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Can Anti-Parasite Medication Be Dangerous to Some Dogs?


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Parasites are a common affliction in our dogs, and we take it for granted that providing anti-parasitic medications to control parasites is necessary to ensure their health. Many parasites that affect our dogs not only have serious effects on our dogs’ health and well being but can be life-threatening, such as heartworm, and require medication for treatment or prevention on a regular basis. Unfortunately, anti-parasitic medications have some side effects, and in certain dogs with impaired health or immune systems or in certain breeds that contain a specific gene which causes serious and life-threatening side effects to occur, dosage with anti-parasitic medication can be fatal.

Many anti-parasitics containing ivermectin, which is used to prevent heartworm, tapeworms, lice, demodectic and sarcoptic mange mites, and ear mites in dogs. Ivermectin works by disrupting the neurological processes in the parasites. This neurological action can cause serious side effects in some dogs.

Potential Effects

Although most dogs do not experience side effects or experience only mild side effects, such as gastrointestinal upset and lethargy, severe reactions in some dogs are possible. Severe reactions are more common in some breeds that carry a gene called MDR1, which is a multi-drug resistant gene. This gene affects the formation of a protein that provides for toxins and drugs to be flushed from brain tissues. The genetic mutation allows ivermectin, and some other medications, to cross the blood-brain barrier and cause severe neurological illness. If the gene mutation is present, toxins are not efficiently removed from the brain, which can result in serious neurological symptoms including drooling, lethargy, loss of appetite, disruptions in heart rate, difficulty breathing, seizures, disorientation, shock, hypothermia, nervous system depression, blindness, dilated pupils, loss of coordination, seizures, and sometimes death. Symptoms of ivermectin toxicity can occur within a few hours of exposure or may take a few days. The gene mutation is particularly prevalent in herding breeds. Breeds that are known to frequently carry this gene are Collies, Long Haired Whippets and Australian Shepherds. Also, Silken Windhounds, Skye Terriers, English Sheepdogs, German Shepherds, Shetland Sheepdogs, and Border Collies have been known to carry this genetic abnormality.

Determining Risk

Fortunately, the gene can be tested for, and it is recommended if you have a dog from an “at risk” breed that you consider testing for the gene before administering an anti-parasitic containing ivermectin. The test involves a simple DNA sample taken with a swab from your dog's mouth and sent for laboratory testing.

If a herding dog is identified as carrying the gene they should not be administered medications with ivermectin if they are exposed to livestock and its feces. However, the dog is still at risk from ingestion of horse or cattle feces containing ivermectin, which is commonly used in livestock. Precautions to prevent ingestion of livestock feces may be necessary.

Prevention and Precautions

It is also advised to test dogs for presence of parasites, such as heartworm, before administering medications to avoid unnecessary use of anti-parasitics. Use of heartworm medication is only necessary at certain times of the year, depending on your geographic location. Understanding the minimum effective medication routine required to prevent your dog from becoming infected with parasites can reduce the chance of your dog experiencing negative side effects unnecessarily. Also, if heartworms at the larval state are present, reactions to ivermectin are more common.

Besides minimizing use of anti-parasitics and checking for genetic mutations that predispose your dog to severe reactions, products that help to detoxify your dog's body after medications are administered and aid in the metabolism of medications are available. A veterinarian with holistic training can recommend several natural products that will support your dog's immune, neurological, and metabolic systems when medication is required.

As pet owners, protecting our dogs from serious conditions such as parasitic infestations is considered part of responsible pet ownership. To provide medication for your dog and have them experience a severe or life-threatening reaction is heartbreaking. Being aware of your dog's possible susceptibility to a reaction to anti-parasitic medications containing ivermectin is critical, to prevent such a reaction. Testing to determine if your dog carries the gene that could cause a severe reaction to ivermectin is recommended for at-risk breeds, and minimizing the effects of anti-parasitic and other medications by determining when medication is necessary and what is the minimum required amount to effectively protect your pet from parasites and disease is recommended for all dogs. Speak to your veterinarian if you have concerns about medications being administered to your dog, and possible adverse reactions, to ensure you have a good understanding of the risks and can take appropriate precautions to prevent adverse reactions.

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