When it comes to assessing your pet’s risk of getting heartworm disease, we must first look at the life-cycle of the worm and the environmental requirements for its development.   Heartworm disease cannot be spread directly from dog to dog, the larvae must develop in the mosquito first. So the first risk factor is how many mosquitoes is your dog exposed to?  If you live in, or take your dog to areas where there are many mosquitoes, then the risk may be higher.  It is also very important to understand the temperature requirements for heartworm development in the mosquito. If these requirements are not met, then the larvae will not be able to infect a dog that it bites. 

The development of the heartworm larvae in the mosquito is dependent on a temperature that remains above 57F (14C) for at least 30 days straight and at least two weeks of temperatures over 80F (26C). If these conditions are not fulfilled, the parasitic life cycle cannot be completed and your dog is safe.  Here in Colorado, it is rare for nighttime temperatures to stay above 57 degrees for 30 days, and as for all the mosquitos in the high country – temps typically dip into  the 30’s and 40’s during the night, so there is zero risk of heartworm, even though the mosquitoes are plentiful.  Next, let’s look at what happens if you dog is bitten by an infected mosquito.

When a mosquito carrying the heartworm larvae bites your dog, the larvae is deposited in the muscle tissue. It’s final destination is the pulmonary artery near the heart, but it takes 2 1/2 to 4 months for it to arrive there.  When the heartworm larvae reaches the pulmonary artery, it then takes about three to four months to reach maturity.

It is easy to see that it takes somewhere between five-and-a-half to eight months for the heartworm larvae to mature into an adult worm and that your dog should be safe if you administer heartworm prevention  once every three months, rather than the standard monthly recommendation, even if you live in an area where heartworm disease is prevalent.

If this is all true, then why is the medication recommended every month?  These recommendations come from the manufacturer of the preventative, and these companies are often owned by large corporations looking to make a profit.  Once they determine that giving the medication monthly is effective in preventing heartworm, they have no incentive to investigate the possibility that giving it less frequently will be just as effective.  After all, this would be encouraging consumers to buy less product, cutting into their profits. While it is convenient to just do the monthly preventative, it is important to remember that these drugs are not benign, and are, in fact toxic to the nervous system, which is how they eliminate heartworm.  It is important that you become an advocate for your pet and carefully consider the recommendations for dosing and determine what is most appropriate. 

If you are interested in getting away from drugs completely, there is yet another option.   There is now a DNA test for heartworm larvae  that is much more sensitive than the more commonly used antigen test, which tests for the adult heartworm.  Doing this test at the end of heartworm season where you live will determine if you dog has been exposed, and then can be given the preventative only if necessary.   If the heartworm season where you live is longer than 4 months, based on the temperature guidelines above, then the test should be repeated at 4 month intervals.  It is very important to test your dog  to be sure there are no adult worms present before beginning this program.

This may sound a bit confusing, but if you want your pet to live the best life possible, it is up to you to become educated enough to make the best decisions for your pet’s health and not just follow recommendations blindly.  These days, there is complete access to the information you need to help your pet, you just need to start doing some investigating.