By Deb Maxwell, ParaBoss Executive Officer

February 2018

 

Scarring and/or the presence of liver flukes in livers at slaughter can result in them being condemned.  In abattoirs sourcing lambs from a higher rainfall region in eastern Australia, it is not unusual for 20–30% of livers to be condemned. This results in a significant financial loss.

A review into on-farm practices that could decrease this incidence of pathology reveals that only grazing management successfully prevents or reduces infection, however, it imposes serious restrictions on producers.

Chemical treatment is an important tool in decreasing the ill-effects of liver fluke on sheep (such as anaemia), but may not prevent liver scarring or the presence of flukes at slaughter. 

Lambs would need to be prevented from grazing liver fluke-infested paddocks for all, or a large part, of their short lives, because long Export Slaughter Intervals for tremacides preclude the use of these products at appropriate times before slaughter. But even with their use, liver scarring from a treated infection may still result in condemnation of livers.

Infection by liver fluke and their effects

Firstly, liver fluke do not occur everywhere across Australia; there are none in Western Australia. While they have been reported in South Australia, they are not in the sheep producing regions. In Queensland, New South Wales, Victoria and Tasmania they are present in the tableland sheep production areas. However, they also occur along the east coast of Australia where sheep are typically not run. Here they affect cattle and goats.

Liver fluke are generally restricted to higher rainfall regions and in paddocks where there is a suitable ecosystem: slow moving springs, streams and channels.

Liver fluke undergo a part of their lifecycle in freshwater snails, and the infective stage (metacercariae) is found on vegetation around springs and streams. Sheep ingest these metacercariae, which release immature flukes into the small intestine. The young flukes penetrate the intestinal wall, make their way to the liver, and then migrate through the liver tissue for 6–7 weeks before entering the bile ducts to become adults. Egg production starts 8–10 weeks after infection.

Disease from liver fluke is due to haemorrhage and tissue damage in the liver from migrating immature fluke and from damage to bile ducts and blood loss due to the feeding activities of the adult fluke.

Figure 1. Juvenile liver fluke (marked x) on the surface of a liver. Source: Bruce Watt.

Chemical control of liver fluke

In sheep, there are currently four commercially available tremacide active chemicals to kill liver fluke and all are short acting, providing no residual protection.  They are available alone and in combination products.

These are triclabendazole, closantel, albendazole and oxfendazole. However, oxfendazole is not effective against fluke by itself at standard dose rates, but may potentiate the effect of the other flukicides with which it is combined. None of these actives kill all stages of fluke. Even the most effective products leave some young immature fluke.

(In cattle, two further actives, nitroxynil and clorsulon are available [in combination] but are not registered for use in sheep.)

All of these chemical treatments are subject to lengthy withholding periods, so chemical treatment cannot be used relatively close to slaughter. Combined with the incomplete kill of immature fluke, it is quite possible that lambs that have been run on fluke-infested country will have live fluke in them at slaughter, regardless of any previous treatment.

Table 1 shows the maximum age fluke may be at slaughter, providing the Export Slaughter Interval (ESI) is observed. The age of the oldest flukes would be the ESI plus the minimum age of fluke killed by the treatment.

For example, the oldest fluke at slaughter following treatment with triclabendazole is 9 weeks (ESI) + 2 weeks (the age of fluke not killed by the treatment) = 11 weeks.

The shortest period is 10–11 weeks, which provides ample opportunity for liver damage to occur before slaughter. The immature fluke not killed by the flukicide can be migrating through and damaging the liver and if the sheep remain on fluke-infested pastures after treatment, then younger fluke can also be present.

The result is that chemical control is unable to be used to prevent the damage caused by immature flukes migrating through the liver if the sheep are ingesting metacercariae in the few months prior to slaughter.

Table 1. Tremacides and the minimum age of flukes killed, Export Slaughter Interval (ESI) and maximum age of flukes that could be present at slaughter provided the product was used according to the ESI (for commercially available sheep products).

Product actives (incl. where they occur in combination drenches)	Minimum age of fluke killed	ESI	2Maximum fluke age at slaughter
Triclabendazole	2 weeks	56­–63 days	70–77 days (10–11 weeks)
Triclabendazole and oxfendazole	2 weeks	63 days	77 days (11 weeks)
Triclabendazole and moxidectin1	2 weeks	63 days	77 days (11 weeks)
Triclabendazole and abamectin1	2 weeks	63 days	77 days (11 weeks)
Closantel	6 weeks	60 days	102 days (14.6 weeks)
Closantel and albendazole, abamectin1, levamisole1	6 weeks	60 days but could   be longer (no ESI   established)	102 days (14.6 weeks)
Closantel and abamectin1	6 weeks	84 days	126 days (18weeks)
Albendazole	12 weeks	10 days	94 days (13.4 weeks)
Oxyclozanide and levamisole1	12 weeks	14 days	98 days (14 weeks)

¹This active does not kill liver fluke.

²Maximum fluke age at slaughter if lambs were killed the day following the ESI is calculated by the addition of ESI and the minimum age of fluke killed.

How else can liver fluke infection be prevented or decreased?

Lymnaeid water snails are an essential part of the liver fluke life cycle. They only exist where there is permanent slow moving water such as springs, slow-moving streams with marshy banks, irrigation channels and seepages.

In regions where fluke occur, there will be farms that do not have any paddocks that support the Lymnaeid water snail. These farms will not have liver fluke in their stock unless the animals were already brought onto the property infected by liver fluke (fluke can live for several years in livestock). And without the snail intermediate host the life cycle cannot be completed, so no other animals can become infected.

On farms where the Lymnaeid snail exists, liver fluke is likely to be present. However, not all paddocks are likely to support the snails.

Also, the risk of infection from fluke is greater during spring (in warmer regions) to autumn as the fluke eggs generally require temperatures over 10°C for hatching. Development to the infective stage for livestock takes at least 5 weeks, but if the conditions are cool it may take months, therefore the fluke-infested pastures may present a lower risk to lambs in winter if grazing of these paddocks is necessary.

Strategies to decrease liver damage from liver fluke:

1. If complete prevention of liver fluke infection and the resultant liver scarring is to be achieved, lambs must be grazed on fluke-free pastures for their entire lives.
2. If the first strategy is too restrictive to grazing management, especially where fluke-infested pastures may provide the best nutrition in early lambing, the next best option would be to move lambs to a fluke-free pasture early in their life (at 4–8 weeks, when they will have had limited opportunity to ingest fluke metacercariae). Then 2–3 weeks after the move treat them with a triclabendazole product. The delayed drench gives time for newly ingested immature fluke to develop to a stage that the drench will kill.
3. The next-best option will be to move lambs to fluke-free pastures at a later age and treat with a flukicide 2–3 weeks after the move (while still adhering to the withholding periods). However, it provides more opportunity for lambs to have gained infections and for liver damage and scarring to have occurred.

During times of higher feed availability, sheep will be less likely to graze fluke-infested areas of paddocks, whereas during dry times and drought, these sheep will be forced to forage in these areas, increasing the risk of them ingesting liver fluke metacercariae. As such, it is likely that higher levels of fluke infestation or more extensive liver damage is seen at abattoirs during drought.