Fasciolosis in Sheep: How Liver Damage Affects Productivity and Reproductive Performance

A sheep flock may experience declining productivity even when only a few animals show obvious signs of liver fluke infection. Reduced growth, poor body condition, impaired fertility and lower milk production can develop as part of the disease burden. These losses are particularly important because subclinical infections may go unnoticed while continuing to affect flock performance. 

Infection with Fasciola hepatica affects more than liver function. The damage caused by migrating immature flukes and established adult parasites can have wider consequences for animal health, productivity and welfare^[1],[2]. 

How Does Liver Fluke Infection Damage the Liver? 

The severity of hepatic damage depends on the parasite's developmental stage and migration through the liver. 

Immature flukes penetrate the liver capsule and migrate through the hepatic parenchyma. Their movement mechanically damages tissue, producing haemorrhagic and necrotic areas. The liver responds through tissue regeneration and inflammatory cell infiltration, particularly involving mononuclear cells and eosinophils. 

As infection progresses, the developing inflammatory response and subsequent tissue repair contribute to fibrosis. Once mature flukes reach the bile ducts, they feed on blood, bile, lymph and tissue. Their feeding activity, together with damage to the biliary structures, contributes to chronic hepatic changes^[3]. 

The fluke tegument also releases immunomodulatory molecules that help the parasite evade host immune responses and persist during infection^[4]. 

Why Does Fasciolosis Affect Productivity?^[2] 

The consequences of liver damage extend beyond the affected organ. Blood loss and reduced protein levels contribute to anaemia and hypoproteinaemia, affecting the animal's overall condition. 

The economic impact is particularly significant in subclinical infections, where production losses may occur without obvious clinical illness. 

Important productivity-related consequences include: 

• Reduced growth: Infected sheep may experience poor growth rates and progressive loss of body condition. 
• Reduced milk production: Liver fluke infection has been associated with decreased milk yield. 
• Impaired fertility: Reduced reproductive performance is another recognised consequence of fasciolosis. 
• Compromised general health: Anaemia and protein loss can contribute to reduced physiological resilience and poorer overall welfare. 

These effects make it important to consider fasciolosis when investigating unexplained reductions in flock productivity, even when characteristic clinical signs are absent. 

Reproductive and Associated Health Complications^[2] 

Fasciolosis can be particularly concerning around lambing. Infected sheep may be more vulnerable to metabolic complications, including hypocalcaemia and pregnancy toxaemia. The infection has also been associated with black disease caused by Clostridium novyi and parasitic gastroenteritis. 

Additionally, liver fluke infection is recognised as a predisposing risk factor for mastitis. Inflammatory mediators released in response to liver damage may also affect early pregnancy. 

These associations highlight the importance of considering liver fluke infection when evaluating reproductive problems and other health complications within affected flocks. 

Clinical Bottom Line 

The impact of fasciolosis should not be assessed solely by the number of clinically affected sheep. Subclinical infections can contribute substantially to economic losses through reduced growth, milk production, and fertility. 

For veterinarians, recognising the relationship between hepatic damage, systemic effects, and declining flock productivity is important when investigating unexplained production losses and developing comprehensive flock health management plans.

References:

1. Mazeri S, Rydevik G, Handel I, Bronsvoort BM, Sargison N. Estimation of the impact of Fasciola hepatica infection on time taken for UK beef cattle to reach slaughter weight. Scientific reports. 2017 Aug 4;7(1):7319. https://www.nature.com/articles/s41598-017-07396-1.pdf
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
3. Lalor R, Cwiklinski K, Calvani NE, Dorey A, Hamon S, Corrales JL, Dalton JP, De Marco Verissimo C. Pathogenicity and virulence of the liver flukes Fasciola hepatica and Fasciola gigantica that cause the zoonosis Fasciolosis. Virulence. 2021 Dec 31;12(1):2839-67. https://www.tandfonline.com/doi/pdf/10.1080/21505594.2021.1996520
4. Ryan S, Shiels J, Taggart CC, Dalton JP, Weldon S. Fasciola hepatica-derived molecules as regulators of the host immune response. Frontiers in immunology. 2020 Sep 2;11:2182. https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2020.02182/pdf