There is no reason why your Australian Shepherds cannot take Heartgard or Iverhart to treat heartworm disease. The only sensitivities reported in this breed are at much higher doses when treated for other conditions like mange. Long term normal use for the prevention of heartworm disease does not affect this breed. The only breed that can be affected adversely with long term use are collies because a percentage of them have a DNA mutation. This is not the case in Aussie's. However, if you do not feel comfortable with using this form of prevention there is no reason why you cannot switch your dogs to Interceptor. If your dogs have been taking this preventive without side effects then it is unlikely that they will have any issues with it in the future.
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