ShapeShapeauthorShapecrossShapeShapeShapeGrouphamburgerhomeGroupmagnifyShapeShapeShapeShape

Ruminating on rumen fluke

by
01 August 2017, at 12:00am

Dr PHILIP SKUCE of the Moredun Research Institute examines the current state of play for sheep and other livestock

RUMEN FLUKE (or paramphistomes) are digenean, 2-host trematode parasites that infect a broad range of mammalian definitive hosts, including sheep, cattle, goats, alpacas and llamas. They have a complicated life-cycle, similar to that of the liver fluke, Fasciola hepatica, involving a molluscan intermediate host and infection as a result of ingesting infectious metacercarial cysts while grazing. However, they undertake a slightly  different journey within the definitive host. After excysting in the small intestine, where they are thought to feed on the intestinal mucosa, the tiny immature rumen fluke migrate “upstream” and settle in the rumen and reticulum, where they mature and lay eggs. There is still some controversy about what the adults actually feed on, but they are typically found attached through their large posterior sucker (acetabulum) with the mouth free to sample ruminal contents. Adult rumen fluke look like small pink maggots on the surface of the rumen, whereas the immatures resemble tiny grains of rice on the mucosal surface of the intestine. The former appear to be relatively well-tolerated, clinical disease being exclusively associated with large infestations of immature parasites and accompanying intestinal pathology, which can be fatal. Rumen fluke are common in tropical countries, where they thrive in the warm, wet climate, and are acknowledged to have a significant impact on livestock productivity. However, in recent years, they have become increasingly common in livestock in temperate countries and are already prevalent in many parts of Europe. Typical herd-level prevalence in mainland Europe would be in the region of 20- 30% in sheep and cattle, whereas Ireland appears to be particularly badly affected, a recent study finding a herd-level prevalence of ~77% in sheep across the country. 

Identification 

Rumen fluke are most commonly detected using conventional coprological techniques, typically faecal egg count by sedimentation, exactly as performed for routine liver fluke diagnosis at regional Veterinary Investigation Centres. However, rumen fluke eggs look very similar to those of liver fluke and for many years were probably counted as such; it  was not until 2010 that a specific VIDA diagnostic code was first entered in the UK. As with liver fluke, a positive rumen fluke egg count only indicates the presence of adult egg-laying parasites and not the more pathogenic immature fluke in the intestine. Unlike liver fluke diagnosis, there are currently no commercially available immunological tests for rumen fluke, e.g. serum and/ or coproantigen ELISA. Post-mortem examination and abattoir inspection both provide good opportunities to detect the adult parasites but there is, typically, no routine inspection of the rumen at slaughter in most abattoirs. Clinical signs of rumen fluke disease (paramphistomosis) are relatively generic, e.g. ill-thrift, diarrhoea, poor body condition, etc., and could be mistaken for any number of other conditions. 

Treatment 

Treatment options for rumen fluke are very limited, but advice from animal health specialists has been to not treat for rumen fluke unless there are clear clinical signs and a positive diagnosis of rumen fluke infection. There is only one flukicide with acknowledged activity against rumen fluke (juvenile and adult) and that is oxyclozanide, although such products have no specific label claim for rumen fluke treatment, at least not in the UK. There have been reports from Spain claiming some activity of closantel against adult rumen fluke, but these have not been substantiated in other studies and would not be recommended here. 

Prevention

Prevention of rumen fluke infection, as with prevention of liver fluke infection, requires a good working knowledge of the rumen fluke life-cycle to help inform when and where things may be happening on-farm. It also requires an integrated parasite control approach aimed at reducing pasture contamination with rumen fluke eggs in spring, reducing snail habitat on-farm in summer (e.g. improved drainage, rolling poached areas, etc.), avoiding exposure of stock to potential cyst challenge in autumn (e.g. housing, fencing) and strategic treatment of stock in winter with oxyclozanide if and when required. 

Clinical relevance 

The clinical relevance and production impact of rumen fluke in temperate regions are still under debate. Recent abattoir studies in Belgium, the Netherlands and the UK found little association between rumen fluke infection in cattle and production effects, other than an association with diarrhoea and a reduced carcase fat coverage. To the best of my knowledge, there are no published reports of production effects in sheep. One of the main complications is that stock infected by rumen fluke are often also infected by liver fluke, so it is very difficult to separate the effects of the respective parasites on their hosts. Also, we hear numerous anecdotal accounts from farmers who see dramatic improvements in condition having treated their stock with oxyclozanide. However, oxyclozanide is a liver fluke drug in its own right, with good activity against adult F. hepatica, so it may be the underlying liver fluke infection that’s being treated rather than the rumen fluke. Often in these cases, no specific diagnosis was obtained, nor any indication of actual treatment efficacy, so it is difficult to draw conclusions about the relative impact of the two fluke parasites on infected animals. 

(Re)emergence of a new/old parasite?

Rumen fluke first came to our attention in the late 2000s, when their eggs started to appear in routine diagnostic samples submitted to regional VI Centres. Diagnoses peaked in 2013, following the exceptionally wet summer and autumn of the preceding year. For decades, it was assumed that the rumen fluke species infecting livestock in GB and Ireland was  Paramphistomum cervi which, as the name suggests, has a natural definitive host in wild deer. This was based on an early morphological identification of adult specimens recovered from Scottish and Irish cattle at Glasgow Vet School in the 1950s. P. cervi was considered a relatively non-pathogenic and incidental finding at post-mortem. However, recent molecular analysis, initially carried out at Moredun, has confirmed that the predominant – if not only – rumen fluke species currently infecting GB livestock is actually Calicophoron daubneyi, the predominant species infecting livestock on mainland Europe. It is distinctly possible that the original Glasgow species identification was correct; C. daubneyi may be a more recent (and more pathogenic) invader. Various theories exist for how and when C. daubneyi entered GB livestock, but it is entirely possible that this happened with the importation of European livestock, most likely cattle, e.g. Limousin or Charolais from central France. A detailed molecular phylogenetic analysis of livestock samples from Ireland revealed one large suprapopulation of rumen fluke across the whole country, indicative of a relatively early incursion and extensive spread by livestock movement. A more recent study of sheep in
Ireland found a small number of animals with Paramphistomum leydeni,
another acknowledged rumen fluke species from deer, indicating that livestock can be infected by wildlife species. Another interesting finding that
overturns our pre-conceived ideas of the rumen fluke life-cycle is the identification of the liver fluke snail, Galba truncatula, as the intermediate host of rumen fluke, at least in GB, as it is in Europe. It was originally thought that rumen fluke utilise planorbid pond snails as their preferred intermediate molluscan host, hence a possible explanation for the increased incidence and geographic spread of rumen fluke with snails being spread on the margins of flooding events. This was first reported in Wales, where liver fluke and rumen fluke stages were even found within the same snail, and has since been confirmed in Scotland. It is maybe no surprise, therefore, that we are seeing a parallel increase in incidence and geographic spread of liver fluke and rumen fluke considering they share many of the same environmental risk factors. In conclusion, there is no doubt that rumen fluke has become more common in UK livestock in recent years. However, that doesn’t make it more important; its clinical and production impacts are still under debate. That said, we’ve only recently confirmed its species identity and determined its main molluscan intermediate host. These new findings are interesting from an epidemiological perspective, especially how they relate to rumen fluke disease risk factors and the interaction with liver  fluke in the same environment. This should not, however, deflect from the fact that liver fluke is the acknowledged pathogen and we should redouble our efforts to improve awareness and promote best practice for sustainable fluke control, whichever species is present!

Further reading

Gordon, D. K. et al. (2013) Identification of the rumen fluke, Calicophoron daubneyi, in GB livestock: possible implications for liver fluke diagnosis. Veterinary Parasitology 195 (1-2): 65-71.
Jones, R., Williams, H., Dalesman, S. and Brophy, P. (2015) Confirmation of Galba truncatula as an intermediate host snail for Calicophoron daubneyi in Great Britain, with evidence of alternative snail species hosting Fasciola hepatica. Parasites & Vectors 8: 656.
Martinez-Ibeas, A. M. et al. (2016) Rumen fluke in Irish sheep: prevalence, risk factors and molecular identification of two paramphistome species. BMC Veterinary Research 12 (1): 143.