Based on an article that first appeared at

This is a question that is commonly posed to me in clinical practice, and to be fair, a fair one.  If you are giving your dog year round heartworm prevention, why do we still ask you to have your dog get a heartworm test once a year to renew the heartworm prevention prescription?  If you live in an area where the mosquito vector responsible for transmission of the disease is seasonal, why the recommendation for year round heartworm prevention?  Why treat at all if you live in a region with no mosquitoes? Below are the answers to these questions.

1.)    No treatment is 100%.

While heartworm preventives are highly effective in preventing heartworm disease, not one single heartworm preventive can claim 100% efficacy, even in controlled settings where product is regularly administered vigilantly. 

2.)    While as in other aspects of life, we all strive to be perfect in giving our pets their medications in a timely manner, not one single person can claim to be perfect.

Heartworm preventives are only effective in killing the infective stage of larvae that infected mosquitos inject into the dog’s bloodstream when feeding.  If even a few days late in administering the heartworm preventive, if a dog is infected, the larvae may have already reached a stage of development that the preventive medication will not kill; and thus, continue to develop into adult worms despite continued timely preventive administration.

As such, I see about 1-2 cases per year of heartworm positive dogs whose owners had previously  purchased 12 months of heartworm prevention and religiously administered product, but in hindsight usually recalled a particular month when they may have been late.

3.)     Although in temperate climates where the temperature regularly drops below freezing in the winter months naturally culling mosquito populations, unseasonably warm weather will commonly lead to pockets of mosquito blooms, thus making transmission of heartworm disease still possible during less mosquito prone months.

4.)    In dry climates like Arizona and Nevada, it is true that mosquitos – and therefore incidence of heartworm disease – are uncommon to see.    

However, anywhere or any time there is the right atmospheric condition, however brief, combined with fresh, standing water (lakes, ponds, puddles, irrigation ditches, etc.), it is possible to have mosquito blooms.  According to statistical data compiled by the American Heartworm Society, Arizona reported 462 cases of heartworm cases in 2001.  With significant ongoing development and irrigation systems necessary to provide water increase in size and scope, it is likely that number is currently higher and poised to rise.

5.)    Most veterinary grade heartworm preventives also have potent killing capability for the most common intestinal parasite worms. 

Worms such as these are ubiquitous nationally and can infect people, especially young children, immune suppressed individuals, and the elderly.  Treating with heartworm prevention helps to prevent these harmful parasites as well.

6.)    With the advent of “all in one” preventives like Trifexis that offer protection from fleas in addition to intestinal parasites and heartworm, there is the added benefit of keeping homes free of fleas as well by keeping your pet on year round heartworm prevention.

So there you have it, no heartworm preventive is 100%, no owner no matter how responsible is perfect in timely administration of heartworm preventive, mosquito blooms and heartworm infection still happens in regions cold winter months during unseasonably cold weather, mosquito blooms and heartworm infection in dry climates occur under the right set of conditions, and year round prevention with modern “all in one” products also protect the pet (and its family) from common intestinal parasites and fleas.

Lastly, once a canine is infected with heartworm disease, it is too late for preventives.  Treatment for heartworm disease is expensive, potentially toxic, and sometimes fraught with complications.  It is a lot less expensive, far less traumatic for the pet, and safer for the human family to just administer year round treatment, while engaging in once yearly screening.

Dr. Roger Welton is the President of Maybeck Animal Hospital and CEO/Chief Editor of the veterinary information and blog online community, Web-DVM.