From Snails to Sheep: The Flatworm Affecting Farms Across The UK
Farm animals are susceptible to a whole range of diseases
and parasitic infections. From the commonly known and well publicised foot and
mouth disease to the lesser publicly recognised bluetongue disease, farmers
must deal with a wide array of viruses, bacteria and parasites that can affect
their livestock. In this blog, I spoke to Bethan John, a 3rd Year
PhD student in IGH about her PhD research into Liver Fluke.
Liver fluke (Fasciola
hepatica) is a flatworm parasite of grazing animals such as cows and sheep.
Though only the size of a 50p piece, this parasite causes weight loss and anaemia
in infected animals and is estimated to cost the UK cattle industry £40.4
million per year. Therefore, it is both an animal welfare issue and an economic
one. This parasite is also known to infect humans (zoonotic) when humans eat infected meat.
The liver fluke takes many forms during its life cycle. Infection
occurs when the animals consume the parasite when it is in the form of a cyst
attached to blades of grass. Once the cysts are in the small intestine they
mature into juvenile parasites which burrow from the small intestine across the
peritoneum and into the liver where they mature into adults. These adults are
hermaphrodites having both male and female characteristics, and are able to
reproduce both sexually (2 parents) and asexually (1 parent) resulting in the production
of eggs.
These eggs are passed from the mammal via its faeces onto
damp pastures where they hatch. They are then able to infect the mud snail
(usually Galba truncatula) which is
the intermediary host. Within the snail the parasite is cloned into thousands of genetically
identical cysts, a process known as clonal amplification. It is these genetically
identical cysts that are shed and become
attached to blades of grass. This process can only occur in boggy areas where
the snail can be found. The cycle is then completed when the parasite cysts are
ingested by grazing animals and mature flukes develop in the liver and go on to
lay eggs.
However, there has been evidence via word-of-mouth that even
animals that have been housed inside and fed on silage, far away from boggy
areas and snails, have still become infected with fluke. Silage, which is grass
that has been dried and fermented in airtight conditions, has also been known
to transmit some bacterial diseases and other parasites. It is, therefore,
thought that the fluke cysts could be transmitted from the silage to the housed
animals. Moreover, it may also be the case that the eggs can survive in slurry,
animal waste mixed with water and runoff, which is commonly used as a
fertiliser on farms.
As part of her PhD Bethan is looking into whether the
various fluke life cycle stages can
survive under certain environmental
conditions and within silage Fluke eggs
are a lot more sensitive to the environment than the cysts, they need moisture
and a temperature over 10°C to hatch, whereas cysts can survive in much cooler
environments; over 50% of cysts on grass can overwinter and still be infectious
to grazing animals.
There is also a big issue with the use of drugs
to control fluke infections. The usual drug used, triclabendazole, is becoming
less effective. If it can be shown that fluke can be passed to livestock via
silage then farmers can change and improve their farming practises to reduce
the incidence of fluke without the need for drugs.
Eleanor Senior is a 3rd
Year PhD student in IGH studying the bovine parasite Tritrichomonas foetus.
Bethan hunting for snails on a UK farm.