Heartworm meds in dog that has had MDR1 reaction

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Barked: Thu Feb 21, '13 10:10pm PST 
Help! Found out the hard way that my foster catahoula has the MDR1 gene. After a week in ICU at Kansas State, he is a healthy 5 mo old terror! I can't find any consistent info on what to use for heartworm prevention. Since he's had the reaction, does that make him even more susceptible? He is going for a neuter next week & I'm already freaking about that. My vets are aware of the condition but its still scary. Every vet tells me that the dose in Heartguard is low enough- 68 mcg. His dose of Ivomec that I gave him was 50,000 mcg. But I'm having a hard time giving him anything that's a poison to him in any dose. Does anyone have experience with this?