Safety of Heartgard®in Dogs that May Have MDR-1 Mutation
(Particularly Collie Breeds)
Dr. Julie Dahlke
October 2012
Some dogs have a genetic mutation commonly known as MDR-12 that causes a
breakdown in the protective blood-brain barrier. With this defect, certain drugs
are allowed access to the brain at much greater levels than normal, resulting in
an increased risk for toxic levels of the drug (or medication) to access the brain
and cause neurotoxicity. The symptoms of such a neurotoxicity could include
lethargy, drooling and coma. Ivermectin is one of the drugs that if prescribed
could result in a toxic dose, or overdose, in sensitive breeds (breeds containing
the MDR-1 gene).
It has been established that in sensitive dogs (dogs who are positive for the
MDR-1 gene mutation), the lowest possible toxic dosage of ivermectin is 80
mcg (micrograms) per kilogram of body weight or 36 mcg/pound. The chart
below shows the dosage of ivermectin in the different strengths of Heartgard as well as
the dosage per weight and total (lowest) toxic dose per weight.
Dosage of
Ivermectin
Heartgard®1 Products
Dosage in
mcg/pound
68 mcg per tablet
Dose: 1
tablet/mouth
Blue (for dogs
weighing
25# or less
14 mcg per pound 181 mcg per 5# dog (2.6 tablets)
for a 5# dog
363 mcg per 10# dog (5 tablets)
Lowest total toxic dose
136 mcg per tablet Green (for dogs
Dose: 1
weighing
tablet/mouth
26-50#)
12 mcg per pound
945 mcg per 26# dog (7 tablets)
for a 26# dog
272 mcg per tablet Brown (for dogs
Dose: 1
weighing
tablet/mouth
50-100#
5.5 mcg per pound 1,818 mcg per 50# dog (13
for a 50# dog
tablets)
SUMMARY
Heartgard is a safe and effective medication to prevent a very serious infection in dogs –
heartworm disease. While sensitivities to medications can occur in any individual pet,
Heartgard is safe for use in proper dosages for any dog, including dogs containing the
MDR-1 gene mutation (frequently collie-breeds though others can be affected as well.)
Any medication recommended for prevention of disease has the potential to cause toxic
side effects if given inappropriately or in overdose. Smaller dogs and younger dogs are
frequently more susceptible to overdose due both to size and to the tendency to search
out and ingest non-food items. Heartgard is intended to taste very yummy to dogs so it
is particularly important to keep the package sealed and out of the reach of your pet
when being stored and administer only according to label instructions.
1 Ivermectin-containing heartworm preventive medication
2 Currently known as ABCB1 gene