My last post talked about the increasing concern for Heartworm disease here in Tucson. I am way overdue in following that blog with one that explains Heartworm prevention and hopefully challenges some misconceptions about these medications.
Heartworm prophylaxis is prescribed by veterinarians and is usually given to our dogs and cats in the form of a monthly pill, or chewable treat, or as a topical drop applied monthly to the coat. There is a long acting (6 month) injectable preventative, that was recalled for safety reasons but may be reintroduced. All of these different forms of medication are actually insecticides – they kill the brand new baby heart worms, or microfilaria, that have been transferred with the bite from a carrier mosquito. If given every month, all of these products are thought to be extremely effective at preventing any of the microfilaria from developing and causing disease in our pets.
Some of our clients have expressed concern about giving their pets a monthly dose of a medication for the duration of their lives to prevent a disease that they might never acquire. There have also been reports that certain breeds of dogs can be harmed by some of the Heartworm preventive medications. As we discussed in the last blog, Heartworm prevention is a decision that should be made based on local or regional incidence of Heartworm bearing mosquitos. I believe the overwhelming evidence is that in our local community of Tucson, we have reached the tipping point when the benefits of monthly Heartworm prevention for all dogs far outweighs the risks of giving the monthly dose. It is clearly easier on the pets and on pet owners to prevent this illness then to treat a patient that has developed a mature Heartworm infection.
Some breeds of dogs do have an increased sensitivity to various drugs (not just Heartworm preventatives) because of a genetic difference, most commonly Collies, Australian Shepherds, Long-haired whippets and Silken Winds. These breeds may be more sensitive to Ivermectin type drugs used for heartworm prevention. If your pet has a this specific genetic mutation (to the MDR1 gene) they may potentially show toxicity to doses of Ivermectin as low as 100 micrograms per kilogram. When used for heart worm prevention, Ivermectin is dosed well below that threshold – 6 micrograms per kilogram – once per month. This same drug is also used to treat Demodex mites – often given at doses up to 600 micrograms per kilogram – daily – sometimes for several weeks in a row. Of course, this type of use is unsafe for those sensitive breeds. The doses used for heartworm prevention are considered safe for use in these sensitive breeds. If you dog is one of these related breeds, talk to your vet about choices for Heartworm prevention as well as possible genetic testing to see if your dog is a carrier of this genetic sensitivity.
What is the reasoning behind annual Heartworm testing? We perform the first test to identify occult disease – patients who may have Heartworms without obvious symptoms. Our treatment for those positive dogs is different than starting a monthly preventative. Although these monthly preventives are almost 100% effective at preventing disease, we almost never have 100% compliance with giving the monthly doses. If a dose is skipped or given late the chance of infection can rise. In addition most major drug manufacturers will guarantee their product – if your dog has been properly tested, has been on Heartworm prevention, as still acquires Heartworms – the company will pay for the treatment of your pet.