Avermectin Toxicosis

Petra A. Volmer, DVM, MS, DABVT, DABT

BASIC INFORMATION coma, and death. Seizures may also occur. Signs may persist for Description days or even weeks in severe cases. Avermectins are a group of antiparasite agents (parasiticides) used Diagnostic Tests in the prevention of heartworm (Dirofilaria immitis ) disease and in the treatment of infestations by mites such as scabies (sarcoptic Diagnosis is based on a history of exposure to an avermectin-con- mange), demodicosis (Demodex mites), and ear mites ( Otodectes taining product and consistent clinical signs. Breed predisposition mites). Certain members of the avermectins can also be found in also increases the likelihood of the diagnosis. There are no rapid ant and roach baits. Some common avermectins are ivermectin tests for the avermectins that can be done in the veterinary clinic. (Heartgard, Ivomec, Tri-Heart), selamectin (Revolution ), mox- Analyses may be performed on various body tissues by outside idectin (Proheart ), milbemycin ( Interceptor ), and abamectin laboratories. Laboratory and other tests may be recommended to (Avomec, Raid, Hot Shot MaxAttrax ). rule out other causes of similar clinical signs. A genetic test for the Causes and Toxicity MDR1 gene mutation is available on a cheek swab sample from a The avermectins exert their effect by mimicking inhibitory neu- dog with suspected P-glycoprotein deficiency. rotransmitters in the central nervous system (brain and spinal cord), which results in depression of brain activity and death of TREATMENT AND FOLLOW-UP the parasite. Avermectins can have the same effect in animals. Most animals are quite resistant to the effects of these compounds, Treatment Options because they do not accumulate in the brains of animals. However, Your veterinarian may induce vomiting in the animal if an aver- dogs with a deficiency in P-glycoprotein, an enzyme that helps mectin-containing product has been ingested and the pet is not remove avermectins from the brain, are exquisitely sensitive to exhibiting clinical signs. This is followed by administration of these compounds. The avermectins accumulate and exert their activated charcoal to decrease absorption of the compound into inhibitory effect on the brain, and this action is reflected by the the body. Administration of activated charcoal also hastens elimi- clinical signs. nation of the avermectin. In general, certain herding breeds of dogs are more likely If the dog is exhibiting clinical signs, treatment is directed at to have the MRD1 genetic mutation that causes a deficiency in controlling those signs and may include assisting breathing (plac- P-glycoprotein. Such breeds include the collie, Australian shep- ing the animal on a ventilator), administering intravenous fluids, herd, Shetland sheepdog, and border collie. Individuals of other maintaining body temperature, and providing nutrition. In severe breeds, such as the Old English sheepdog, German shepherd dog, cases, the animal may be in a coma-like state for weeks and will long-haired whippet, and a variety of mixed-breed dogs, may also require good nursing care to prevent the formation of bedsores. have the MRD1 mutation and exhibit sensitivity to avermectins. Most cases of avermectin toxicosis have resulted from admin- Follow-up Care istration of high doses of the drug (often of the oral livestock prod- The status of nervous and heart systems must be monitored con- uct) or from ingestion of horse dewormer paste by farm dogs. In tinuously while the animal is showing clinical signs. Frequent some cases, the avermectin compound was injected subcutane- follow-up visits may be needed after your pet is discharged to ously as a treatment for mange. monitor its progress and to ensure that it continues to recover with- out complications. Clinical Signs Prognosis Signs can occur within several hours after exposure. Initial signs A complete recovery is expected in animals receiving prompt include lethargy, pupil dilation, and staggered gait. These signs and aggressive treatment, especially if the clinical signs are mild. may be followed by apparent blindness (which is often reversible Although signs may persist for prolonged periods (days to weeks), with time), tremors, decreased body temperature, slow heart rate, recovery can occur in many cases with appropriate treatment.