CD4 and Opportunistic infection: Euthanasia was considered to be AIDS related if the animal exhibited peripheral blood CD4+ T-cell depletion (<200/mm3), greater than 25% loss of body weight and combinations of the following conditions: diarrhea unresponsive to treatment, opportunistic infections, peripheral lymph node atrophy, and abnormal hematology (e.g., anemia, thrombocytopenia, or leukopenia).