Today's Veterinary Nurse

SEP-OCT 2017

Practical, peer reviewed, state-of-the-art companion animal nursing and technical educational articles with CE. Promotes better health for animals and career growth and development for veterinary technicians and veterinary assistants.

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PEER REVIEWED 60 | TODAY'S VETERINARY TECHNICIAN | September/October 2017 also cause adverse reactions (BOX 3) . 5 Because of the public's awareness of ivermectin-associated problems, many breeders and websites recommend avoiding all products containing this drug, including heartworm preventive agents. It is true that higher concentrations of ivermectin, such as those used for treating mange and other parasites, should not be administered to MDR1-affected dogs. However, dog owners should know that the doses of US-produced heartworm preventive agents recommended by manufacturers and approved by the Food and Drug Administration are safe for dogs with the mutation. Conversely, off-label use of higher concentrations of large-animal ivermectin formulations for heartworm prevention has led to fatal consequences and should be avoided. In addition, care should be taken to keep dogs away from large animals that have been wormed with ivermectin. Even dogs that are unaffected by the MDR1 mutation may be exposed to toxic doses of this medication if they consume the feces of recently wormed cattle, horses, and sheep. Other Affected Drugs Veterinary staff may not be able to avoid using other medications on the known substrate list, especially when anesthesia or cancer treatments are necessary. With some analgesics and chemotherapeutic agents, a reduced dose is recommended. A common over-the-counter antidiarrheal (loperamide) is also a known substrate. Veterinary technicians should be aware of the patient's MDR1 status when an owner is giving this medication to treat diarrhea. The veterinary team may need to suggest alternatives until MDR1 testing is complete. Drug Misconceptions A common misconception found on many breed websites is that metronidazole is a problem drug for MDR1-affected dogs. Metronidazole is prescribed for protozoal and anaerobic bacterial infections. This drug is known to cause dose-related neurologic side effects, but this is not related to P-glycoprotein function. Additionally, vaccination reactions are caused by immune-mediated issues, which are unrelated to the MDR1 mutation. However, clients with affected dogs may be apprehensive about giving any medications or vaccinations because of misleading information on public websites. This is a case where the veterinary technician can play an important role in preventing false information from keeping pet owners from making good treatment decisions. CLINICAL SIGNS Dogs exhibiting clinical signs of drug toxicosis should be evaluated quickly. MDR1-related drug toxicosis of the CNS often presents as vague neurologic signs, such as weakness, lethargy, ataxia, and disorientation. The owner may report that the pet has been stumbling and bumping into things. In severe cases, the patient will become nonambulatory and even comatose, and ventilator-assisted respiration may be necessary. If the inducing medication has a reversal agent, such as naloxone for loperamide, it should be administered per the manufacturer's recommendations. However, for affected dogs, the most important treatment is intensive supportive care, including IV fluids, nutritional support, and diligent patient monitoring. Recovery may be a lengthy process requiring the owner to be aware of the extent of the nursing care and expenses involved. MDR1 -affected dogs receiving chemotherapeutic agents that are P-glycoprotein substrates are at increased risk for neutropenia, thrombocytopenia, and adverse gastrointestinal effects. Therefore, other options, such as alternative medications or dose reductions, should be investigated. PREVENTION Unless a patient has a clear pedigree from a non–herding breed line, caution is advised before administering a known MDR1 toxic drug to any dog, especially mixed breeds. As mentioned, MDR1 Genetic Testing: What You Need to Know Acepromazine Apomorphine Butorphanol Doramectin Doxorubicin Erythromycin Ivermectin Loperamide Milbemycin Moxidectin Paclitaxel Selamectin Vinblastine Vincristine Vinorelbine BOX 3 Drugs Affected by the MDR1 Gene Mutation 5 T E C H P O I N T Coat color, markings, and sex are not predictors of dogs affected by MDR1, despite the handy "white feet" rule.

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