Collies and related hearding breeds have been know to have been killed by ivermectin in doses approximately 5 times the theraputic dose but other breeds of dog do not appear as sensitive.
Which Heartworm Preventative Is Safe For My Border Collie? Heather's other dog web site has info on the gene mutation that cause, increased sensitivity to ivermectin.
". Levels of ivermectin shown to cause toxicity in beagles, a breed that does not have the mdr1 mutation are 2.5 - 40 mg/kg which is greater than 200 times the therapeutic dose. "
all important to keep in my the theraputic level is lower in dogs than it is in cattle. You will over does a dog by using cattle/sheep doseage.
"The normal dose of ivermectin used for prevention of heartworm in products such as Heartguard (6 micrograms/kg) is not documented to cause the toxicity associated with this mrd1 mutation in collies or any other dog. It is only when higher doses are used, often by people mistakenly thinking the cattle/sheep dose is appropriate for dogs, that toxic symptoms appear in susceptible dogs. "
The Life cycle of ivermectin is not that as truely a preventative. It does not stay in the blood stream that long. Theoretically dog could go every 60 days with treatment an still be covered. The problem is if one is off a bit as usually happen from time to time the margin of safety is gone.
Harworm Society guide lines for Vets
"The single dose retroactive efficacy of all these macrocyclic lactones is assured for one month, and remains high for at least an additional month. However, efficacy against older larvae declines and requires progressively longer-term administration as the worms age to achieve a high level of protection. The extended post-infection efficacy of the macrocyclic lactones is a safeguard in the event of inadvertent delay or omission of regularly scheduled doses and does not justify lengthening the recommended one month interval of administration for the oral and topical formulations. "