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Common heartworm preventive, other antiparasitics can be deadly for some cats

Closeup of a cat
The American Heartworm Society recommends heartworm prevention for all cats.

Warmer and wetter weather inevitably means the return of mosquitos and their insatiable thirst for blood. 

The tiny bugs aren’t just a nuisance to humans — they also carry and spread the deadly parasite that in dogs and cats causes heartworm disease, which can result in severe lung disease, heart failure, other organ damage and death.

The American Heartworm Society recommends heartworm prevention for all cats. While preventatives are available, some can be dangerous for cats that have a rare gene mutation that can only be identified with a genetic test.

Dr. Katrina Mealey, a Regents professor and veterinarian in the College of Veterinary Medicine at Washington State University, and her team in the Program in Individualized Medicine have identified a growing list of common drugs and products that can cause serious and potentially fatal reactions in cats with the feline MDR1 mutation. Recent research published in the Journal of the American Veterinary Medical Association led to eprinomectin, a main ingredient in the heartworm and common intestinal parasite preventative Centragard, being added to the list. 

“It is not a problem with Centragard — the problem lies with the 1% of cats that are affected by the genetic mutation, and that is a large number when you consider 42 million households in the U.S. have at least one cat,” Mealey said. “This is a widely used preventive drug, and if your cat has this mutation, even when the product is used according to the label, it could cause serious and life-threatening reactions.”

WSU veterinarian Katrina Mealey provides a checkup for her dog Bumpus at the Veterinary Teaching Hospital. 

Cats with the MDR1 mutation have a defect in a protective mechanism called the blood-brain barrier that makes them highly susceptible to neurological toxicity when exposed to certain drugs, including those frequently used for parasites, diarrhea and chemotherapy. Drugs known to cause adverse reactions include emodepside, ivermectin, loperamide, milbemycin, moxidectin, selamectin, vinblastine, vincristine and vinorelbine. A veterinarian can administer alternative dosages or treatments if a cat is known to be positive for the mutation.

All cats have two copies of the MDR1 gene. In most cats, the genes are normal, however, 4% have at least one mutated copy of the gene. Roughly 1% of all cats are homozygous for the mutation, meaning they have two copies of the mutated gene, and these animals are at serious risk of having an adverse reaction to the drug. Cats that are heterozygous, those with one normal and one mutated copy of the gene, would be predicted to experience milder adverse effects. Cats can pass the mutation to their offspring. 

Genetic testing is the only reliable way to determine if a cat has the defect. 

“While the mutation is rare, I recommend the test for all cats, preferably when they are kittens,” Mealey said. “It’s just better to know your pet is safe before it has an adverse reaction to one of these common drugs. If your veterinarian knows your pet has the MDR1 mutation, he or she can ensure only safe medications and doses are administered.” 

While multiple companies offer tests online, Mealey recommends reading the fine print and legal limitation language before ordering. She noted pet owners will find that many companies specifically state their results are not intended to be used for medical diagnosis or treatment. WSU’s test, which can be ordered for $70, is specifically intended to be used to help veterinarians determine safe drugs and safe doses for pets.

Tests are simple to complete and typically involve owners collecting a cheek swab sample from their cat at home and then mailing that sample to a testing lab.

When testing with WSU, owners usually receive test results within one week. Owners and their veterinarians also gain access to an online portal in which they will have access to a board-certified veterinary pharmacologist who can answer questions about what drugs or dose adjustments are necessary to safely treat cats with the MDR1 mutation.

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