1. Progressive anaemia was observed in 30 White Leghorn hens obtained from a poultry farm in England, where the ailment became noticeable through the yellowish-paleness of the combs and gills of the diseased hens and the heavy mortality following on the appearance of these symptoms.Only hens of one stock were affected, other breeds on the same farm did not develop the same or a similar condition.2. The anaemia was characterised by a lasting diminution in the normal number of erythrocytes, a Hb index less than 1, the preponderance of immature and deformed erythrocytes and the presence of marrow-cells of the erythrocyte series in the circulating blood. The leucocytes were normal or slightly below normal in numbers; the monocytes were diminished.3. The essential pathological lesion is an intravascular proliferation in the marrow of the stem cell of erythrocytes, which results in the blood-stream being overrun with erythroblasts and immature erythrocytes; these are rapidly destroyed in the spleen and liver, whilst the issue of normal erythrocytes in the marrow is hindered. The myeloblastic elements do not proliferate actively. These lesions explain the pathogenesis of the anaemia.4. This intensified activity of the bone-marrow, the resulting congestion and increased localised pressure, induce the production of osteoid tissue; the femur and other bones of the fowls thicken, so that the medullary canal is nearly obliterated.5. Attempts to detect microbes in the blood or isolate bacteria from the organs of anaemic hens failed. The spontaneously diseased hens all harbour a minute cestode,Davainea proglottina, in the duodenum, where the helminth causes round-celled infiltration, petechiae and thickening of the intestinal peritoneum.6. InDavainea-free fowls of the same and other breeds, the injection of blood and cellular emulsions of liver and spleen from erythromyelotic fowls caused a condition of hypercythaemia which lasted several months. This was related to congestion and hyperplasia of the red-blood-marrow and accompanied by considerable enlargement of the liver and spleen.7. Typical erythromyelosis was reproduced in one hen of the same stock from the same farm, by the intravenous inoculation of blood and tissue from spontaneously diseased fowls. In other hens only a transient type of anaemia was obtained, corresponding to the experience of other observers, more particularly Bedson and Knight (1924).The experimental results agree with the view that a proliferative micro-plasm is the specific cause of this condition.8. It is suggested that the development of this outbreak is induced by the massive infestation of the duodenum withDavainea proglottina. This irritative cestode produces a receptive condition in the bone-marrow of the fowl, which allows the microplasm to unfold its pathogenic properties. The duodenal parasite therefore plays the part of a preparatory cause. The association of cestode and microplasm affords an example of a “compound” disease.9. The microplasm is specific and multiplying solely in certain definite blood-marrow cells ofGallus domesticus. This proliferative action can be compared to that of the microbes of epitheliosis contagiosa and Rous fowl-sarcoma; the resulting cell-reaction is then similar to tumour formation in these and other tissues of the fowl.10. Since this haemopathy in fowls is characterised by specific pathological features in the erythrocyte-producing cells of the marrow, the name erythromyelosis is suggested, to mark clearly the position of the principal cellular lesion.11. This disease (erythromyelosis) has been distinguished from other ailments of fowls, in the course of which anaemia may also occur,e.g.avian tuberculosis with involvement of the marrow. The leucocythaemic type is differentiated by diagnosis of the predominant type of cell found in active multiplication in the blood and the position of the cell-proliferation in the marrow.12. Bone-marrow tumours and neoplasms of the lymphatic system occur in fowls, without causing progressive alterations in the cellular contents of the blood. The perivascular aggregations of cells in different organs may also hypertrophy, without any cytological changes being noticeable in the blood, except diminution of the numbers of erythrocytes. Such lesions are therefore distinguishable from different forms of leucocythaemia, where a persistent and progressive increase of genetically related cells is noticeable in the bloodstream and is concurrent with certain diffuse lesions of the marrow.13. The haemoblastic diseases of fowls, such as erythromyelosis, leueo-cythaemia, lymphocythaemia, differ from tumours of the blood-producing tissues, since the former are the result of a pathological disturbance of the mechanism of haemocytopoiesis, whilst the latter are the localised proliferations of cellular elements, which then may spread by a process identical to neoplastic metastasis. There is a distinction in mode and degree.14. The many resemblances between erythromyelosis and certain blood diseases in man and animals may indicate that their pathogenesis. though not identical, is comparable. A similar analogy may be applicable to fowl-leuco-cythaemia and lymphocythaemia.
