Treatment of a mature heartworm infection can be very dangerous. When the arsenic-based drug is given to an infected dog, the massive die-off of the worms can cause severe inflammation and even respiratory failure. Not all dogs survive treatment. Clearly, prevention is the best option!
Alternatively, many veterinarians advocate simply giving the regular heartworm preventative to kill off any microfilariae already present and keep newly deposited larvae from developing, while waiting for the adult worms to die. This may be a more practical alternative for cats, or for dogs that do not have a severe infestation.
"There is no reason to give heartworm medicine to most pets year-round (except to make money for those who make and sell it!)"
Except for a the warmest parts of the U.S. (mainly in the southeast), heartworms are a completely seasonal problem. There is no reason to give heartworm medicine to most pets year-round (except to make money for those who make and sell it!).
In many areas of the country (northern and mountain states, for instance), such warm temperatures simply don't exist for most of the year, and sustained warm temperatures don't occur until at least June. In fact, only in Florida and south Texas is year-round heartworm transmission possible. Within 150 miles of the Gulf Coast, heartworm risk exists 9 months out of the year. In the rest of the country, heartworm transmission is possible between 3 and 7 months out of the year. Hawaii and Alaska have each had a few cases of canine heartworm, but the incidence in those states is very low.
It should be obvious that during seasons where there are no mosquitoes, there is no risk of heartworm. Evidently that little fact escaped the attention of the veterinarian who prescribed heartworm protection—in December–for a puppy living high in the Colorado mountains. At that altitude, temperatures are never warm enough for heartworms!
A debate about when to give heartworm preventatives was published in an April 2009 journal article ("Ask the Expert: Year-Round Heartworm Prevention: Two Viewpoints," by By Dwight Bowman and James Lok, published in NAVC Clinician's Brief, the official publication of the North American Veterinary Conference, 2009/04/01). Both authors are university professors in parasitology.
The argument presented by Dr. Bowman in favor of year-round heartworm medication focused on just two points: (1) the speculation that "scenarios can arise where transmission may occur in cooler climates in the 'off season;' and (2) the completely unrelated issue of prevention of internal parasites by additional drugs added to the heartworm preventative.
Arguing on the other side, Dr. Lok lays out the case for appropriate seasonal control, and concludes, "Besides incurring unnecessary costs for the client, indiscriminate application of broad-spectrum medications can engender further confusion about the primary imperative for these medications—heartworm prevention—and when they are most crucial—during the season of heartworm transmission."
Of course, if in any given year the weather is unseasonably warm for long enough, exceptions to those recommendations should be made.
Having looked at both sides of the issue, I have to agree with those who suggest that giving year-round treatment to animals in states where year-round transmission does not occur is doing an injustice to both the animals being given drugs they don't need, as well as the pocketbooks of their guardians. This argument is rarely presented since the drug companies have the resources to widely promote their views (and products) to consumers as well as veterinarians.