I have an immune compromised dog. He is 7.5 yrs old and had his spleen removed in Oct 07, followed with a year of anti-rejection drug. We would like to travel with him, but the area we're planning on visiting has a high incidents of heartworm. I have been advised by 2 Vets that medicine for heartworm is not an option, as well as any topical repellent. I asked about using a herbal substitute and was told that was not an option either. Can you help me?

dog,compromised immune system. What can  use for heartworm- flea prevention,wnat to travel with him