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Editorial Information

John Graham-Brown BVSc (hons), MSc (hons), PhD, MRCVS

Published 2018

Parasite Forecast - August 2018

Capture 18-08

May 2018 was one of the hottest on record, with a mean temperature of 12.1oC, 1.7oC above the long-term average (1981-2010 data). These above average temperatures were noted across all regions in May, with increased average temperatures also observed for the previous 3 months (March-May) with the exception of Northern Ireland, where temperatures were similar to the regional long-term average.

Rainfall was 31% below the long-term national average for May. Dry conditions were experienced across the majority of the UK for the first two weeks, with showers and thunderstorms becoming increasingly widespread in the second half of the month. This equated to below average rainfall in most regions in May with the exception of the Midlands and southern England, which both experienced rainfall close to their long-term regional averages. Average rainfall for the previous 3 months (March-May) was more varied between regions, with increased average rainfall for most of England and Wales except for northwest England and north Wales, where rainfall was just below the long-term regional average. Rainfall for the previous 3 months was below the long-term average in Scotland and Northern Ireland.

August Parasite Forecast/Update


When compared to common intestinal round worms, the extended life cycle of liver fluke (completion takes 17 - 18 weeks under optimal conditions) means peak risk for infection occurs later in the grazing season.

Both the free living stages of liver fluke and its intermediate host (the mud snail Galba truncatula) are heavily dependent on temperature and rainfall. Warm, wet summers provide the optimal conditions for development and propagation. The NADIS fluke forecast uses UK Met Office data for temperature and rainfall to calculate fluke risk for the coming season, which is generally considered to start in late summer/early autumn.

The provisional fluke forecast for 2018 is based on rainfall and temperature for the months of August-October 2017, and May-June 2018, and predicts moderate risk in the north and west of Scotland, and low risk in all other regions (Figure 1). A more comprehensive fluke forecast will be provided later in the year, so please stay posted for future parasite forecasts.

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Figure 1: Provisional fluke forecast for summer 2018 by region. A risk value >300 is considered moderate. A risk value >375 is considered high.

Both sheep and cattle are susceptible to infection with liver fluke.

Acute disease outbreaks may occur at any time from late summer onwards. This type of infection is more common in sheep, and results from the consumption of large numbers of infective cysts (metacercariae) as they emerge onto contaminated pastures (Figure 2). Signs include:

  • Sudden death in heavy infections
  • General dullness, anaemia and shortness of breath
  • Rapid weight loss, fluid accumulation (ascites) and bottle jaw
  • Large numbers of animals may be affected

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Figure 2: Infective stages of liver fluke (metacercariae) start to accumulate on "flukey" pastures from late summer/ early autumn onwards as they are shed with increasing frequency from their snail intermediate hosts.

Advised actions include:

  • Monitoring for signs of disease.
  • Routine diagnostic testing to give a greater insight into the current infection:
    • Post-mortem in acute outbreaks allows for a definitive diagnosis (Figure 3).
    • Worm egg counts can be used to diagnose infection in individuals, or groups of animals when using a composite sample.
      • Egg counts cannot detect pre-patent infection and should not be relied upon for the diagnosis of acute disease.
      • Where acute disease occurs, treatment with triclabendazole is recommended as this is the only product effective against both adult and immature stages of the parasite.
        • Due to growing concerns over drug resistance it is also advised such treatments are accompanied by resistance testing to monitor efficacy.
        • If triclabendazole resistance has been confirmed on farm, alternative control strategies proposed under the sustainable control of parasites in sheep (SCOPS) guidelines involve administering two treatments with either closantel 6 weeks apart, or nitroxynil 7 weeks apart.


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Figure 3: Tracks on the surface of this lamb liver are typical lesions associated with acute fasciolosis. These are caused by juvenile flukes penetrating and migrating through the tissue. Heavy infections can result in catastrophic liver damage and internal bleeding.
  • Risk of infection can be reduced by identifying high risk fluke pastures and avoiding grazing them during peak risk periods.
    • Mud snails are generally found in damp, muddy areas such as the borders of permanent water bodies, wet flushes (identified through presence of rushes and other water loving plant species), ditches, boggy areas etc.
    • Pastures previously grazed by fluke infected sheep should be considered a risk to cattle and vice versa.


Parasitic Gastroenteritis (PGE)

PGE is a disease of lambs in their first grazing season, resulting from the accumulation of large infectious burdens of gastrointestinal roundworms over the course of the grazing season. Typical signs include:

  • loss of appetite
  • diarrhoea
  • dehydration
  • weight loss

Pasture contamination usually peaks over the summer months, falling off into the late summer and autumn. The hot, dry conditions experienced over previous months are likely to increase larval mortality and therefore reduce disease risk in most instances. However, groups of lambs grazing "dirty" pastures may still be at risk. Weaning can provide an opportunity to reduce risk of disease in these animals through a combination of anthelmintics and grazing strategies.

Advised actions include:

  • Monitoring for signs of PGE.
  • For lambs currently grazing dirty pasture:
    • Dose and move at weaning to safe pasture (eg. silage aftermath) where available.
      • Leave animals on dirty pasture for 2-3 days prior to moving.
      • Aim to leave at least 10% of the flock untreated.
  • Where safe pasture is unavailable:
    • Use targeted selective treatments (TSTs) based on monthly liveweight gain.
      • Generally only 40-60% of lambs should require treatment.
      • Treatment based on faecal egg counts where average worm egg counts in samples taken from 10-12 lambs are greater than >500-700 epg.
      • Where anthelmintic treatments are administered, it is strongly advised to check efficacy through worm egg counts:
        • Re-test 10-12 individuals at 7-14 days post treatment depending upon the product used.
        • Where resistance to one or more products is suspected consider strategic use of group 4-AD or 5-SI wormers.
          • Such strategies should be planned under veterinary guidance. For more information see the SCOPS guidelines, or seek the advice of a veterinary professional.
          • After weaning ewes should not require further anthelmintic treatment for roundworms until late pregnancy.
          • Rams are more susceptible to roundworm infections than ewes, and it is generally advisable to check burdens in these animals through a composite faecal egg count at this time of year.


Due to the ability of female Haemonchus contortus worms to produce huge numbers of eggs (up to 10,000 per worm per day) and the potential for these to develop rapidly on pasture to infective stage larvae mean risk of haemonchosis should not be ruled out based on recent weather patterns. Cases have been reported in the UK in July (Figure 4), and outbreaks may continue to occur into August. Outbreaks of haemonchosis are sporadic and difficult to predict, so vigilance is advised:

  • Acute onset disease is characterised by:
    • Anaemia caused blood feeding of adult worms, observable as pallor of the tissues around the eyes and general fatigue.
    • Oedema or fluid accumulation, including bottle jaw.
    • Sudden death in heavy infections.
    • Chronic infections may also occur, characterised by progressive weight loss, anaemia and loss of appetite.
    • Both lambs and ewes are considered at risk for haemonchosis.
    • Given the presenting signs described, haemonchosis can appear similar to fasciolosis. Diagnosis to further distinguish can be achieved through:
      • Post-mortem in cases of sudden death associated with severe outbreaks.
      • Faecal egg counts. These are generally very high and can be differentiated from other species of intestinal roundworm through special staining techniques. For more information please speak to your vet.


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Figure 4: A case of haemonchosis identified at post mortem in Dorset, July 2018. (Photo credit: Emily Gascoine, Synergy Farm Health)

Where outbreaks occur, treatment using most anthelmintic products are effective, although some evidence of resistance to white drenches (1-BZ) has been reported previously in the UK. Some flukicidal products, such as nitroxynil and closantel are also effective against Haemonchus worms.

Blowfly strike

Blowfly may continue to be a problem over the summer months. Hotter temperatures increase fly activity, although the drier conditions should reduce risk to some extent by reducing both maggot survival and risk of diarrhoea from PGE as previously described.

Animals with soiled fleece around their back ends, footrot lesions and/or open skin wounds are particularly at risk for blowfly strike due to the attractiveness of these to female blowflies (Lucilia) as sites to lay their eggs. Appropriate worm control will help keep fleece soiling to a minimum, whilst dagging provides a practical way of reducing risk in animals with diarrhoea and soiled fleece.

Control and treatment of blowflies and their maggots should be pursued through the application of appropriate products such as spot-ons, pour-ons or dipping. For a more thorough overview of blowfly risk, control and prevention please see the dedicated NADIS blowfly forecast (


Parasitic Gastroenteritis (PGE)

The recent hot, dry weather is likely to have decreased the survival of larvae on pasture. Larval pasture burdens for most cattle roundworms tend to peak in early to mid-July, although levels of Ostertagia ostertagi larvae can remain high until the end of the grazing season. Young stock in their first grazing season may therefore still be at risk of type-1 ostertagiosis, particularly if set stocked on permanent pasture. Animals should continue to be monitored for signs of disease and/or poor performance.

Advised actions include:

  • Monitoring for signs of PGE:
    • Loss of appetite
    • Loss of weight and body condition
    • Profuse diarrhoea
    • Where outbreaks of ostertagiosis occur treat all calves in the affected group.
    • Move calves to safe grazing such as hay or silage aftermath where available.
    • Continue targeted selected treatments based on liveweight gain, or faecal egg counts where weighing apparatus is unavailable.
    • Set-stocked pastures where calves have received strategic dosing through the season should have relatively low larval burdens. However, faecal egg counts to monitor the success of such anthelmintic dosing as outlined in the control of worms sustainably (COWS) group guidelines is advised:
      • Perform counts on faecal samples (10g) taken from 10-15 calves selected for treatment.
      • Repeat sampling of animals with a previously high egg count (e.g. more than 200 eggs per gram) at 14-17 days post treatment (7-10 days for 2-LV group products) to assess the reduction in egg count post-treatment. For more information, please speak to your vet.


Lungworm infection may continue to be a risk during the summer months, with cases reported earlier in the season (Figure 5). The recent dry, hot weather is likely to reduce pasture contamination as infective larvae die off, although larvae shed by already infected cattle may survive these drier periods within faecal pats to be dispersed onto pasture following periods of rainfall. It is therefore advisable to continue monitoring for signs of disease in at risk animals such as unvaccinated calves in their first grazing season on farms with a known history of disease. Early signs of infection include:

  • Widespread coughing in the group, initially after exercise then at rest.
  • Increased respiratory rate and difficulty breathing.
  • Rapid loss of weight and body condition.
  • Milk drop in lactating cattle.
  • Death in heavy infections.

Where infection is suspected treat animals with an anthelmintic (most products are effective) and remove affected cattle to "safe" pasture (e.g. aftermath) or house in a well ventilated building. Infection can be confirmed by post-mortem or detection of larvae in saliva or faecal samples. In lactating dairy cattle a milk sample antibody ELISAs is also available. It is also important to consider milk withdrawal periods when treating these animals.

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Figure 5: Lungworm infection diagnosed in Cheshire, late June 2018. (Photo credit: LLM Farm Vets)

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