Liver Fluke Life Cycle
Thefarmpage brings to your attention an article on animal health which was recently published in the AMTRA web page. The farm page hope this article is helpful in your animal management in the fight against this deadly and costly parasite.
What is liver fluke?
Liver fluke disease, or fasciolosis, is caused by the parasite Fasciola hepatica. The parasite is not host specific and can affect cattle and sheep, as well as other grazing animals such as deer and rabbits. Fasciolosis occurs when animals ingest infective cysts (metacercariae) from the pasture. The disease can be fatal in sheep, while in cattle, it rarely causes clinical symptoms. However, liver fluke can have a significant effect on the performance and productivity of all affected species.
The life-cycle of liver fluke
In order to understand how a parasite can infect a host species, it is critical to know its life-cycle. In the case of liver fluke the life-cycle is indirect, involving an intermediate mud snail host. Details of this life-cycle are summarised below:
- Adult fluke live in the bile ducts of the host animal’s liver and produce eggs, which are passed out in the faeces onto the pasture. Each adult fluke is capable of laying up to 50,000 eggs per day
- .Eggs hatch to release miracidia which have a limited lifespan and must infect the mud snail (Galba truncatula) intermediate host within three hours after hatching.
- Miracidia burrow into the mud snail, where they multiply several times, ultimately developing into cercariae. Each miracidium entering a snail can result in 600 cercariae leaving it.
- Cercariae attach themselves to blades of grass where they encyst to form metacercariae. It is these cysts on the blades of grass which are the infective form of fluke, picked up by grazing animals
- Once eaten, the cysts break open to release immature fluke which travel to the liver. They tunnel through the liver tissue, enter the small bile ducts where they further mature, then ultimately move into the larger bile ducts and occasionally the gall bladder, where they complete their development to adult fluke and begin to lay eggs
- The time taken from picking up fluke infection to the adult egg-laying stage being reached is approximately 10 – 12 weeks.
- Similarly, the time taken from fluke eggs being deposited on the pasture to the infective cysts being formed is about 12 weeks. All free living stages of the fluke life-cycle are affected by temperature and moisture, so the rate of completion is increased when conditions are warm and wet.
Effects of Liver Fluke
Adult fluke live in the bile ducts of the liver and each fluke can cause 0.5ml of blood loss per day from the infected host. While sheep tend to show clear signs of fluke infection, such as anaemia and bottle jaw, cattle often do not show such obvious signs of infection at all. However, liver fluke is responsible for significant loss of productivity in both species.
In sheep, low levels of fluke infection can cause a 10% reduction in weight gain of ewes, 10% reduction in multiple births and a 5% reduction in the birth weight of lambs. It can also result in a 30% reduction in the weight gain of lambs.
Research shows that even low levels of infection can cause significant reductions in cattle production parameters, such as growth rates and feed conversion efficiency. Indeed, mild fluke infections have been shown to reduce weight gain by approximately 1kg per week and feed intake by up to 11% compared to treated cattle.1 These factors add days to the time it takes farmers to finish their cattle, with every extra day costing them money.
In addition to affecting productivity, fluke infection can also increase the susceptibility of animals to other infections and metabolic disorders. With all these points in mind, treating for fluke is therefore extremely cost effective.
Treatment Advice, Timings and Options
Clearly any treatment regime would benefit from preventing the livestock from grazing snail habitat, or removal of the snail habitats through drainage. However, neither of these options is generally practical on farm. Therefore, the strategic use of flukicides tends to be the accepted way to manage liver fluke.
In terms of treatment, many farmers are aware of the risks of fluke disease during the autumn and winter, and will routinely treat their cattle at housing. However, many are unaware of the increased risk from fluke during the grazing season and the benefits of administering a treatment at this time.
An ‘at grass’ fluke treatment will help to reduce the levels of fluke eggs shed onto the pasture in the spring and summer, thus helping to break the fluke life-cycle and subsequently reducing the risk of winter disease. An ‘at grass’ treatment may also contribute to improved growth rates and enable farmers to maximise their animals’ growth from grass. In fact, cattle treated against fluke at grass were shown to have 31% better weight gain compared to untreated animals, and 8% compared to cattle that were treated with a wormer only.5
In terms of treatment timing, cattle can start to pick up infective fluke cysts as soon as they are turned out to grass, and because it takes approximately 10 to 12 weeks for these infective cysts to become adult egg-laying fluke, treatment should be administered 10 to 12 weeks after turnout to minimize fluke eggs reaching the pasture. However, in some cases from a practical management perspective, treatment may be advisable eight weeks after turnout to tie in with the worming regime on farm. Therefore, the general advice would be to treat eight to 10 weeks after turnout/exposure to infection.
As well as a pre-tupping and mid-winter dose of flukicide, sheep will benefit from a pre-lambing treatment. In high risk situations, a dose in the summer months to reduce pasture contamination from fluke eggs, and subsequent risk of disease in animals later in the year, should also be considered.
Remember that liver fluke is not host specific; it is the same parasite that affects both cattle and sheep. Therefore where cattle and sheep are grazing the same pastures they should both be treated against fluke.
These include either fluke-only products, wormers at increased dose rates (to have activity against liver fluke) or combination products containing a flukicide.
The only products with activity against immature fluke are those that contain triclabendazole, closantel or nitroxynil.
For any advise on the treatment of fluke please ask at your local AMTRA registered Agricultural Merchant or Veterinary Practice.