Objective—To determine the frequency of the MDR1
gene mutation (polymorphism) associated with ivermectin
sensitivity in a sample population of Collies in
Washington and Idaho.
Animals—40 healthy client-owned Collies.
Procedure—A blood sample (8 ml) was collected
from each dog and used for RNA extraction. Reverse
transcriptase was used to generate MDR1 cDNA.
Polymerase chain reaction (PCR) primers were
designed to amplify a 1,061-base pair region of the
MDR1 gene. The PCR products were sequenced to
determine whether the Collies had 0, 1, or 2 mutant
alleles. Pedigrees of some dogs were available for
analysis to determine relatedness of affected dogs.
Results—Of the 40 Collies, 9 (22%) were homozygous
for the normal allele (normal), 17 (42%) were
heterozygous (carrier), and 14 (35%) were homozygous
for the mutant allele (affected). Pedigree analysis
revealed that some, but not all, affected dogs
were related to each other within the 4 most recent
Conclusions and Clinical Relevance—A high percentage
of a sample population of Collies in
Washington and Idaho are affected or carriers of the
mutant MDR1 allele associated with ivermectin sensitivity.
A similar frequency of this mutation may be
detected in dogs from other geographic areas.
Pharmacologic treatment with ivermectin, loperamide,
vincristine, and other drugs that are substrates
of P-glycoprotein, the MDR1 gene product,
may result in neurologic toxicosis in a high percentage
of Collies. (Am J Vet Res 2002;63:479–481)