Just quoting this to make sure it gets noticed.
This is wrong.... But vets do try to sell this load of hogwash
The first step in treatment is clearing the migrating immature worms. If we were to jump directly to killing the adult worms first, the adult worms we remove could be readily replaced shortly afterwards by those that were in the process of migration at the time of treatment. By addressing the migrating immature worms first, we minimize the number of adult worms we must kill in the second step. Fewer adult worms dying at once means less risk.
Happily, the microfilariae, L3, and L4 larvae can all be killed by monthly ivermectin-based heartworm preventive products (i.e. Heartgard
, Tri-Heart
etc.). The milbemycin based products (Sentinel
and Interceptor
) will also do the same job but will kill the microfilariae much faster, which can create circulatory shock if there are large numbers of microfilariae dying all at one time. The newer products using selamectin and moxidectin do not clear microfilaria well enough to be used in the treatment of an active infection, so right now the ivermectin based products seem to be the best for this use. The American Heartworm Society recommends 1 to 3 months of a preventive prior to treating the adult worms. How long you choose to wait depends on how urgent the dogs need is to have the adult worms removed. After all, it is the adult worms that cause heartworm disease, not the immature worms addressed by the preventives.
http://pet-diseases.suite101.com/article.cfm/heartworm_treatment_in_dogs
Other Alternatives to Heartworm Treatment in Dogs Infected with Heartworms
Some dogs exist which are not good candidates for heartworm treatment even though they may be infected. Often these are the dogs which are experiencing severe clinical signs of heartworm disease and Immiticide
injections have been deemed to be too dangerous to the dog's health. For these dogs, a viable option is beginning a monthly routine with an ivermectin-based heartworm preventive medicine.
Ivermectin is chosen over other heartworm preventive medications because it has a lower risk for complications in heartworm infected dogs. However, heartworm positive dogs should be observed closely for at least 8-12 hours after administering the ivermectin heartworm preventive in case an adverse reaction occurrs.
Ivermectin-based monthly heartworm preventive medications kill the microfilaria form of the heartworm which circulates in the blood stream of infected dogs, making the dog non-infectious for other dogs in the immediate area.
Eventually, the adult worms living inside the dog's heart will die and disappear. However, this process can take months to years and the risk of further damage to the dog's heart exists while adult heartworms are still present.