog should be safe if you reduce the heartworm drug frequency to once every 3 to 4 months.

3. Why do the drug companies recommend monthly heartworm prevention?

The answer to this question is simple.  Drug companies recommend monthly heartworm preventive either because it doubles or triples their revenues or because they don’t understand the heartworm cycle (which is highly unlikely).


4. Safe alternatives to conventional heartworm drugs

Advances in heartworm diagnostics now offer DNA testing on the basis of PCR (polymerase chain reaction) technology. This test allows you screen for any presence of heartworm and is very reliable with practically no false negatives.

The great news is that DNA testing can eliminate the need for heartworm medication completely unless a dog is infected with heartworm.

In order to help you make a more informed decision, I have created a simple chart bellow with a suggested number of DNA heartworm tests that need to be done to eliminate the need for preventive drugs completely.

To determine the length of heartworm season, refer to the two maps below.

Map A: the start of heartworm season


Map B: the end of heartworm season


(The season is less then four months in all regions of Canada and only dogs living in regions with heartworm incidence need to be tested).

Next, establish the number of tests needed from the table below.


How to evaluate heartworm DNA test results

Heartworm DNA tests can be performed in most veterinary practices by collecting a blood sample and sending it to a veterinary laboratory. Please ensure that the actual DNA test is run as other tests are not as reliable and have a higher margin of error.
Negative DNA test means that your dog has NO heartworm in the body and all you need to do is to retest as recommended above.

Positive DNA test means that your dog has been infected with heartworm. If your last test was done 4 months ago or less, your dog does not have a mature parasite in the heart and heartworm preventive can be safely administered.  However, I strongly recommend that you consult with your veterinarian before administering heartworm preventive to perform general blood work and consider an ultrasound, just to be sure.

CAUTION!!!   Preventive heartworm medication must not be given if your dog missed a test. Using preventive drugs on adult heartworm can cause serious problems and a different treatment protocol must be used.


When it comes to heartworm prevention, it appears that it is possible to dramatically reduce or completely eliminate heartworm drug use if DNA testing is done on a regular basis.

I hope this information will help you make the best decision for your canine friend and create a healthy and long life, naturally.

(Related: Heartworm Facts: Prevalence, Medication And Alternative Treatment)