25 Jul 2016
Emma Holmes and Phil Elkins offer guidance on clinical manifestations of fasciolosis, followed by various treatment and control measures to minimise the impact of this parasite on UK farms.
Figure 1. Increased rainfall and environmental stewardship schemes favouring wetland development have contributed to the increase in habitats for the mud snail – the intermediate host for Fasciola hepatica.
The infamous liver fluke, Fasciola hepatica, is the most economically and clinically significant fluke parasite affecting cattle and sheep in the UK.
Traditionally, fasciolosis was more commonly diagnosed in the wetter areas of countryside; recent diagnoses are more widespread. Global warming, increased rainfall, warmer winters and longer summers, increased animal movements and, perhaps, environmental stewardship schemes favouring the development of wetlands, have all contributed to the increase in habitats for the mud snail (Galba truncatula) – the intermediate host for F hepatica (Figure 1).
Sustainable Control of Parasites in Sheep and Control of Worms Sustainably (sheep and cattle parasite stakeholder groups, respectively) recognise the importance of this parasite and produce valuable control guides (www.scops.org.uk and www.cattleparasites.org.uk).
An integrated approach is of upmost importance due to the distinct lack of new therapeutic approaches and the increasing emergence of flukicide resistance.
This article will review clinical manifestations – in particular, acute clinical disease and how to treat such cases. It will also discuss how treatment and control measures can be instigated to minimise the impact of this parasite on the UK farming industry.
Figure 2 illustrates the well-documented and complicated life cycle of F hepatica, involving an intermediate host, the mud snail (G truncatula).
Three clinical manifestations exist: acute, subacute and chronic fasciolosis. Disease severity varies depending on the species affected. Disease pathogenesis depends on timing, the number of metacercariae ingested and the period over which they were ingested.
Acute infection generally affects sheep and can have devastating consequences. Cattle are rarely affected, perhaps because they have a more robust liver structure. Infection occurs seasonally after a warm, wet period – typically autumn/early winter, where high numbers of metacercariae are present on pastures. Treatment protocols should target this immature stage. With milder winters and wetter summers, the parasite epidemiology is changing and, as such, we are seeing earlier reports of acute disease.
Sheep affected with acute disease present with a painful, distended abdomen, anaemia and sudden death, two to six weeks after ingestion of large numbers of metacercariae (usually greater than 2,000) over a short period (Merck, 2014). Overt disease follows the mass migration of immature fluke through the liver parenchyma to the bile ducts, causing fatal haemorrhage.
This acute syndrome can be further complicated with concurrent Clostridium novyi, causing infectious necrotic hepatitis, or “black disease”. This is now a lot less common due to clostridial vaccinations.
Sudden death in previously healthy sheep from August to October should sound alarm bells as it could be the first sign of a devastating flock problem. All animals should be urgently inspected and may reveal signs of lethargy and reduced grazing activity.
Rounding up a group of sheep may be problematic due to pain causing a reluctance to run – 10% of a flock may be affected with sudden death, yielding massive financial losses to the farmer (NADIS, 2016a).
Differentials to consider when presented with sudden death in sheep include:
Subacute and chronic disease commonly affects sheep and cattle. Subacute disease presents when large numbers (500 to 1,500) metacercariae are ingested over a longer time frame. Affected animals tend to survive longer (7 to 10 weeks) despite considerable liver damage. Chronic disease occurs following the ingestion of low to moderate numbers of metacercariae (200 to 500) over prolonged periods.
Presenting signs in sheep are weight loss and poor fleece quality, despite adequate nutrition. Some sheep have submandibular oedema (bottle jaw) due to hypoproteinaemia and are severely depressed, anorexic and possibly unable to stand. While rare, affected sheep may die in an emaciated condition – especially if complicated by metabolic demands of advanced pregnancy/early lactation.
These losses occur secondary to significant liver haemorrhage, commonly from November/December onwards, but may be earlier with severe challenge.
Differential diagnosis for poor condition of a group of sheep include:
In cattle, symptoms include:
Some are asymptomatic, despite being heavily infected. Immune suppression associated with fasciolosis may predispose to some clostridial diseases and some authors have reported interferences with intradermal tuberculin testing in cattle (Claridge et al, 2012).
A thorough history and clinical examination of individuals/groups of animals should be highly suggestive of fasciolosis.
Correct diagnosis is essential to limit production losses and to ensure appropriate treatments are used. The most recent fluke forecasts can be found in the NADIS parasite forecast (www.nadis.org.uk). Various diagnostic options (Panels 1-4) exist, with accuracy increasing as the parasite matures.
Multiple treatment options are available, with the product choice (Table 1) governed by:
In a nutshell, there is:
Animals should be moved to low-risk areas post-treatment to reduce the chance of reinfection as no residual activity exists with flukicide treatments.
When administering medicines, it is good practice to adhere to the five Rs (Panel 5).
Use a product based on the parasite life cycle that needs to be targeted and therapeutic objectives. Only use a combined worm/fluke preparation, if appropriate. Flukicides have no persistent effect necessitating repeat therapy.
Accurate dosing is paramount. Weigh or weigh-band individuals ensuring equipment is correctly calibrated. Underdosing has an inefficient kill rate, increasing the likelihood of resistance developing. Overdosing could result in toxicity – particularly with already damaged livers – and increase drug residue depletion time.
Always check withhold periods. This is particularly important in milk-producing animals due to current restrictions. Any suspect treatment failure should be reported and investigated by the market authorisation holder. Faecal egg count reduction tests (FECRTs) can be used to assess treatment efficacy.
Composite FECRTs have been validated for sheep (2 composites of 10 individuals, pre-treatment and post-treatment, 3 weeks apart; Daniel et al, 2012).
When presented with acute disease, the immature fluke is the target for treatment. Triclabendazole (TBZ) is the drug of choice, killing the immature fluke from two weeks of age. The main risk period is from mid-summer. Alternatives should be considered particularly in late winter/early spring, to reduce the emergence of resistance.
All stock should be treated. Animals should be moved to “low-risk”, snail-free pasture or housed after treatment. If this is not possible a repeat dose should be given three weeks later (Skuce, 2009). Further deaths can occur, despite treatment due to liver damage.
Handle affected individuals with care to prevent infected livers from rupturing. Some animals may require euthanasia on welfare grounds; others will need nutritional and hepatic support to aid recovery.
Antibiotics and NSAIDs should be considered if concurrent clostridial infections are suspected or for abdominal discomfort, respectively. Blood transfusions could be an option in severely anaemic and valuable animals.
When presented with an ill-thrifty animal affected with chronic disease in the winter/early spring, treatment should target late immature and adult fluke. Nutritional and hepatic support will also be required.
Many farmers use a strategic adulticide treatment for animals over winter or while housed to reduce pasture contamination in the spring/early summer. Various products are available.
A quarantine treatment of animals purchased from endemic areas is advisable before mixing with the rest of a flock/herd. These animals will need to be housed or isolated on carefully chosen pastures. TBZ should be avoided unless acute disease is confirmed, due to the unknown resistance status of the farm of origin.
Subsequently, a repeat treatment will be required at a later date (depending on product used) to allow juvenile parasites to reach a susceptible age.
Control measures include the following:
Pasture contamination and risk should be reduced and access limited to snail habitats by improving fencing and drainage, using an alternative water source and sensible grazing rotation to avoid peak.
F hepatica is here to stay. The complex life cycle of the parasite necessitates a flexible plan to be actioned on individual farms.
With no vaccine on the horizon, we need to constantly monitor on-farm risk factors and use parasite forecasting to employ therapeutics responsibly and effectively.
