Hematological evaluation in monitoring health of fish

Hematologic evaluation can be useful in monitoring the Health status of the fish as long as interpretation accounts for intrinsic and extrinsic factors that can influence the appearance of cells and the quantitative values obtained.

  • Hematologic disorders are marked by aberrations in structure or function of the blood cells or the mechanisms of coagulation.
  • Although many other diseases may be reflected by the blood and its constituents the abnormalities of erythrocytes, leukocytes, thrombocytes, clotting factors are considered. Diseases of fish may result in anemia, leukopenia, Leukocytosis, thrombocytopenia

Normal erythrocytes

  • Erythrocytes of teleost fishes have similar appearance and ultra structure to those of other non mammalian vertebrates.
  • The cells are oval to elliptic in shape with abundant pale, eosinophilic cytoplasm and centrally positioned oval to elliptic nuclei, which is basophilic.
  • Elasmobranch erythrocytes are similar in appearance to those of teleosts but considerably larger.
  • A small amount of erythropoiesis occurs in the peripheral blood of both classes of fish, so it is common to find a small percentage of immature erythrocytes when examining a hemogram from a normal fish.
  • Immature erythrocytes tend to be more rounded than oval with a blue-tinted cytoplasm and larger, more heterochromatic nucleus, thus a higher nucleus to cytoplasm ratio.

Normal leukocytes

  • Lymphocytes are the most common and probably most variable leukocyte.
  • Small cells with basophilic nuclei and blue cytoplasm, the nucleus ratio is high.
  • They sometimes are rounded but often irregular in shape and often with ‘‘blebs’’ or blister-like Out pocketings of the outer membrane.
  • Reactive lymphocytes are larger and plasma cells do occur and can appear 2 to 3 times larger than regular.


  • The most common granulocyte of teleosts is the neutrophil.
  • These are rounded cells that have nuclei that may or may not be segmented and their cytoplasm varies from extremely pale to gray or slightly blue depending on the species.
  • The nucleus-cytoplasm ratio is low and cells tend to be similar to or larger than erythrocytes rarely if ever smaller.
  • Immature neutrophils occur in the circulating blood of teleosts.
  • In typical neutrophils and heterophils, the nucleus may be segmented or non segmented in mature cells.
  • Neutrophils also are found commonly in elasmobranch species.
  • The nuclei can be lobed or round. The cytoplasm is colorless with no visible granulation.


  • True eosinophils are less common in teleosts whereas basophils are rare.
  • The granules of eosinophils tend to be larger and more distinct than those of the neutrophil, heterophil series, round or rod-shaped and prominently eosinophilic.


  • Monocytes of teleosts and elasmobranchs are similar to those of other vertebrates.
  • Irregular in shape with an eccentric, large, heterochromatic nucleus  and  basophilic cytoplasm replete with vacuoles and other membrane-bound organelles.

Abnormalities of erythrocytes and leukocytes

  • Hematologic abnormalities involving the erythrocytes of fish include;
  • Polycythemia
  • Anemia
  • Abnormal morphology
  • Nuclear or cytoplasmic inclusions


  • Fish with a PCV of 45% or greater generally are considered to have a relative Polycythemia resulting from
  • Dehydration
  • Elevation in serum osmolality, total protein, sodium or chloride.
  • Hypoxia, in stressed fish resulting from release of catecholamine during splenic contraction and with erythrocyte swelling.


  • Anemias are well documented in fish. PCVs less than 20% .
  • There are three primary types of anemia:
  • Hemorrhagic (blood loss)
  • Hemolytic (erythrocyte destruction)
  • Hypoplastic (poor erythropoiesis).
  • The basic descriptive terminology used for anemias in other animals applies equally to fishes and may refer to cell size (microcytic,normocytic, or macrocytic)
  • Hemoglobin concentration (hypochromic or normochromic)
  • Cell loss (hemolytic or hemorrhagic)
  • Hemopoietic status (regenerative or non regenerative)

Causes of non regenerative anemia include;

  • Inflammatory disease
  • Nutritional disorders
  • Toxins
  • Renal or splenic disease

Immature cells may be present in response to environmental stressors such as;

  • hypoxia
  • toxins
  • temperature change

 Hemorrhagic anemia in fish may be associated with;

  • trauma
  • cutaneous ulceration
  • parasitism
  • nutritional deficiencies (vitamin K and B, inositol, and choline)
  • viral or bacterial septicemia

Non infectious diseases

  • Associated with husbandry practices and environment
  • Among husbandry-related diseases, a microcytic normochromic anemia can result from;
    • Environmental stress
    • Increases in population density.

Infection-induced abnormalities

  • Bacterial disease

Gram-negative bacteria such as Aeromonasspp and Pseudomonas spp, are common causes of septicemia and hemorrhagic anemia in freshwater fish.

  • hemorrhagic septicemia characterized by
  • Anemia,
  • Leukocytosis
  • Reticulocytosis
  • A severe hemorrhagic anemia caused by cold-water vibriosis (Hitra disease)
  • Salmon and Brook trout affected by columnaris disease (Flavobacterium columnare) developed macrocytic hypochromic anemia with evidence of fragmented erythrocytes noted on blood smears

Gram-positive septicemia caused by Streptococcus and can result in

  • Increases in granulocytes
  • Decreases in lymphocytes.
  • Septicemia caused by streptococcus

Viral diseases

  • In viral infection there is intra erythro cytic inclusions or profound hemorrhagic or hemolytic anemia
  • Erythro cytic necrosis virus (VEN) causes hemolytic anemia and is associated with intra cytoplasmic inclusions and nuclear changes in the erythrocyte of marine fish
  • Erythrocytic inclusion body syndrome virus also results in intracytoplasmic inclusion
  • Infectious hematopoietic necrosis virus (IHNV) infects a range of salmonid , wherein it obliterates the lymphoid tissues of the spleen and kidney
  • Viral hemorrhagic septicemia (VHSV) cause anemia.
  • Spring viremia of carp (SVC) is a disease  like VHSV, it causes hemorrhagic anemia
  • Channel catfish virus causes acute hemorrhage at the bases of the fins and in liver, kidney, and the gastrointestinal system

Parasitic diseases

  • Several parasites, are present in the blood of teleosts;
  • These may be intra erythrocytic or occur in the plasma.
  • Trypanosoma probably is the best-known extra erythrocytic parasite in fish.
  • In salmonids and may result in severe anemia because of destruction of hematopoietic tissue
  • These usually occur as small to larger cellular inclusions in the cytoplasm of erythrocytes


  • Occupying more than half of the cytoplasmic space and curving around the erythrocyte nucleus
  • They also may cause anemia by disruption of hematopoeisis.
  • Develop a chronic normocytic hypochromic anemia with distinct poikilocytosis

Authors: Aneela Amin*1, Aisha Khatoon1, Muhammad Bilal1, Zain- Ul- Abidin2, Sana Majeed1, Ashiq Ali1, Muhammad Jehanzaib3

1 Department of Pathology, Faculty of Veterinary Science, University of Agriculture  Faisalabad, Pakistan. 2 Veterinary Research Institute, Lahore, Pakistan, University of Veterinary and Animal Science, Lahore 

* Corresponding Author Email: anee4405@gmail.com