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NADIS Parasite Forecast - September

PF Capture 09-16

At the start of July the weather was breezy and showery, with low pressure in charge. Unsettled conditions persisted for much of the first half of the month, with fronts frequently bringing rain to the north and west, although rainfall amounts were generally small in the south. It was generally warmer and sunnier between the 17th and 23rd, with a brief hot spell from the 18th to 20th, which triggered a thundery breakdown in the north and across parts of East Anglia. Changeable weather resumed during the last week, with further frequent belts of rain, but again many southern areas often stayed dry.   (

September Parasite Forecast/Update

The most recent version of this monthly parasite forecast may be accessed at


This year summer weather conditions have been such that the risk of liver fluke disease is likely to be high in Scotland and north-western regions of England, and broadly similar to the high challenge faced in 2015. The risk of liver fluke disease in 2016 may indeed be higher because it follows a year where there was a high prevalence of liver fluke disease leading to pasture contamination with eggs last winter which will have developed through the intermediate snail stage this summer to infest sheep in the autumn.


Normal liver (left) and chronic liver fluke (right) - develop a liver fluke control plan with your veterinary advisor now; this farmer did not and paid a heavy price.


Normal liver (top) and chronic liver fluke (bottom); flukicide treatment will not repair such severe liver damage.  This situation resulted because purchased sheep were not correctly treated for immature flukes.

A full and accurate assessment of the liver fluke risk is based upon monthly weather data compiled from May to October but waiting until early November to learn of a high risk of acute/subacute fluke in September would be too late; this provisional forecast of a high risk for 2016 is the best compromise.

A more accurate prediction of fluke risk will be provided during September once August weather conditions are known in detail but it is likely that prophylactic treatment against immature flukes with triclabendazole will be necessary in Scotland during September to prevent acute and subacute liver fluke disease. Farmers should consult their veterinary surgeons now so they have a plan in place, and the correct drugs, to prevent acute and subacute liver fluke disease this autumn. Advice will also be needed where triclabendazole resistance has been previously susepcted on farms.

The early predicted liver fluke risk for 2016 is:

Scotland, northwest England and north Wales a 'high risk' is predicted

Central and eastern regions of England are forecast to be at 'low risk'.

It is now time to review liver fluke control measures on your farm.


On high risk farms move sheep from wet pastures where possible from late August.


Migrating flukes predispose to the clostridial disease "black disease". As well as prophylactic fluke drenching, ensure all lambs are fully vaccinated (twice, one month apart) to prevent clostridial diseases.


Postmortem examination is essential to establish a diagnosis of acute and subacute liver fluke disease, and other potential causes of sudden death.

Liver fluke disease can be largely controlled by effective drugs administered at the correct time as directed by the farmer's veterinary surgeon as part of the flock health plan.  However, evasion strategies should also be adopted wherever possible by not grazing potentially contaminated, poorly-drained areas from late August onwards.

  • Sheep with acute fluke may simply be found dead without prior signs of illness therefore it is important to have all sudden deaths investigated to allow immediate preventive measures. Postmortem examination is essential to establish a diagnosis of acute fluke.
  • Ensure all lambs are vaccinated against clostridial diseases because migrating flukes predispose to black disease.
  • Farms with a known liver fluke problem especially in Scotland should consult their veterinary surgeon about treating sheep with triclabendazole in September.
  • Triclabendazole is the drug of choice because it is effective against very young immature flukes.
  • The coproantigen ELISA test can be used to monitor triclabendazole efficacy two to three weeks after dosing and farmers should contact their veterinary practitioner for further advice where drug resistance is suspected.
  • Later fluke treatments could use closantel or nytroxynil especially if triclabendazole resistance is suspected/proven on the farm.
  • Where closantel and nytroxynil  are used to prevent liver fluke disease, accurate dosing is essential. Do NOT drench sheep based upon the heaviest animal in the group as toxicity is possible.  Weigh a representative range of sheep and  treat according to weight.  Be very careful not to accidentally  treat sheep twice.
  • Flocks with no previous evidence of fluke disease must maintain their farm's biosecurity especially with respect to purchased sheep.

Quarantine treatments of purchased stock

September is the month of most breeding stock sales and quarantine treatments are essential for all introduced stock.

  • Quarantine arrangements are essential to reduce the risk of introducing anthelmintic-resistant worms with purchased sheep.
  • Current best practice involves sequential full dose treatments with either monepantel (Zolvix) or derquantel and abamectin (Startect), and injectable moxidectin upon arrival on farm.
  • Confine sheep for 24-48 hours, then turnout onto pasture recently grazed by sheep.
  • Maintain purchased stock in separate groups and monitor closely for disease for at least 30 days before mixing with home flock.
  • Liver fluke must be considered in quarantine treatments.



Despite their appearance and absence of diarrhoea, quarantine treatments are essential for replacement rams to reduce the risk of introducing anthelmintic-resistant worms.

Parasitic gastroenteritis (PGE)

  • PGE is likely to become a problem on contaminated grazing in many areas as larval challenge continues.
  • Monitor lamb FWECs or liveweight gain to direct anthelmintic treatments.



Parasitic gastro-enteritis affecting fattening lambs is more common following wet weather which enables egg hatching and early larval development.


  • August and September are  peak months for lungworm disease (husk; hoose).
  • Adult cattle which have not built up immunity through natural challenge in previous years are also susceptible to lungworm.
  • Any animal showing coughing at rest and an increased breathing rate should be investigated for the presence of lungworm.
  • The presence of Dictyocaulus larvae in faeces can be readily undertaken by your veterinary practice with results available within 24 hours but note that clinical signs of lungworm may be present before the infestation becomes patent.


  • Type 1 ostertagiosis presents in growing cattle with profuse diarrhoea suddenly affecting a large percentage of the group.


Diarrhoea and rapid weight loss caused by type 1 ostertagiosis.

Local farm conditions may vary so consult your veterinary surgeon. Parasite control should be part of your veterinary health plan.