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Diagnose and assess fluke status in cattle to prevent reinfection at grass

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Understanding the liver fluke lifecycle, assessing potential peak periods of infection on pasture, and detecting the parasite in cattle are vital to ensure appropriate fluke treatment and prevent continual pasture and herd re-infection.

Liver fluke can be found in cattle livers at slaughter all year round1. In untreated cattle, adult fluke are able to survive for at least six months2 and fluke eggs can be shed throughout the year. Peak concentrations are normally found from January to March in Europe3 which coincides with the presence of overwintered adult fluke in untreated cattle, infected during the previous season.

The encysted, infective stage ingested by grazing cattle - the metacercaria - is found on pasture in high numbers from late summer into autumn, but depending on conditions they can be found in low numbers on pasture all year round4.

Callum Blair, Veterinary Adviser for Merial Animal Health, says: "Cattle are at risk from fluke infection for a large percentage of the year. Animals can be re-infected many times and there is little evidence that cattle develop immunity or resistance to the parasite.

"Fluke in growing cattle have been shown to reduce liveweight gain by up to 1.2kg/week5. Leaving infected cattle untreated not only reduces their ability to make the most of grass, it could cost more in the longer term with increased feed costs and finishing times."

Mr Blair continues: "As well as improving the health and performance of animals, treating cattle for fluke during the summer will ensure any adult fluke present are removed, which in turn limits the numbers of eggs shed onto the pasture, reducing re-infection."

The fluke lifecycle depends on the presence of an intermediate host - usually Galba truncatula. This snail feeds on the algae on the surface of mud and favours slow moving bodies of water, such as that around water troughs, ponded areas (whether permanent or temporary) and poached areas around gateways and other wet areas of pasture.

Eggs released in cattle faeces from adult fluke develop on pasture, releasing short-lived miracidium which infect the snail. The majority of the free-living lifestages and those within the snail host take place between May and October, with large numbers of the infective metacercariae released from snails onto the pasture during August, September and October, ready to be ingested by grazing cattle.

However, snails can also carry infection over winter and release metacercariae onto pasture in the early spring. Furthermore, eggs shed in autumn can survive winter temperatures below 10oC, remaining on pasture for several months, ready to begin development once temperatures rise.

Cattle grazing at-risk pasture in spring and early summer can therefore pick up infection earlier than anticipated.

On farms where there is a high risk of fluke infection from overwintered eggs or snails on pasture that might have developed since turnout or the last treatment, a summer treatment in June or July may be advised6. Trodax® (nitroxynil) kills adult and immature fluke (>8 weeks post-infection) and can be used as part of a strategic control programme to reduce pasture re-contamination and ensure animals maximise the production benefit from grass.

If left untreated, adult fluke will start producing eggs which will be passed back onto the pasture to continue the lifecycle, re-infecting animals in late summer and autumn before housing.

Animals bought in, especially from known fluke endemic areas, should be assessed for the risk of fluke infection. If treatment is advised, an alternative product to triclabendazole such as Trodax® (nitroxynil) or products containing clorsulon should be used.  A follow up treatment will be required to ensure all life stages of fluke are removed. The timing of this treatment will vary from 7 - 10 weeks, depending on the product used. Ideally, cattle should be kept off pasture or at least away from snail habitats during the quarantine period. This approach will also ensure any triclabendazole-resistant fluke are not brought onto farm.

Mr Blair says: "Identification of fluke infection is important to determine if production losses, increased finishing times or lack of thrive are due to fluke damage. However, this is not straight forward and in the first instance, reports from abattoirs will be helpful in detecting adult fluke and damage from migrating immature fluke."

Tests at a herd level are useful for investigating the presence of fluke on farm and for monitoring infection once a control programme is underway4. Composite faecal egg counts (FEC) from ten animals is a relatively inexpensive method to determine the presence of fluke. However, FECs lack sensitivity and with fluke eggs passed erratically, it may be necessary to repeat the test a few weeks later. As fluke eggs may be passed for a period of time after treatment, FEC samples should not be collected in the first three weeks after dosing.

FECs are still useful in individual animals, though they can only detect patent infection. Increasing the amount of faeces sent for analysis to over 30g per animal will improve the test sensitivity7.

An ELISA test is available which can detect early, pre-patent infection (2-4 weeks after infection) via antibodies in serum and milk samples. However, serum antibodies persist for 4-10 weeks after treatment so a positive result demonstrates the animal was exposed to the parasite, but not that they are necessarily currently infected8.

A new copro-antigen detection ELISA test has been developed which can detect fluke infection two to four weeks before eggs are detected in faeces. In sheep, this test has been useful for detecting resistance to triclabendazole but its use in cattle is less well developed at the present time.

In all cases the farm and herd history, time of year and weather conditions over winter and summer months should be considered, as well as diagnostic results, since these factors directly influence the risk of fluke infection and disease.

References:

1. Froyd, G. (1975) Liver fluke in Great Britain: a survey of affected livers. Veterinary Record, 97: 492-5

2. Ross, J.G., Todd, J.R. (1968) Epidemiological studies of fascioliasis: a third season of comparative studies with cattle. Veterinary Record, 84: 695-699

3. Gonzalez-Lanza, C., Manga-Gonzalez, Y., Del-Pozo-Carnero, P., Hidalgo-Argüello, R. (1989) Dynamics of elimination of the eggs of Fasciola hepatica (Trematoda, Digenea) in the faeces of cattle in the Porma Basin, Spain. Veterinary Parasitology, 34: 35-43

4. Williams, D.J.L., Howell, A., Graham-Brown, J., Kamaludeen, J., Smith, D. (2014) Liver fluke - an overview for practitioners. Cattle Practice. Vol 22(2): 238-244.

5. Johnson, E.G., (1991) Agri-practice.

6. Forbes, A., (2013) Liver fluke control in cattle: why, when and how? Cattle Practice. Vol 21(2): 150-156.

7. Rapsch, C., Schweizer, G., Grimm, F., Kohler, L., Bauer, C., Deplazes, P., Braun, U., Torgerson, P.R. (2006) Estimating the true prevalence of Fasciola hepatica in cattle slaughtered in Switzerland in the absence of an absolute diagnostic test. Int J Parasitol. Sep; 36(10-11):1153-8.

8. Salimi-Bejestani, M.R., McGarry, J.W., Felstead, S., Ortiz, P., Akca, A., Williams, D.J. (2005a) Development of an antibody-detection ELISA for Fasciola hepatica and its evaluation against a commercially available test. Res Vet Sci. Apr; 78(2):177-81.

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