Anaemia is the condition in which there are fewer red blood cells than normal. Hemolytic anaemia describes the condition in which this loss of red blood cells occurs because the red cells break up (lyse).

This can happen inside the blood vessels (intravascular hemolysis) or outside of the blood vessel (extravascular hemolysis). Hemolytic anaemia can occur for many reasons, including heat stroke, parasites, viral infections. toxins and other conditions. One of the things that can cause hemolytic anaemia is immune system disease --- in this case the disorder is known as immune mediated hemolytic anaemia (IMHA). This is the term currently preferred for the condition formerly known as auto-immune hemolytic anaemia (AIHA). The reason for the subtle name change is the recognition that much of the time the immune system really is attacking an invader -- it just happens to be one that is stuck on the red blood cell so both get destroyed.

Anaemia occurs for a number or reasons but they break down into two major categories. Either blood is not being made in the first place (nonregenerative anaemia) or else it is being destroyed in some manner even though it is being made by the body. If the destruction outpaces the production, anaemia occurs.

Kidney failure, bone marrow disorders, some cancers, some toxins, inherited disorders, ehrlichiosis, drug reactions and iron deficiency can all lead to non-regenerative anaemia. Liver failure, heat stroke, iron deficiency, trauma, immune mediated hemolytic anaemia, drug reactions, babesiosis, hemobartenollosis, inherited hemolytic anaemia, intestinal parasites (hookworms, esp.) and bleeding disorders (such as von Willebrand's disease) can all lead to regenerative anaemia. Even severe flea infestation can cause significant anaemia in young dogs.