Dogs with HCM cannot absorb vitamin B12 in the gut. Cobalamin malabsorbtion can be secondary to (caused by) other diseases but the primary form is hereditary. Symptoms are highly varied and can mimic many other diseases including exocrine pancreatic insufficiency, porto-systemic (liver) shunt, and early-onset intractable epilepsy. Severity of the disease is also highly variable; in the worst cases it can be fatal. Treatment is relatively simple and inexpensive: Periodic Vitamin B12 shots.
HCM is caused by a single gene and is recessive. If a dog has HCM both of its parents carry the mutation that causes it. Dogs affected with HCM should not be bred. An HCM mutation specific to Aussies has been identified but it is possible that other HCM-causing mutation in this same gene may also be present. There is a DNA test available so siblings of the affected dog and the grandparents should be tested if they are to be bred. Those which have the mutation and the parents of affected dogs, which must have it, should be bred only to clear-tested dogs. If a dog is found to have HCM or carry one of the mutations, its first-step (parents, offspring, and full siblings) relatives which will be used for breeding need to be tested. Those that are carriers should be bred to clear-tested mates.