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Anemia: blood loss



  • Anemia is a sign of disease, not a diagnosis.
  • Blood loss anemia causes an absolute reduction in the number of red blood cells, it's usually accompanied by hypoproteinemia. It can be divided into external or internal, and acute or chronic.
  • Cause: overt or concealed hemorrhage. The most common cause of chronic blood loss anemia is probably parasitism; acute blood loss anemia can be a sequela to trauma, guttural pouch mycosis, rupture of the middle uterine artery close to parturition, or diffuse intravascular hemolysis.
  • If the cause is corrected, the bone marrow will usually respond quickly returning red cell numbers to normal within 2-6 weeks.
  • Signs: acute blood loss is associated with marked tachycardia, weakness, pale mucous membranes, colic and collapse. Chronic blood loss is associated with poor performance, weight loss, pale mucous membranes, lethargy and weakness.
  • Diagnosis: external blood loss is easily identified but quantifying the loss can be challenging. Internal blood loss may require ultrasonography, peritoneal/pleural fluid analysis or an exploratory laparotomy.
  • Treatment: in most cases blood loss anemia does not require treatment beyond control of cause of the blood loss, but in acute blood loss resuscitative fluid therapy may be required.
  • Loss of blood from the body over a long period may lead to iron deficiency anemia.
  • Prognosis: depends on ability to control underlying problem and prevent development of hemorrhage.



  • Acute (significant non-apparent bleeding) may occur into major cavity or other internal structure:
    • Bleeding into lungs (EIPH), thorax (rupture of aorta/pulmonary artery), abdomen (verminous arteritis, parturient rupture of middle uterine artery into broad ligament, broad ligament hematoma, ruptured liver or spleen) or retroperitoneal (renal artery rupture) or intestinal lumen (ulceration).
    • Idiopathic renal hematuria.
    • Femoral artery rupture in cases of femoral/pelvic fractures.
  • Chronic blood loss:
    • Overt bleeding from nose/respiratory tract:
      • EIPH.
      • Ethmoid hematoma.
      • Guttural pouch mycosis/bronchial myxogranuloma.
    • Overt bleeding from gut or kidney - may be obvious or may be detected microscopically/biochemically.
    • Occult bleeding seldom sufficient to result in anemia, but can be debilitating eventually with development of primary iron deficiency anemia:
      • Gastric/intestinal ulceration or neoplasia.
      • Idiopathic renal hemorrhage (may be overt from time to time).
      • Cystic or ureteral calculus.
  • Parasitic consumption of blood is in effect hemorrhage occurring into ecto- and endoparasites. The extent of blood loss that a horse is able to tolerate depends on the time period over which it is lost with horses with chronic and internal blood loss able to cope much better than those with acute external hemorrhage.

Predisposing factors

  • Intestinal parasites.
  • External parasites:Haematopinus asini, lice   Pediculosis  and ticks.
  • Guttural pouch mycosis   Guttural pouch: mycosis  .
  • Iatrogenic, eg castration.
  • Coagulation defects, eg thrombocytopenia or disseminated intravascular coagulopathy.
  • Hemophilia A (factor VIII deficiency) is a very rare sex-linked recessive occurring in Arabs   Arab  , TBs   Thoroughbred  and Standardbreds   Standardbred  .


  • Loss of red blood cells leads to anemia - acute or chronic.
  • Venous bleeding rarely causes significant anemia unless complicated by clotting abnormalities.
  • Arterial bleeding can rapidly lead to significant blood loss.
  • Acute or chronic loss of red blood cells in excess of the bone marrow's capacity to replace them.
  • Fall in red blood cell count, hematocrit and hemoglobin levels.
  • Chronic blood loss may lead to iron deficiency anemia.
  • Horses with internal hemorrhage are able to re-absorb some of the red blood cells and re-use the iron.


  • Horses with acute external blood loss can rapidly deteriorate (within hours), whereas horses with chronic blood loss may go many months before any rapid deterioration.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Harris M, Nolen-Walston R et al (2012) Effect of sample storage on blood crossmatching in horses. J Vet Intern Med 26 (3), 662-667 PubMed.
  • Hart K A (2008) Evaluation and management of anemia in the post-operative colic patient. Equine Vet Educ 20 (8), 427-432 VetMedResource.
  • Wilson E M, Holcombe S J et al (2009) Incidence of transfusion reactions and retention of procoagulant and anticoagulant factor activities in equine plasma. J Vet Intern Med 23 (2), 323-328 PubMed.
  • Jones P A, Bain F T, Byars T D et al (2001) Effect of hydroxyethyl starch infusion on colloid oncotic pressure in hypoproteinemic horses. JAVMA 208 (7), 1130-1135 PubMed.
  • Taylor E L, SEllon D C, Wardrop K J et al (2000) Effects of intravenous administration of formaldehyde on platelet and coagulation variables in healthy horses. Am J Vet Res 61 (10), 1191-1196 PubMed.
  • McCarthy P E, Hooper R N, Carter G K et al (1994) Postparturient hemorrhage in the mare: managing ruptured arteries of the broad ligament. Vet Med 45, 121 VetMedResource.

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