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Monocytosis refers to an increase in the number of monocytes (haematogenous macrophages) in the blood. It may be found in conjunction with other changes indicative of a stress leucogram or it may occur independently of other changes to the haematological profile. The blood monocytes themselves represent a brief transitional stage as they quickly move into tissues and differentiate further to tissue macrophages. As such, monocytes rarely perform any notable functions but they been found to phagocytose red blood cells in cases of immune-mediated haemolytic anaemia. The major causes of monocytosis are:

  • Stress leucogram mediated by the production or administration of glucocorticoids. This phenomenon is also characterised by neutrophilia, eosinopenia and lymphopenia.
  • Chronic inflammatory processes, including chronic bacterial infections and chronic inflammatory diseases of the liver (chronic hepatitis and cholangitis) and pancreas (chronic pancreatitis). Serum fibrinogen may also be elevated in such chronic processes and there may be a variable neutrophilia depending on the level of active inflammation.
  • Granulomatous disease results in monocytosis as monocytes are recruited to contain certain types of bacteria (namely Mycobacteria spp., Nocardia spp., Actinomyces and Rhodococcus equi) or foreign objects. Severe granulomatous disease may also result in hypercalaemia as differentiated macrophages acquire the ability to produce activated vitamin D metabolites.
  • Certain types of monocytic or myelomonocytic chronic myeloid leukaemia may result in the presence of large numbers of monocytes in the blood.

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