Ivermectin is a medication which is used in dogs to prevent heartworm infection and treat various parasitic infections, including ear mites, demodectic mange and sarcoptic mange. In addition, ivermectin is used in dogs that already have heartworm infections to kill the larval form of heartworms that are found in the blood stream (microfilaria).
Ivermectin is found in various formulations, including Heartgard Plus® and similar heartworm preventive medications, Acarexx® and Ivomec®.
Most Common Side Effects of Ivermectin in Dogs
In most dogs, ivermectin is a safe product with relatively few side effects. When side effects do occur, the most commonly seen symptoms are lack of appetite, lethargy, vomiting and diarrhea. These symptoms often resolve without medical intervention within a few days.
Any dog that is to receive ivermectin should be tested for heartworms prior to administration. In dogs that have microfilaria (a larval form of heartworms) in their blood, ivermectin administration can cause a serious shock-like syndrome that can be fatal. Symptoms seen with this type of reaction include depression, vomiting and hypothermia (a low body temperature).
In some dogs, ivermectin can also affect the central nervous system, leading to side effects such as incoordination and seizures. This reaction can be fatal as well.
If your dog has been given ivermectin and you believe he is suffering side effects from the medication, you should contact your veterinarian immediately.
MDR1 Gene Defect and Ivermectin Sensitivity in Dogs
The dogs most likely to experience serious side effects with ivermectin are those dogs that have a genetic gene mutation known as the deletion mutation of the MDR1 (multi-drug resistant) gene. Dogs most likely to be affected by this gene mutation are those of the collie breeds. Specific breeds known to affected are:
- Collies
- Australian Shepherds
- Shelties
- Long-haired Whippets
The MDR1 gene mutation can be found in other breeds as well as in mixed breed dogs also.
There is a test that can identify dogs with the MDR1 gene defect. The test is available through the Washington State University Clinical Pharmacology Laboratory and can be performed on either a blood sample or a cheek swab sample.
The symptoms most likely to be seen with dogs that are sensitive to ivermectin due to the MDR1 gene defect are those involving the central nervous system. These symptoms include incoordination, dilated pupils, seizures and coma.
The signs are dose related. In most cases, the low dosages of ivermectin present in heartworm preventive medications such as Heartgard Plus® are safe. However, higher dosages of ivermectin are often used to treat other conditions and these higher dosages may be problematic. Many veterinarians prefer to test for the gene mutation in those breeds that are most susceptible before administering these higher dosages.
Dogs receiving higher dosages of ivermectin, such as those used to treat demodectic mange or sarcoptic mange or to kill microfilaria, should be observed closely for at least eight hours after administration of the medication. Dogs exhibiting signs of side effects within this period should be seen immediately by a veterinarian.
Other Drugs to Use Cautiously with Ivermectin in Dogs
There are several drugs that can potentiate the side effects of ivermectin and make complications even more severe, especially in dogs with the MDR1 gene defect. These drugs include:
- benzodiazepams
- amiodarone
- carvedilol
- clarithromycin
- cyclosporine
- diltiazem
- erythromycin
- itraconazole
- ketoconazole
- quinidine
- spironolactone
- tamoxifen
- verapamil
- amitraz
- Comfortis®
Ivermectin is commonly used in dogs to treat many different conditions and is usually quite safe. However, side effects can occur and some dogs may suffer a gene mutation that makes them even more sensitive to the side effects of ivermectin.
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