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→Migration and Pathology
*'''Black disease (Infectious Necrotic Hepatitis)''' - is caused by a toxin produced by ''Clostridium novyi'' type B. It is commonly associated with liver fluke infestation because:
*'''Black disease (Infectious Necrotic Hepatitis)''' - is caused by a toxin produced by ''Clostridium novyi'' type B. It is commonly associated with liver fluke infestation because:
**migrating flukes → liver necrosis → anaerobic conditions → clostridial multiplication → toxin production → disease.
**migrating flukes → liver necrosis → anaerobic conditions → clostridial multiplication → toxin production → disease.
=== Adults in Bile Ducts ===
*'''Bile duct damage''' - adult flukes (2-5cms long) in bile ducts feed on epithelium and blood; chronic inflammatory responses → fibrosis of bile duct wall (and, in cattle, calcification).
*'''Bile duct damage''' - adult flukes feed on biliary epithelium (gamma glutamyl transpeptidase released by damaged cells) and blood, leading to ulceration and haemorrhage, which causes epithelial hyperplasia and increased mucosal permeability.
*'''Anaemia''' - 250 flukes can cause up to 50ml of blood loss daily → 10 times increase in rate of erythropoiesis → normochronic anaemia until iron stores are exhausted → hypochromic anaemia.
*'''Hypoalbuminaemia''' - anaemia (and other plasma proteins) lost into bile duct because of:
a) whole blood loss, and
b) increased epithelial permeability
→ catabolic rate incereased by 2.5times → increased nitrogen loss via urine.
*Depending on magnitude of nitrogen loss:
a) no obvious effect (although animal is still in abnormal physiological state)
b) reduced weight-gains and/or wool growth and/or milk production
c) loss of body tissue (i.e. weight-loss); hypoalbuminaemia → reduced plasma oncotic pressure → oedema.
=== Disease Manifestations ===
*Large numbers of metacercariae ingested over a short period of time leads to acute disease.
*Smaller numbers ingested over a longer period of time leads to chronic disease.
*'''Feed intake''' - fluke infections causes reduced food intake. Note: chronic fasciolosis occurs at a time of year when animals are on a low plane of nutrition. This combined with the reduced food intake causes significant effect on the development and severity of clinical and subclinical fasciolosis.
*'''Species susceptibility''' - the proportion of flukes that reach the bile ducts is determined mainly by the fibroplastic potential of the liver and the effectiveness of the protective immune responses (which are ineffective in sheep). Therefore, establishment rate: '''sheep>cattle>pig'''.