Effects of dermal application of 10.0% imidacloprid-0.08% ivermectin in ivermectin-sensitive Collies

OBJECTIVE: To evaluate the safety of dermal application of 10.0% imidacloprid-0.08% ivermectin in ivermectin-sensitive Collies at dose rates of 3 to 5 times the proposed maximum therapeutic dose. ANIMALS: 15 Collies (5 males and 10 females) that were confirmed as ivermectin-sensitive dogs. PROCEDURE: Dogs were assigned to 3 treatment groups (control, 3X, or 5X group) in a randomized block design on the basis of the maximal ivermectin-sensitivity score obtained during preliminary screening. Dogs in groups 3X and 5X were treated at 3 and 5 times the maximum label dose, respectively. Control dogs received an application of an equal volume of a nonmedicated solution. Observation and scoring on all days were conducted to specifically include neurologic signs typical of ivermectin toxicosis, including lethargy, ataxia, abnormal mydriasis, and abnormal salivation. RESULTS: None of the dogs had clinical abnormalities during the study period. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis of results of this study indicates that dermal application of 10.0% imidacloprid-0.08% ivermectin is safe for use in ivermectin-sensitive Collies at dose rates of 3 or 5 times the proposed maximum therapeutic dose.     

Novel insertion mutation of ABCB1 gene in an ivermectin-sensitive Border Collie

P-glycoprotein (P-gp) is encoded by the ABCB1 gene and acts as an efflux pump for xenobiotics. In the Border Collie, a nonsense mutation caused by a 4-base pair deletion in the ABCB1 gene is associated with a premature stop to P-gp synthesis. In this study, we examined the full-length coding sequence of the ABCB1 gene in an ivermectin-sensitive Border Collie that lacked the aforementioned deletion mutation. The sequence was compared to the corresponding sequences of a wild-type Beagle and seven ivermectin-tolerant family members of the Border Collie. When compared to the wild-type Beagle sequence, that of the ivermectin-sensitive Border Collie was found to have one insertion mutation and eight single nucleotide polymorphisms (SNPs) in the coding sequence of the ABCB1 gene. While the eight SNPs were also found in the family members'' sequences, the insertion mutation was found only in the ivermectin-sensitive dog. These results suggest the possibility that the SNPs are species-specific features of the ABCB1 gene in Border Collies, and that the insertion mutation may be related to ivermectin intolerance.     

Frequency of the nt230 (del4) MDR1 mutation in Collies and related dog breeds in Germany

MDR1 (ABCB1) P-glycoprotein exerts a protective function in the blood–brain barrier thereby limiting the entry of many drugs and other xenobiotics to the central nervous system. A nonsense mutation has been described for Collies and related dog breeds which abolishes this function and is associated with increased susceptibility to neurotoxic side effects of several drugs including ivermectin, moxidectin and loperamide. In order to evaluate the occurrence and frequency of this nt230 (del4) MDR1 mutation in Germany, we screened 1500 dogs. Frequency of the homozygous mutated genotype was highest for Collies (33.0%), followed by Australian Shepherd (6.9%) and Shetland Sheepdog (5.7%). Thirty-seven percent of the Wäller dogs and 12.5% of the Old English Sheepdogs were heterozygous for the mutant MDR1 (−) allele. Considering the predominant role of MDR1 P-glycoprotein in drug disposition and in particular for blood–brain barrier protection, MDR1 genotype-based breeding programs are recommended for improving the safety of drug therapy in these canine breeds.     

Loperamide‐induced expression of immune and inflammatory genes in Collies associated with ivermectin sensitivity

This study evaluated the impact of the ABCB1‐1Δ mutation in Collies which exhibited toxicity toward ivermectin, on changes in gene expression when given the unrelated ABCB1 substrate loperamide, to identify potential biomarkers predictive of drug safety. Thirty‐two healthy intact Collies consisting of dogs with either a wild‐type, heterozygous mutant, or homozygous mutant genotype were used. Whole blood samples were collected from Collies at 0 or 5 h following administration of loperamide at a dose of 0.10 mg/kg. Whole‐genome gene expression microarray was conducted to examine for changes in gene expression. Microarray analysis identified loperamide‐induced changes in gene expression which were specifically associated with ivermectin‐sensitive phenotypes in Collies possessing the ABCB1‐1Δ mutation. Gene pathway analysis further demonstrated that the altered genes are involved in immunological disease, cell death and survival, and cellular development. Thirteen genes, including CCL8 and IL‐8, were identified. Collie dogs harboring ABCB1‐1Δ mutation which also exhibited toxicity toward ivermectin demonstrated systematic responses following loperamide treatment exhibited by altered expression of genes involved in immune and inflammatory signaling pathways. Genes such as CCL8 and IL‐8 are potential biomarkers in whole blood that may predict the safety of loperamide in dogs with ABCB1‐1? mutation associated with ivermectin sensitivity.     

Evaluation of the safety of ivermectin administered in a beef-based formulation to ivermectin-sensitive Collies

Twenty-four Collies sensitive to the toxic effects of ivermectin, when administered at high dosages, were studied to evaluate the effects of repeated monthly treatment with an ivermectin beef-based formulation at amounts up to 10 times the dosage recommended for heartworm prevention in dogs. Collies were treated 3 times at 30-day intervals at rates of 12, 36, or 60 micrograms of ivermectin/kg of body weight, or with vehicle. Complete physical and neurologic examinations were performed on all dogs prior to the first treatment and after the final treatment. Clinical observations and ivermectin reaction scores were recorded daily for each dog throughout the study. Clinical or neurologic signs characteristic of ivermectin toxicosis were not observed for any dog during the study. Single episodes of vomiting were recorded for 2 vehicle-treated dogs and 2 dogs treated with ivermectin at 12 micrograms/kg from 6 to 21 days after treatment. At the end of the study, all dogs were challenge-exposed with ivermectin at 120 micrograms/kg to reconfirm their sensitivity to this class of compounds. All dogs developed signs typical of ivermectin toxicosis during the subsequent 48- to 72-hour period. Results of this study demonstrated that ivermectin can be administered repeatedly without adverse effects at rates up to 60 micrograms/kg (10 times the recommended use level) to Collies known to be sensitive to this drug.     

Clinical observations in collies given ivermectin orally

An oral liquid form of ivermectin was administered to 14 purebred Collies (12 rough coated, 2 smooth coated). All Collies were given ivermectin at dosages of 100 and then 200 micrograms/kg of body weight. Three of the dogs developed mild clinical signs of toxicosis (salivation, vomiting, confusion, ataxia, and tremors) with the 100 micrograms/kg dosage. After the 200 micrograms/kg dosage, 7 dogs (including 1 smooth-coated Collie) developed severe toxicosis (seizure-like activity, recumbency, nonresponsiveness, and coma). Because dogs that developed severe toxicosis were not retreated, only the 7 remaining dogs were given ivermectin at 600 micrograms/kg. Severe toxic signs were not observed in the dogs given the 600 micrograms/kg dosage, and only 1 of these 7 dogs developed severe toxicosis when given ivermectin at 2,500 micrograms/kg. Dogs that developed severe toxicosis were given supportive care while in the comatose state. All dogs recovered completely. The results indicated that Collies (including the smooth-coated Collies) have a wide range of sensitivity to ivermectin-induced toxicosis.     

Development of a PCR-based diagnostic test detecting a nt230(del4) MDR1 mutation in dogs: verification in a moxidectin-sensitive Australian Shepherd

A subpopulation of dogs of the Collie and Australian Shepherd breeds show increased sensitivity to central nervous actions of ivermectin, doramectin, loperamide, and probably several other drugs. The molecular background for this greater sensitivity is a nonsense mutation in the MDR1 efflux pump, which is part of the functional blood-brain barrier and normally limits drug penetration into the brain. This report describes a rapid PCR-based method for detection of this nt230(del4) MDR1 mutation using a small amount of genomic DNA from blood cells. Thereby, homozygous intact, homozygous mutated, and heterozygous mutated MDR1 genotypes can be clearly differentiated by high resolution polyacrylamide gel electrophoresis. Using this diagnostic test two Collies and one Australian Shepherd were screened for the nt230(del4) MDR1 mutation. The Collies had no history of altered drug sensitivity and showed homozygous intact and heterozygous mutated MDR1 alleles, respectively. However, the Australian Shepherd developed clear signs of neurotoxicity including ataxia, crawling, acoustic and tactile hyperexcitability, and miosis after a single dose of moxidectin (400 microg/kg). For this dog two mutated MDR1 alleles were detected. This report describes for the first time moxidectin neurotoxicosis in a dog with a homozygous MDR1 mutation.     

Increased toxicity of P-glycoprotein-substrate chemotherapeutic agents in a dog with the MDR1 deletion mutation associated with ivermectin sensitivity

Lymphoma was diagnosed in a 4-year-old spayed female Collie, and treatment with a combination chemotherapy protocol incorporating prednisone, L-asparaginase, vincristine, vinblastine, doxorubicin, and cyclophosphamide was initiated. The dog had signs of gastrointestinal tract toxicosis and myelosuppression after treatment with P-glycoprotein-substrate drugs (vincristine, vinblastine, and doxorubicin) even when dosages were reduced, but did not have signs of toxicosis after treatment with cyclophosphamide, a non-P-glycoprotein-substrate drug, even when administered at the full dosage. It was postulated that a deletion mutation in the canine MDR1 gene (deltaMDR1 295-298) could be responsible for the drug toxicoses in this dog. This mutation has been identified as the cause of a functional P-glycoprotein defect in Collies susceptible to the toxic effects of ivermectin, another P-glycoprotein-substrate drug. The MDR1 genotype of this dog consisted of 1 normal and 1 mutant MDR1 allele. Because P-glycoprotein contributes to renal, biliary, and intestinal excretion of P-glycoprotein-substrate drugs, it is possible that drug excretion was delayed in this patient, resulting in clinical signs of toxicosis.     

Brain penetration of ivermectin and selamectin in mdr1a,b P-glycoprotein- and bcrp- deficient knockout mice

P-glycoprotein, which is encoded by the multi-drug resistance gene (MDR1), highly restricts the entry of ivermectin into the brain by an ATP-driven efflux mechanism at the blood–brain barrier. In dogs with a homozygous MDR1 mutation though, ivermectin accumulates in the brain and provokes severe signs of neurotoxicosis and even death. In contrast to ivermectin, selamectin is safer in the treatment of MDR1 mutant dogs, suggesting that selamectin is transported differently by P-glycoprotein across the blood–brain barrier. To test this, we applied selamectin to mdr1-deficient mdr1a,b ^−/− knockout mice and wild-type mice. Brain penetration, organ distribution, and plasma kinetics were analyzed after intravenous, oral, and dermal spot-on application in comparison with ivermectin. We found that in vivo both macrocyclic lactone compounds are substrates of P-glycoprotein and that these strongly accumulate in the brain of mdr1a,b ^−/− knockout mice compared with wild-type mice at therapeutic doses of 12 mg/kg selamectin and 0.2 mg/kg ivermectin. However, selamectin accumulates to a much lesser degree (5–10 times) than ivermectin (36–60 times) in the absence of P-glycoprotein. This could explain the broader margin of safety of selamectin in MDR1 mutant dogs. In liver, kidney, and testes, ivermectin and selamectin accumulated less than four times as much in mdr1a,b mutant mice as in wild-type mice. Breast cancer resistance protein (Bcrp)-deficient bcrp ^−/− knockout mice were also included in the application studies, but showed no differences in brain concentrations or organ distribution of either ivermectin or selamectin compared with wild-type mice. This indicates that Bcrp is not a relevant efflux carrier for these macrocyclic lactone compounds in vivo at the blood–brain barrier.     

Assessment of toxicosis induced by high-dose administration of milbemycin oxime in collies

Fifteen Collies, previously having mild reactions to ivermectin challenge (120 micrograms/kg of body weight; 20 times the recommended dosage level), were studied to evaluate the effects of milbemycin oxime administration at 5 and 10 mg/kg (10 and 20 times the manufacturer's recommended dosage). Five replicates, comprising 3 dogs each, were formed on the basis of body weight. Within replicates, each dog was randomly allocated to treatment with 5 or 10 mg of milbemycin/kg or served as a untreated control. Dogs were examined repeatedly for signs of toxicosis for 4 days after treatment and daily thereafter. Two of 5 dogs treated at 5 mg/kg (10x) developed signs of mild depression on the day of treatment, but were normal 24 hours after treatment. All 5 dogs treated at 10 mg/kg (20x) developed signs of mild depression and ataxia by 6 hours. Signs persisted for 24 hours in 3 dogs. Two of these dogs also had mydriasis, whereas 3 salivated excessively. All dogs recovered completely by day 2 after treatment. The results of this study demonstrated that Collies sensitive to the effects of 120 micrograms of ivermectin (20x)/kg show similar sensitivity to the effects of milbemycin oxine administered at 10 mg/kg (20x). We conclude that ivermectin and milbemycin commercial formulations have similar margins of safety and that milbemycin toxicosis appears to be dose-dependent in Collies with a demonstrated sensitivity to ivermectin.     

The ABCB1-1Δ mutation is not responsible for subchronic neurotoxicity seen in dogs of non-collie breeds following macrocyclic lactone treatment for generalized demodicosis

P-glycoprotein (P-gp), encoded by the multiple drug resistance gene ABCB1 (also known as MDR1 ), is an integral component of the blood brain barrier crucial in limiting drug uptake into the central nervous system. Altered expression or function of P-gp, as seen in dogs of the collie lineage homozygous for the nt228(del4) mutation of the ABCB1 gene ( ABCB1-1Δ ), can result in potentially fatal neurotoxicosis, especially following administration of systemic macrocyclic lactones (SML). Occasionally, dogs from unrelated breeds develop subchronic signs of neurotoxicity when receiving SML to treat generalized demodicosis. It is possible that these dogs are heterozygous carriers of the ABCB1-1Δ mutation, resulting in decreased P-gp activity and central neurotoxicosis. Cheek swabs were collected from 28 dogs with generalized demodicosis that had shown subchronic signs of neurotoxicity following daily oral administration of ivermectin or other SML. Ten of these animals received concurrent systemic treatment with other confirmed or putative P-gp substrates. After DNA extraction, the relevant portion of the ABCB1 gene was amplified by polymerase chain reaction, and sequenced. Twenty-seven dogs were homozygous normal while one dog was heterozygous for the ABCB1-1Δ mutation. Therefore, with the exception of one dog, the observed neurotoxicity could not be attributed to the ABCB1-1Δ mutation. Possible explanations for the adverse reactions observed include pharmacological interactions (administration of SML with other P-gp substrates or inhibitors), excessively high doses, polymorphisms in P-gp expression, uncharacterized mutations in the ABCB1 gene or in another gene, or phenomena unrelated to the SML–P-gp interaction.     

Safety of moxidectin in avermectin-sensitive collies

OBJECTIVE: To evaluate the safety of moxidectin administration at doses of 30, 60, and 90 microg/kg of body weight (10, 20, and 30 times the manufacturer's recommended dose) in avermectin-sensitive Collies. ANIMALS: 24 Collies. PROCEDURE: Collies with mild to severe reactions to ivermectin challenge (120 mg/kg; 20 times the recommended dose for heartworm prevention) were used. Six replicates of 4 dogs each were formed on the basis of body weight and severity of reaction to ivermectin test dose. Within replicates, each dog was randomly allocated to treatment with oral administration of 30, 60, or 90 microg of moxidectin/kg or was given a comparable volume of placebo tablet formulation. Dogs were observed hourly for the first 8 hours and twice daily thereafter for 1 month for signs of toxicosis. RESULTS: Signs of toxicosis were not observed in any control group dog throughout the treatment observation period. Likewise, signs of toxicosis were not observed in any dog receiving moxidectin at 30, 60, or 90 microg/kg. CONCLUSIONS AND CLINICAL RELEVANCE: The moxidectin formulation used in the study reported here appears to have a wider margin of safety than ivermectin or milbemycin in avermectin-sensitive Collies.     

Ivermectin toxicity in 17 collies

Ivermectin is widely used in veterinary medicine as an anthelminthic and generally has a wide margin of safety, but Collies are prone to ivermectin toxicity. Two groups of Collies were presented to the University of California Veterinary Medical Teaching Hospital (VMTH) with ivermectin toxicity. The medical records of the 2 groups of Collies were reviewed retrospectively. Group I comprised 5 adult Collies that received at least 400 microg/kg ivermectin p.o. and were presented to the VMTH 3 hours after intoxication. These Collies showed marked clinical signs on presentation. Three of these dogs required mechanical ventilation and were euthanized for financial reasons; the remaining 2 dogs were comatose but recovered in 5-7 days. Group II was comprised of 12 adult Collies presented to the VMTH 2 days (n = 10) and 5 days (n = 2) after subcutaneous injection of 200-250 microg/kg ivermectin. These animals showed greater variation in severity of illness among individuals; 5 animals progressed to stupor or coma, whereas 4 animals remained ambulatory. Most of these dogs' clinical signs deteriorated from the day of intoxication until approximately day 6, from which time they showed gradual but steady improvement. All of the Collies in this group survived, but it took 3 weeks for most of them to recover. Collies suffering from ivermectin toxicity can have a severe and prolonged clinical course requiring intensive nursing care. Respiratory, cardiovascular, and nutritional support may all be required. With appropriate care, however, the prognosis for complete recovery is good.     

Determination of homovanillic acid, 5-hydroxyindoleacetic acid and pressure in the cerebrospinal fluid of Collie dogs following administration of ivermectin

Twelve adult Collie dogs were studied to determine the effects of ivermectin on neurotransmitter metabolites released from the brain into the cerebrospinal fluid (CSF) and on CSF pressure. Ten of the 12 Collies were given ivermectin orally at a concentration of 200 g/kg body weight. Three of these 10 Collies showed clinical signs of ivermectin-induced toxicosis which progressed into a state of unresponsive recumbency in 2 dogs.Cerebrospinal fluid pressures and neurotransmitter metabolite concentrations in cisterna magna CSF were analysed 49 to 50 hours after administration of ivermectin in 6 of the 10 treated dogs, and in the 2 untreated control Collies. Cerebrospinal fluid pressures were within normal limits in all dogs. However, compared to the CSF concentrations in the 2 untreated and 3 non-reactive Collies, the 2 ivermectin-reactive Collies still in recumbency had elevated CSF concentrations of homovanillic acid (HVA), a metabolite of dopamine, and 5-hydroxyindoleacetic acid (5-HIAA), a metabolite of serotonin. These findings suggest an association between altered neurotransmission and severe ivermectin-induced neurological abnormalities. No evidence of elevated intracranial pressure was found.     

ABCB1-1Delta polymorphism can predict hematologic toxicity in dogs treated with vincristine.

BACKGROUND: Dogs that harbor the naturally occurring ABCB1-1Delta polymorphism experience increased susceptibility to avermectin-induced neurological toxicosis as a result of deficient P-glycoprotein function. Whether or not the ABCB1-1Delta polymorphism affects susceptibility to toxicity of other P-glycoprotein substrate drugs has not been studied. HYPOTHESIS: Dogs that possess the ABCB1-1Delta mutation are more likely to develop hematologic toxicity associated with vincristine than ABCB1 wild-type dogs. ANIMALS: Thirty-four dogs diagnosed with lymphoma were included in this study. METHODS: Cheek swab samples were obtained from dogs diagnosed with lymphoma that were to be treated with vincristine. DNA was extracted from cheek swabs and the ABCB1 genotype was determined. Hematologic adverse drug reactions were recorded for each dog and graded according to the Veterinary Comparative Oncology Group's criteria for adverse event reporting (Consensus Document). In order to avoid possible bias, ABCB1 genotype results for a particular patient were not disclosed to oncologists until an initial adverse event report had been submitted. RESULTS: Dogs heterozygous or homozygous for the ABCB1-1Delta mutation were significantly more likely to develop hematologic toxicity, specifically neutropenia (P= .0005) and thrombocytopenia (P= .0001), after treatment with vincristine than ABCB1 wild-type dogs. CONCLUSIONS AND CLINICAL IMPLICATIONS: At currently recommended dosages (0.5-0.7 mg/M(2)), vincristine is likely to cause hematologic toxicity in dogs with the ABCB1-1Delta mutation, resulting in treatment delays and unacceptable morbidity and mortality. Assessing the ABCB1-1Delta genotype before vincristine administration and decreasing the dosage may prevent toxicity and treatment delays resulting from neutropenia or thrombocytopenia.     

Establishment of a cell line for assessing drugs as canine P‐glycoprotein substrates: proof of principle

P‐glycoprotein (P‐gp), encoded by the ABCB1 (MDR1) gene, dramatically impacts drug disposition. P‐gp is expressed in the intestines, biliary canaliculi, renal tubules, and brain capillaries where it functions to efflux substrate drugs. In this capacity, P‐gp restricts oral absorption, enhances biliary and renal excretion, and inhibits central nervous system entry of substrate drugs. Many drugs commonly used in veterinary medicine are known substrates for canine P‐gp (vincristine, loperamide, ivermectin, others). Because these drugs have a narrow therapeutic index, defective P‐gp function can cause serious adverse drug reactions due to enhanced brain penetration and/or decreased clearance. P‐gp dysfunction in dogs can be intrinsic (dogs harboring ABCB1‐1Δ) or acquired (drug interactions between a P‐gp inhibitor and P‐gp substrate). New human drug candidates are required to undergo assessment for P‐gp interactions according to FDA and EMA regulations to avoid adverse drug reactions and drug–drug interactions. Similar information regarding canine P‐gp could prevent adverse drug reactions in dogs. Because differences in P‐gp substrates have been documented between species, one should not presume that human or murine P‐gp substrates are necessarily canine P‐gp substrates. Thus, our goal was to develop a cell line for assessing drugs as canine P‐gp substrates.     

Allele-specific polymerase chain reaction diagnostic test for the functional MDR1 polymorphism in dogs

The major multidrug transporter P-glycoprotein (Pgp) contributes to the barrier function of several tissues and organs, including the brain. In a subpopulation of Collies and seven further dog breeds, a 4 base pair deletion has been described in the Pgp-encoding MDR1 gene. This deletion results in the absence of a functional form of Pgp and loss of its protective function. Severe intoxication with the Pgp substrate ivermectin has been attributed to the genetically determined lack of Pgp. An allele-specific polymerase chain reaction (PCR)-based screening method has been developed to detect the mutant allele and to determine if a dog is homozygous or heterozygous for the mutation. Based on this validation, the allele-specific PCR proved to be a robust, reproducible and specific tool, allowing rapid determination of the MDR1 genotype of dogs of at risk breeds using blood samples or buccal swabs.     

ABCB1-1Δ Polymorphism Can Predict Hematologic Toxicity in Dogs Treated with Vincristine

Background: Dogs that harbor the naturally occurring ABCB1-1Δ polymorphism experience increased susceptibility to avermectin-induced neurological toxicosis as a result of deficient P-glycoprotein function. Whether or not the ABCB1-1Δ polymorphism affects susceptibility to toxicity of other P-glycoprotein substrate drugs has not been studied.
Hypothesis: Dogs that possess the ABCB1-1Δ mutation are more likely to develop hematologic toxicity associated with vincristine than ABCB1 wild-type dogs.
Animals: Thirty-four dogs diagnosed with lymphoma were included in this study.
Methods: Cheek swab samples were obtained from dogs diagnosed with lymphoma that were to be treated with vincristine. DNA was extracted from cheek swabs and the ABCB1 genotype was determined. Hematologic adverse drug reactions were recorded for each dog and graded according to the Veterinary Comparative Oncology Group's criteria for adverse event reporting (Consensus Document). In order to avoid possible bias, ABCB1 genotype results for a particular patient were not disclosed to oncologists until an initial adverse event report had been submitted.
Results: Dogs heterozygous or homozygous for the ABCB1-1Δ mutation were significantly more likely to develop hematologic toxicity, specifically neutropenia ( P = .0005) and thrombocytopenia ( P = .0001), after treatment with vincristine than ABCB1 wild-type dogs.
Conclusions and Clinical Implications: At currently recommended dosages (0.5–0.7 mg/M²), vincristine is likely to cause hematologic toxicity in dogs with the ABCB1-1Δ mutation, resulting in treatment delays and unacceptable morbidity and mortality. Assessing the ABCB1-1Δ genotype before vincristine administration and decreasing the dosage may prevent toxicity and treatment delays resulting from neutropenia or thrombocytopenia.     

ABCB1-1Δ (MDR1-1Δ) genotype is associated with adverse reactions in dogs treated with milbemycin oxime for generalized demodicosis

Twenty-two dogs diagnosed with generalized demodicosis were treated with milbemycin oxime (MO) because of poor response to previous therapies or because the dog was a breed known to be susceptible to ivermectin toxicosis. Fifteen of the 22 dogs were herding breeds. Doses of MO ranged from 1.0 to 2.2 mg kg⁻¹ day⁻¹ per os. Cheek swab samples were obtained in order to determine each dog's ABCB 1 genotype. Adverse drug reactions were recorded for each dog by the owners and/or veterinarians. The ABCB 1-1Δ genotype was significantly associated with the development of an adverse reaction (neurological toxicity) after treatment with MO. None of the 19 dogs with the wild-type ABCB1 allele experienced adverse reactions, whereas two dogs homozygous for the ABCB1-1Δ mutation developed ataxia. Assessing the ABCB1-1Δ genotype prior to MO administration may prevent neurological toxicity in these patients.