Haemorrhagic Disease Pathophysiology

Introduction

Haemorrhagic diseases can be categorised into two different causal mechanisms:

• Increased vessel fragility which leads to a non-thrombocytopenic purpura.
• Inadequate haemostatic response. This may be caused by platelet dysfunction or deficiency, which leads to a primary or secondary thrombocytopenic purpura, or deficiencies/derangements of clotting factors.

Syndromes caused by vascular fragility and platelet dysfunction tend to be purpuric and acquired. Syndromes caused by clotting factor defects tend to cause more severe bleeding and are usually congenital.

Vascular Fragility

There are several ways that vascular fragility may arise. Capillaries can be damaged by toxins - this occurs in severe bacterial infections in all species. Streptococcus and Pasteurella infections and anthrax would be good examples of organisms that feature capillary damage as a symtom of infection.

Drug-induced damage to the vascular system does occur, although the mechanisms of damage induced by drugs are not clearly understood. Examples include heavy metals, e.g. lead, bismuth, mercury, iodides, fluorides, chlorinated hydrocarbon pesticides and salicylates.

Vascular fragility can occur as a consequence of an underlying disease state, which is seen in allergy or anaphylactic reactions, connective tissue diseases such as Ehlers-Danlos syndrome in dogs, Diabetes mellitus and antibody-antigen complex reactions.