Acute myelomonocytic leukaemia (AMML) in cats

© Mylonakis, ME et al (2008) AVJ 86(6): 224-228

Acute myeloblastic leukaemia without (M1) or with (M2) maturation and erythroleukaemia (M6) are considered the most common variants of myeloid dysplasia recognised in cats. In contrast to this disease in dogs, acute myelomonocytic leukaemia (AMML) is rare in cats. The majority of cats with myeloid neoplasia are young adults and are usually seropositive to feline leukaemia virus ( FeLV) and/or feline immunodeficiency virus ( FIV).

The prognosis for acute myeloid neoplasia is uniformly poor because it progresses rapidly, leading to life-threatening peripheral blood cytopenias as a result of the profound bone marrow effacement by the neoplastic cells. Cytomorphological evaluation of peripheral blood and bone marrow aspiration smears is often inconclusive, thus making cell-line specific cytochemical staining and/or immunophenotyping invaluable diagnostic modalities.

A new WHO proposed classification guideline applied to humans has reduced the minimum level of blast cells in bone marrow for the diagnosis of acute myeloid leukaemias from 30% - 20% of all nucleated cells, in recognition of the fact that patients with 20% - 30% blast cells have a prognosis similar to that of patients with more than 30% blasts. The latter guideline may be applicable to leukaemic animals. The majority of blast cells. in both bone marrow and peripheral blood, apparently originate from the monocytic lineage because they express lysozyme.

Leukemoid reactions may occasionally resemble true leukaemias. In cats, there is usually evidence of an inflammatory response with left shift and toxic changes, or Mycoplasma haemofelis or Ehrlichia spp infections, which have been associated with this condition.

Long lasting clinical and laboratory remission from myelodysplastic syndrome or chronic myelomonocytic leukaemia associated with apparent elimination of FeLV antigenaemia has been reported occasionally. The largely unexpected clinical improvement highlights the possibility that cats with acute AMML, even when infected with FeLV, may rarely exhibit a spontaneous although transient clinical recovery.

Treatment

Chemotherapeutic agents such as cytosine arabinoside, adriamycin, cyclophosphamide, prednisolone and vincristine, singly or in combination, have been unsuccessfully employed in a few cases of acute myeloid neoplasia in cats, with survival times ranging from 3 to 10 weeks.