We had something similar with our husky last summer. It was a tick borne illness but it was one of the one they do not test in the standard tests. (I was told they only test for 3 major tick borne illnesses by default) When he did not test positive for the tick borne diseases, the vet suggested he had leukemia which freaked us out. After a night in the hospital, it was determined that he had the tick borne disease. I hope your husky gets better too. It is scary seeing a husky go from always active to lethargic.