Fasciolosis

What it is

Fasciolosis is infection with Fasciola hepatica, the liver fluke. Its life cycle requires a freshwater snail of the family Lymnaeidae (in Europe, mainly Galba truncatula) as intermediate host:

1. Eggs are passed in the faeces and, in water, release a miracidium that penetrates the snail.
2. It multiplies in the snail, and cercariae emerge and encyst on the grass as metacercariae, the infective stage.
3. Cattle ingest metacercariae while grazing; the larvae excyst in the intestine, cross the intestinal wall and the peritoneal cavity, and penetrate the liver through its capsule.
4. Immature flukes migrate through the parenchyma for several weeks, feeding on tissue and leaving haemorrhagic and necrotic tracts, and then enter the bile ducts, where they mature, feed on blood and lay eggs.

In cattle the chronic form predominates: adults in the bile ducts cause cholangitis, hyperplasia and fibrosis of the duct wall and, characteristically in this species, calcification of the bile ducts. Hepatic fibrosis and loss of blood and protein cause anaemia and hypoalbuminaemia in heavy infections. Acute fasciolosis, from massive migration of immature flukes, is much more typical of sheep than of cattle. Migration tracts favour necrotic hepatitis caused by Clostridium novyi. F. hepatica also infects humans, mainly through raw aquatic plants such as watercress.

Who it affects

Cattle of any age and production type grazing wet areas where the snail lives: waterlogged pastures, stream banks, irrigated land. Transmission depends on temperature and moisture, so it is seasonal and varies between regions and years. Young animals in their first grazing season suffer the greatest effect on growth. In tropical regions Fasciola gigantica causes a similar disease.

Anatomy involved

• Liver: in cattle it lies entirely to the right of the median plane, under the ribs. Flukes penetrate through the capsule and migrate through the parenchyma; chronic lesions are seen on cut section as thickened, whitish, calcified bile ducts that grate when cut, with surrounding fibrosis.
• Bile ducts and gall bladder: adults live in the bile ducts and eggs reach the intestine with the bile through the common bile duct; this is why looking for eggs in the faeces or in gall bladder bile is useful for diagnosis.

Clinical signs

Most infections are subclinical: lower weight gain, poorer feed conversion, a possible drop in milk yield and condemned livers at slaughter. Heavy infections cause weight loss, anaemia with pale mucous membranes, submandibular oedema from hypoalbuminaemia and, sometimes, diarrhoea, especially when gastrointestinal nematodes are also present.

Diagnosis

• Faecal sedimentation to look for the operculate, golden-yellow eggs. It is only positive once egg-laying adults are present, shedding is irregular and its sensitivity is low, especially with small amounts of faeces. The eggs must be distinguished from those of paramphistomes (rumen flukes), which are colourless.
• Coproantigen ELISA on faeces: detects antigens from adult flukes.
• Antibody ELISA on serum or milk: detects exposure earlier than coprology, but antibodies persist for months after treatment. Bulk tank milk ELISA is used to monitor the herd's level of infection.
• Liver inspection at slaughter: provides information on the herd of origin.
• Biochemistry: GGT rises with bile duct damage.

Treatment and prevention

• Flukicides: efficacy varies by drug and by parasite stage. Triclabendazole is the only one that also acts against early immature flukes; others (closantel, nitroxynil, clorsulon, oxyclozanide, albendazole) act mainly against later stages or adults. Many flukicides are not authorised for lactating cows or have long milk withdrawal periods, so in dairy cows treatment is usually given at drying off.
• Resistance: resistance to triclabendazole is widespread in some regions; treatment efficacy should be checked (fall in egg counts or coproantigen after treatment).
• Integrated control: fence off or drain wet areas with snails, avoid grazing them in the highest-risk seasons, treat and quarantine purchased animals, and plan treatments according to climatic risk and abattoir feedback.
• Drug use in food animals is subject to each country's regulations and to withdrawal periods for milk and meat.

Key studies

• Hayward et al. (2021), meta-analysis of 233 comparisons between infected and uninfected sheep and cattle. Infection reduced daily weight gain by 9%, live weight by 6% and carcass weight by 0.6%, with greater effects in young animals; it found no statistically supported effect on milk production or on total weight gain. It mixes species and very heterogeneous studies. The milk loss that is usually taken for granted rests mainly on observational studies and on individual trials, not on a pooled estimate.
• Charlier et al. (2012), blinded, randomised, placebo-controlled trial in 11 exposed dairy herds (402 animals): closantel at drying off increased 305-day yield by 303 kg, with a peak 1.06 kg higher and 9% greater persistence, with no change in fat or protein. The response was greater in first-lactation animals and smaller in thin cows. It was co-funded by the drug's manufacturer.
• Takeuchi-Storm et al. (2021), observational study of 218 Danish organic dairy farms. A positive bulk tank milk ELISA was associated with 580.5 kg less 305-day energy-corrected milk, and a positive individual ELISA with 919.5 kg less in cows in their third or later lactation. The bulk tank ELISA value was related to within-herd prevalence, which can be classed as negative, low, medium or high. Being observational, it does not prove that the fluke causes the loss.
• Charlier et al. (2008), diagnostic study in 200 slaughtered cows, with fluke counts in the liver as the reference. Sensitivity and specificity were 43% and 100% for sedimentation of 4 g of faeces, 64% and 93% with 10 g, 94% and 93% for the coproantigen ELISA, and 87% and 90% and 88% and 84% for the two serum antibody ELISAs. A positive 4 g sedimentation made a heavy infection (more than 10 flukes) at least 11 times more likely.
• Kelley et al. (2016), narrative review of triclabendazole resistance; it is the usual reference on the subject. It notes that triclabendazole is the only drug that kills early immature and adult flukes, that resistance is widespread in livestock and that its mechanism is unknown, so it recommends integrated control that does not rely on drugs alone.