Neutrophilia refers to an increase in the number of neutrophils in the blood. In many cases, neutrophilia is accompanied by the release of substantial numbers of immature neutrophils into the blood stream from the bone marrow. The nuclei of these Band cells are not segmented and they appear to be elongated bent tubes in the cytoplasm. The presence of band cells in the blood indicates that there is an intense need for neutrophils to maintain normal blood levels as they are lost in a purulent process somewhere in the body. When the percentage of the total neutrophil population that is contributed by these immature forms is high, the neutrophilia is described as having a regenerative left shift.
Neutrophilia: degenerative right shift
Very severe diseases that deplete the neutrophils may also exhaust the reserve supply of these cells in the bone marrow. In such situations, there is a neutropenia without band cells in the circulation and degenerate neutrophils are seen - this is known as a degenerative right shift. Degenerate neutrophils have a highly segmented nucleus, often in the shape of the 'Victoria Cross'. Some severe localised infections (including pyometras) may cause a neutrophilia of much greater magnitude than is usually observed. This so-called leukaemoid response may precede a right shift.
Causes of Neutrophilia
There are numerous causes of neutrophilia and it is the most common and least specific abnormal finding when blood samples undergo haematological analysis. Common causes include:
- Purulent foci and abscesses in the body can produce a substantial neutrophilia. Examples include pyometra, severe pyoderma and empyema (purulent inflammation of a body cavity). In severe cases of purulent inflammation, neutrophils may appear toxic when observed on cytological slides. These toxic cells have evident, blue-staining cytoplasmic granules and some cytoplasmic vacuolation. These cells have decreased bactericidal capability and do not respond as quickly to chemotactic stimuli. In contrast, abscesses that have become walled off may fail to induce the expected neutrophilia.
- Bacterial infection may result in neutrophilia as neutrophils are the major type of leucocyte that act to control this type of pathogen. Some bacteria, particularly Gram positive species, are particularly pyogenic due to their ability to resist intracellular killing by neutrophils.
- Necrosis of any tissue will result in neutrophilia. Common causes of tissue necrosis include burns, malignancies and infections.
- Fungal infection with, for example Aspergillus spp..
- Viral infection may cause a neutrophilia but neutrophils are often not the major type of inflammatory cell responding to the infection and they may be reacting to the threat of secondary bacterial infections rather than the primary viral pathogen. Examples include canine distemper, equine rhinotracheitis virus (EHV-1) and infectious bovine rhinotracheitis (IBR or BHV-1).
- Parasitic infection may cause a neutrophilia but neutrophils are often not the major type of inflammatory cell responding to the infection and eosinophilia is more likely. Examples include fasciolosis and toxoplasmosis.
- Immune-mediated disease frequently causes a neutrophilia, often with a concurrent monocytosis. Examples include rheumatoid arthritis, systemic lupus erythematosus and immune-mediated haemolytic anaemia (IMHA).
Miscellaneous causes of neutrophilia
Miscellaneous factors that can cause neutrophilia include:
- Excitement, fear and strenuous exercise in young animals.
- Corticosteroids as part of a stress leucogram with concurrent monocytosis, eosinopenia and lymphopenia. The corticosteroids may be endogenous (with pain, stress or hyperadrenocorticism) or exogenous with medical therapy for a variety of other diseases.
- Neutrophilia can occur in response to any of the causes of anaemia.