Fluke is a commonly encountered problem, especially here in the West Coast of Scotland where the wet mild weather is ideal for fluke development. Fluke or Fasciola hepatica is a trematode parasite that can infect a number of hosts including sheep, cattle and man. In fact it is estimated some 30 million humans are infected with fluke worldwide and infection is associated with the eating of uncooked aquatic plants. However human infection is very rare in Scotland. Much more common is infection of cattle and sheep. In fact economic loss due to fluke in cattle and sheep is estimated to be greater than £18 million a year in the UK and Ireland (Mulcahy and Dalton 2001).
Life Cycle of Fluke
Liver fluke pass through an intermediate host before infecting cattle and sheep. Without this intermediate host the fluke would be unable to develop into the form that is infective for cattle and sheep. The intermediate host for the most common liver fluke in the UK is an aquatic snail called Lymnaea truncatula. Due to its aquatic nature the snail thrives on waterlogged fields and as such fluke is more common in areas which tend to be heavily saturated. This includes the majority of the West Coast of Scotland. Liver fluke development starts to occur when the average day and night temperature is above 10°C. This allows development of the eggs passed out from sheep and cattle through the intermediate host (snails) into a form known as metacercariae, which is infectious to cattle and sheep. If conditions are ideal a single egg can produce 600 infectious metacercariae in just 6 weeks. Traditionally the two main periods of infection for cattle and sheep were May to June and August to November. However due to a number of factors including the changing climatic conditions infection can occur at a number of other times throughout the year.
The infective stage for cattle and sheep (metacercariae) attach themselves to the grass where they are often then eaten. Once they are ingested the juvenile fluke migrate through the liver of the host and eventually move to the bile ducts where they mature into adults and start laying eggs. These eggs then pass out in the droppings to infect more snails and so the life cycle continues. It is important to note that from the point of infection of cattle and sheep until the laying of eggs by adults takes 12 weeks. This means an animal can be infected with fluke and show clinical signs but test negative for fluke if tested by faecal egg count.
Fluke is more severe in sheep than cattle. In sheep the clinical signs seen depend on the number of infectious matacercariae that are ingested. In the most severe cases sudden death is the outcome. If slightly less metacecariae are ingested then animals are weak, with pale gums due to damaged red blood cells. Animals will also have difficulty in breathing and may have a swollen abdomen filled with fluid. Sometimes the presence of swelling under the jaw will also be appreciable. Some animals will only carry a few fluke and not show any direct clinical signs but will have general 'ill thrift'. For example they will carry less condition and carry lambs to a lower birth weight etc.
In cattle fluke is very rarely fatal. Cattle mount a stronger immune response to fluke than sheep and therefore their liver is not as severely affected. Clinical signs are similar to the less severe infections of sheep with animals showing poorer BCS, lower milk yields and swelling under the jaw.
Diagnosis and Treatment
The gold standard diagnosis is a post mortem of an infected animal and the isolation of fluke from the liver. However there are a number of ways of diagnosing fluke in living animals. A faecal sample can be used to look for eggs, however as mentioned previously this will only pick up infections that are at least 12 weeks old. Furthermore fluke eggs are not that numerous in the droppings, therefore a negative test result does not guarantee the animal is not infected. On dairy farms a bulk milk test is available to give an idea of the level of infection on the farm. There is also an individual blood test available which looks at antibodies produced when an animal is infected. However these antibodies can remain in the blood for up to 6 months after infection and therefore a positive result does not confirm that the infection is current. A number of other tests are currently being evaluated and research into a vaccine is also being undertaken.
If an animal is found to be positive for fluke then there are a number of treatments available. Triclabendazole is the drug of choice and at the time of writing is available in a number or preparations including:
* Combinex Cattle (Novartis Animal Health)
* Combinex Sheep (Novartis Animal Health)
* Fasimec Duo S 0.1/5% (Novartis Animal Health: Sheep)
* Fasinex 5% (Novartis Animal Health: Sheep)
* Fasinex 10% (Novartis Animal Health: Cattle)
* Tribex 5% Oral Suspension for Sheep (Chanelle Vet UK)
* Tribex 5% Oral Suspension for Cattle (Chanelle Vet UK)
All of these products are administered as an oral drench. Triclabendazole is active against all forms of fluke from the immature fluke migrating through the liver to the mature adults in the bile ducts. Closantel is the other drug available that has activity against immature fluke and is available in the following formulations;
* Flukiver Oral Suspension (Sheep)
* Mebadown Super Oral Suspension (Sheep)
* Supaverm Oral Suspension (Sheep)
and are available from Janssen Animal Health. Nitroxynil is another commonly used flukicide and is available as Trodax 34% (Merial) for use in both cattle and sheep. It has the advantage over the other products in that it comes in an injectable form. However nitroxynil is less effective against fluke and does not prevent the damage they cause as they migrate through the liver.
Prevention is better than cure, and there are a number of steps that can be taken to reduce fluke burdens in your stock. To get an idea if fluke is a problem in your herd it is a good idea to get feedback from your abattoir. they will be able to inform you if any fluke were found in the liver during the meat inspection. In sheep routine treatment of fluke is recommended. This is not usually necessary in cattle herds unless fluke is known to be a problem on the farm. Treatment on dairy units can be a problem as none of the products listed are licensed for use in lactating cows if the milk is for human consumption. Therefore treatment involves either drying off the cow early or waiting till the dry period to treat.
Some attempt can also be made to control the environment of the fluke and snails and make it less favorable therefore decreasing their numbers. Drainage can be improved on fields to make them less liable to water logging and therefore decreasing the snail's habitat. Furthermore areas prone to flooding can be fenced off to prevent stock from grazing the area. Although these measures can be effective they are often expensive and impractical