I live in CO at 9000 ft so HW's are a non-issue. We visited family in TX for 2 wks - my mom gave me some heartgard + for my 12 yr old dog (prescription for her dog, same size & weight) W/in hours he became lethargic, couldn't seem to lift his tail, depressed, lost weight, etc. We took him to the vet & had a "weak" pos. on the HW test. Due to age we decided not to treat. 30 days after the HW pill he's doing great, back to normal. Vet says the pill had no side effects. Not sure what to do now.

  • Vetinfo

    By: Kara McCarty El Segundo, CA

    Replied on 04/19/2011

    Heartworm medications like Heartgard work by killing any microfilaria in the system at the time the medication is given. They don't have a residual effect so the medication given today kills any exposure from the previous month. Ideally the medication should have been given a week after you returned home to prevent heartworm microfilaria from developing into adults.

    It take approximately 6 months after exposure to a mosquito carrying heartworm for an adult heartworm to develop. The test done by veterinarians tests for adults so if he was a weak positive that means he was exposed at least 6 months previously which doesn't sound possible. Maybe he was a false positive. These do happen and in your case I would have him retested to make sure.

    What you are describing may be a reaction to ivermectin, which is the medication found in Heartgard. Toxicity is possible even at normal doses in dogs that are collie type breeds—Shetland Sheepdogs, Smooth and Rough Coated Collies, Border Collies and Bearded Collies or their mixes. Toxicity symptoms are depression, drooling, vomiting, diarrhea, and neurologic symptoms like ataxia, dilated pupils, tremors, blindness and very rarely coma and death.

    If you were in a heartwormendemic area I would also think about a reaction to dying heartworm microfilaria. Ivermectin is used to kill the microfilaria (baby heartworms) after the adults are killed. Some dogs have an allergic type shock reaction to the microfilariadying and will drool, vomit, and be very lethargic. They can die from the reaction but most are treated successfully with supportive care with fluids and steroids.

    Given your dog's history I don't think he has heartworm I think he is sensitive to ivermectin which should be noted on his medical chart. I would have another test done for heartworm but I suspect it will be negative.

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