Fasciolosis in Sheep: Recognising Clinical Presentations and Disease Progression

A sheep presenting with progressive weight loss, pale mucous membranes and submandibular oedema may have chronic fasciolosis. However, another animal from the same flock may die suddenly following extensive liver haemorrhage. Why can the same parasitic infection produce such different clinical presentations? 

The clinical picture of fasciolosis depends largely on the number and developmental stage of Fasciola hepatica present in the liver, as well as the extent of tissue damage. Recognising these differences is important for identifying affected sheep and distinguishing fasciolosis from other conditions with similar presentations. 

Acute Fasciolosis: When Liver Damage Develops Rapidly 

Acute fasciolosis is associated with the migration of large numbers of immature flukes through the liver over a short period. Their movement causes mechanical damage to hepatic tissue and can result in extensive haemorrhage. 

Affected sheep may die suddenly, sometimes without showing prominent clinical signs beforehand. Other animals in the flock may exhibit lethargy, pale mucous membranes, and dyspnoea. Rough handling of affected sheep can increase the risk of liver rupture and subsequent death^[1]. 

When sudden deaths occur, differential diagnoses include haemonchosis, clostridial infections, and acute pasteurellosis^[2]. Examining other animals in the flock may help identify additional cases. 

Subacute Fasciolosis: Recognising Progressive Ill-Thrift^[2] 

Unlike the sudden deterioration associated with acute disease, subacute fasciolosis often presents with gradual loss of body condition. 

Affected sheep may develop ill-thrift and pale mucous membranes. Haematological and biochemical findings can include: 

• Hypochromic macrocytic anaemia and reduced packed cell volume. 
• Hypoalbuminaemia. 
• Elevated liver enzymes, including AST, GGT and GLDH. 

At necropsy, numerous immature flukes may be present within the liver parenchyma, although adult flukes can also occur in the major bile ducts. 

Importantly, extensive hepatic lesions do not always correspond to obvious clinical illness. Lambs slaughtered during autumn may have considerable liver damage without displaying recognisable clinical signs. 

Chronic Fasciolosis: A Progressive Disease with Systemic Consequences^[2] 

Chronic fasciolosis is the most common manifestation of the disease complex. It develops as mature flukes inhabit the bile ducts and feed on blood, bile, lymph, and tissue. 

Progressive weight loss, poor body condition, and anaemia are characteristic findings. In severe cases, mucous membranes may become extremely pale. Submandibular oedema, commonly referred to as bottle jaw, is another important clinical feature. 

Laboratory findings may include hypoalbuminaemia, eosinophilia, and elevated liver enzymes. Fluke eggs may also be detected in faeces. 

Differential diagnoses include haemonchosis, malnutrition, cobalt deficiency, Johne's disease and scrapie. These conditions should be considered when investigating sheep with chronic wasting and anaemia. 

Chronic infection can also have serious consequences during late pregnancy, when affected sheep may die. Associated complications include black disease caused by Clostridium novyi, parasitic gastroenteritis, hypocalcaemia and pregnancy toxaemia. 

What Does Necropsy Reveal? 

Post-mortem examination can provide useful clues about the stage and severity of infection. 

• Acute disease: Haemorrhagic areas, liver rupture, blood clots, and free blood in the abdominal cavity may be evident. 
• Subacute disease: Pale necrotic areas, haemorrhagic migration tracks, and immature flukes may be found within the liver. 
• Chronic disease: Fibrotic scars, dilated bile ducts with thickened walls, and mature flukes in the bile ducts or gallbladder are characteristic findings. 

Histopathologically, chronic cases may show portal fibrosis, bile duct hyperplasia, and infiltration by mononuclear cells and eosinophils. 

Clinical Bottom Line 

Fasciolosis should remain a differential diagnosis when sheep present with sudden death, progressive ill-thrift, anaemia or submandibular oedema. The absence of prominent clinical signs does not necessarily exclude substantial hepatic damage. 

Recognising the differences between acute, subacute and chronic presentations helps veterinarians interpret clinical findings, identify affected flock members and plan further diagnostic evaluation.

References:

1. Corrales JL, Cwiklinski K, Verissimo CD, Dorey A, Lalor R, Jewhurst H, McEvoy A, Diskin M, Duffy C, Cosby SL, Keane OM. Diagnosis of sheep fasciolosis caused by Fasciola hepatica using cathepsin L enzyme-linked immunosorbent assays (ELISA). Veterinary parasitology. 2021 Oct 1;298:109517. https://www.sciencedirect.com/science/article/pii/S030440172100176X
2. Stuen S, Ersdal C. Fasciolosis—an increasing challenge in the sheep industry. Animals. 2022 Jun 8;12(12):1491. https://www.mdpi.com/2076-2615/12/12/1491