The liver fluke parasite causes cattle production and profit losses. However, there are possibilities to reduce the fluke burden in a cattle herd.
The liver fluke (Fasciola hepatica) is a significant parasite of cattle, causing losses in production and profits (1,2,3). The parasite is common to grazing land but it is possible to reduce the fluke burden in the herd with appropriate anthelminitic control alongside pasture management techniques, according to Sioned Timothy, ruminant technical manager for Boehringer Ingelheim Animal Health.
Adult fluke are able to survive for at least six months in untreated cattle(4) and fluke eggs can be shed year-round. In Europe, the greatest numbers of liver fluke eggs are found in animals in January to March5, which is most likely a result of over wintered adult fluke in untreated cattle that were infected the previous grazing season.
Fluke eggs shed in autumn can survive freezing temperatures(6) and will continue to develop in warm, wet winters with temperatures above 10°C. Therefore in relatively mild winters there is an increased risk of egg survival and a high chance of infectivity in the early summer grazing period.
The liver fluke completes its life cycle via an intermediate host – the mud snail (Galba truncatula) – commonly found in standing water and wet pastures. By understanding the habits of the mud snail and the liver fluke lifecycle it is possible to identify periods where the burden of eggs and infective stages of the parasite on pasture can be reduced.
Adult liver fluke live within the bile ducts of the host animal’s liver. Host animals include most ruminants, and the same type of liver fluke infects cattle and sheep. Eggs are produced by adult fluke and are passed out of the animal in faeces. Eggs are produced sporadically, which means they do not always show up in faecal egg count diagnostic tests.
Once eggs have been deposited onto the pasture in dung they can take up to one month to develop into the next stage of their lifecycle. The rate of development is dependent on the external temperature, and is more rapid in the warmth of summer. In the winter when temperatures fall below 10°C, there is little development but eggs may still survive.
Liver fluke eggs release miracidia. These motile parasite stages must find and enter their intermediate host, the mud snail, within three hours of hatching. Wet environments help the miracidia swim greater distances and the intermediate host is more likely to be present, increasing the potential for infection.
Once inside the mud snail host, the miracidia multiply several times over a period of around six weeks, depending on temperature, to form cercariae, which leave the snail host to complete the next stage of the cycle. The multiplication stage within the snail host causes large numbers of cercariae to be present on pasture, with each individual miracidium capable of producing 600 cercariae.
Once on the pasture, cercariae attach themselves to blades of grass where they encyst to form metacercariae. These are the infective stage of the liver fluke, ready to be ingested by grazing animals.
Once ingested the cysts break open to release the immature liver fluke, which travels through the digestive system and to the liver of the host animal. They tunnel through the liver tissue, entering the small bile ducts where they further mature, migrating to the larger bile ducts and occasionally the gall bladder. Here they complete their development to adult fluke and begin laying eggs.
The lifecycle from initial ingestion at pasture to hosting egg-laying adult liver fluke is around 10 to 12 weeks in cattle, and the time from egg laying to infective cysts being formed is around 12 weeks. Therefore, it’s easy to see how a complete lifecycle can take place within a grazing season if the conditions are right.
Breaking this lifecycle requires removing one or more of the environments needed for parasite development.
Preventing cattle from grazing wet pasture would be the ideal prevention from liver fluke infection, but is mostly impractical. However, there are steps which can be taken to break the liver fluke lifecycle by reducing exposure to certain stages of the parasite. These steps include limiting egg output onto the pasture in the spring, which can prevent liver fluke infection for much of the grazing season, and preventing ingestion of infective cysts on pasture.
1. Consider treating cattle before turnout. This reduces egg output onto the pasture in the spring, dramatically reducing the numbers of infective cysts that cattle will ingest later in the season.
While triclabeldazole treats nearly all stages of fluke, it is important to use an alternative when possible, and particularly if sheep are present on the same farm. This is because sheep are more susceptible to acute fluke disease caused by early immature flukes, for which triclabendazole is the only effective treatment.
At turnout it is likely only adult fluke will be present in cattle so a product which only treats late immature and adult stages of the parasite, such as Trodax® (nitroxynil) can be used.
2. Use clean pasture where available. The likelihood of fluke infection is low if cattle graze pasture which is less likely to support liver fluke or was not been grazed by cattle or sheep in the previous year.
Silage aftermath is a good option where rotations allow. Treating around 90% of animals before moving onto clean grazing will remove the majority of adult fluke from the herd and reduce egg output, while reducing the likelihood of selecting for resistant fluke. Remaining juvenile fluke will develop and can be removed at housing to prevent disease and productivity losses over winter.
4. Move outwintering cattle to drier ground or fluke-free pastures over winter.
Infective cysts can remain viable on the pasture for several months, even over winter in low temperatures, so moving cattle will reduce the likelihood of becoming infected. Treating a proportion of cattle for adult fluke before moving will also reduce egg output over winter.
5. Quarantine bought-in stock. House and treat all new stock with a flukicide which removes nearly all stages of fluke (triclabendazole). If there is a likelihood of animals carrying resistant fluke a follow up treatment at a suitable interval with an alternative product, such as Trodax, may be required before turnout.
The survival and infectivity of larvae on the pasture is affected by a number of environmental factors. If, however, young or naïve cattle are exposed to high levels of larvae in the pasture without prior vaccination or sufficiently low level exposure to have developed immunity, clinical disease will be seen. In older animals which have an established level of immunity, low levels of larvae in the pasture will serve to boost this, but in the face of a heavy, uncontrolled challenge, outbreaks of severe potentially fatal disease can also be seen in adult cows.
A control strategy should be in place at turnout and throughout the summer. Vaccinating before the spring turnout is an effective method of preventing clinical disease in both youngstock and adult cows, although this immunity may wane as the grazing season progresses.
Strategic worming programmes are often implemented to control gastro-intestinal parasites in young cattle, and this approach will also provide protection against lungworm. In adult cattle targeted treatments are often used in response to the early signs of disease. This approach demands vigilance to ensure cattle are treated before outbreaks of severe disease and associated lung damage occur.
Many wormers used to treat cattle gutworm (Ostertagia ostertagii), such as EPRINEX® (eprinomectin) and IVOMEC® Classic (ivermectin), will also treat any lungworm present, so they can be used as part of a programme for lungworm control and for treating outbreaks by providing protection against reinfection for up to 28 days.
The effect of lungworm at its most extreme is death, but even in cases where this does not occur, lungworm can cause clinical disease of varying severity, significantly affecting the productivity of cattle. A planned strategy, vigilance and swift action will go a long way to reducing productivity losses throughout coming months.
Trodax 34% w/v solution for injection contains nitroxynil. UK: POM-VPS. Further information available in the SPC or from Boehringer Ingelheim Animal Health UK Ltd, RG12 8YS, UK. UK Tel: 01344 746960 (sales) or 01344 746957 (technical), IE Tel: 01 291 3985 (all queries). Ivomec and Trodax are registered trademarks of the Boehringer Ingelheim Group. ©2019 Boehringer Ingelheim Animal Health UK Ltd. All rights reserved. Date of preparation: Apr 2019. AHD12103. Use Medicines Responsibly.
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