Parasite Forecast

Issue: November

Weather report

The average UK temperature in September 2018 was 12.4oC, 0.2oC below the long-term national average (1981-2010). Regionally, temperatures were similar to their respective long-term averages in September, with cooler than average conditions experienced further north; Northern Ireland, Scotland, NW England and N Wales. However, regional temperatures for the previous 3 months (July – September) were still above the long-term average for all regions.

Total rainfall in September 2018 was 108% of the long-term national average, although this varied greatly between regions, with rainfall in NW Scotland 150% of the long-term average for the region, whilst Northern Ireland, SE England and East Anglia all experienced lower than expected rainfall for the month. Regional rainfall for the previous 3 months was below the long-term average across all regions with the exception of NW and SW Scotland.

Liver fluke: provisional Autumn forecast

The provisional fluke forecast for November 2018 is based on monthly rainfall and temperature data from May-September 2018.

This provisional forecast is currently predicting Medium risk for liver fluke in SW Scotland and Low risk across all other areas of the UK (Figure 1), although it should be noted that some parts of western Scotland have been flagged as medium to high risk. The definitive risk forecast for liver fluke in 2018 will be provided in next month’s forecast.


Figure 1: Current regional risk for liver fluke in Autumn 2018. Remember it is important to take local conditions into consideration when evaluating on farm risk.

Whilst regional fluke risk is low, it is important local factors are taken into account when considering on farm risk. Development of liver fluke and its intermediate host, the mud snail (Galba truncatula; Figure 2) on pasture is dependent upon temperature and moisture, with warm wet conditions optimal. On farms with permanently wet pastures and/or permanent water bodies snails may have continued to thrive in hot weather over the summer, potentially leading to high pasture infectivity in the autumn and winter months.

Figure 2: The intermediate host of liver fluke, the mud snail (Galba truncatula), can be found in damp, muddy areas such as the borders of ponds, streams and ditches, and in areas of boggy pasture and wet flushes. Such areas should therefore be considered potential sources of liver fluke infection.

Despite the overall low predicted risk, continued vigilance for signs of disease and on farm contingency plans in the event of an outbreak are of great importance.

Advised actions include:

  • Monitoring for signs of disease:
    • Sudden death in heavy acute infections
    • General dullness, anaemia and shortness of breath
    • Rapid weight loss, fluid accumulation
  • Animals with chronic infection may show no obvious signs of disease, yet such infections can adversely affect productivity by reducing fertility, growth and milk yields. Chronic infection should be considered where animals have been grazing at risk pastures. In such instances, the decision to treat animals should be informed by diagnostic testing such as worm egg counts, evidence of poor body condition score and/or sub-optimal productivity.
  • Routine diagnostic testing to give a greater insight into current infection status:
    • Post-mortem (Figure 3).
    • Antibody ELISAs are available for testing individual sheep and cattle through blood sampling, or to monitor herd-level infection status in dairy cattle through testing bulk milk tank samples.
    • A faecal antigen test is also available for testing individual animals.
    • Worm egg counts can be used to diagnose chronic infection in individuals, or groups of animals when using a composite sample. Egg counts cannot used for the diagnosis of acute disease.
    • For more information on diagnostic options, please speak to your vet.


Figure 3: Liver fluke infection can be detected at post-mortem as parasitic tracks on the liver in acute disease (left), which is common in sheep in late summer and autumn. Chronic fluke infection may also be detected at post-mortem in sheep (centre) and cattle (right). Chronic infections may have profound impacts on animal productivity, yet show very little outwards signs of disease.

 Where fluke infection is identified:

    • Treatment with triclabendazole is recommended for acute disease, as this is the only product effective against both adult and immature stages of the parasite.
    • With chronic infections, a product other than triclabendazole should be considered, as this type of disease is caused by adult fluke.
    • It is advised to test for treatment efficacy through pre- and post-treatment diagnostic testing.
    • For more information about treatment options and efficacy testing, please speak to your vet.
  • Risk of infection can be reduced by identifying high risk fluke pastures and avoiding grazing these during peak risk periods.
    • Both sheep and cattle are susceptible to infection with liver fluke, meaning pastures previously grazed by either species should be considered a potential risk to the other.
    • Mud snails are generally found in damp, muddy areas (Figure 4).

Figure 4: Areas of permanently wet pasture can provide optimal conditions for mud snails, and by extension liver fluke. Large numbers of mud snails were recovered from the wet boggy areas pictured above, which were created by a constant supply of flowing surface water and poaching by cattle.


Parasitic Gastroenteritis (PGE) and haemonchosis

The larvae of many disease-causing roundworm species will remain infective on grazing pastures to the end of the season. There have been reports from across the UK in September and October showing roundworm infections continue to persist, with high worm egg counts and outbreaks of disease. It is therefore important to maintain vigilance for signs of disease in at risk animals grazing potentially contaminated pastures.

Outbreaks of PGE resulting from trichostrongylosis may be seen from late October onwards in store and replacement lambs and, sometimes, yearlings (Figure 5).

Risk of haemonchosis may also continue to be an issue. Female worms are prolific egg-layers, meaning pastures can become contaminated rapidly under optimal conditions. As the weather becomes colder, newly acquired Haemonchus larvae will begin to encyst in the abomasal wall of the host sheep resuming development in the spring if left untreated. It should be noted that in such infections FECs may be negative.

Figure 5: Trichostrongylosis is a common problem in store and replacement lambs in the autumn and winter months

It is also important to consider the infection status of rams post breeding, as the combination of a high workload and parasite burdens may contribute to a significant loss in body condition in these animals just ahead of winter (Figure 6). Worm egg counts can help to determine whether anthelmintic treatment is necessary.

Figure 6: It is important to remember worm control in rams. High parasite burdens in combination with the high workload during tupping can have a profound impact on health and body condition ahead of the winter months.

Advised actions include:

  • Monitoring for signs of disease:



·         loss of appetite

·         Diarrhoea (black scour)

·         Dehydration

·         Rapid weight loss

·         Sudden death (acute infections)

·         Anaemia and general fatigue (Figure 4)

·         Oedema or fluid accumulation (e.g. bottle jaw)

·         Progressive weight loss and loss of appetite (chronic infections)

  • Signs of chronic haemonchosis can be very similar to liver fluke infection. If you are in any doubt over which parasite is causing disease, please seek veterinary advice regarding diagnosis and treatment.
  • Consider worm egg counts and monitoring weight gain in lambs to determine infection status and need for treatment.
  • Where anthelmintic treatments are required:
    • Move to safe pasture (eg. silage aftermath) after treatment if available.
      • Leave animals on dirty pasture for 2-3 days prior to moving.
      • Aim to leave at least 10% of the flock untreated
    • Check efficacy through worm egg counts:
      • Re-test 10-12 individuals at 7-14 days post treatment depending upon the product used.


Dosing at housing

Housing presents an opportunity in the ongoing control of parasitic disease on farm. Worm burdens acquired over the grazing season may be targeted effectively at this time without risk of animals becoming re-infected. There are also benefits on growth and feed efficiency associated with treating cattle for fluke at housing. As is the case during the grazing season, any treatments administered should be based on evidence of infection, such as clinical disease and/or diagnostic testing (e.g. worm egg counts).

Treatment efficacy should also be considered at this time since ineffective treatments may result in a continued parasitic burden (with associated reductions in productivity) and potential pasture contamination with resistant parasites at turn-out the following grazing season.

  • For growing cattle housed after their first or second season treatment with products containing either a Group 3-ML or Group 1-BZ anthelmintic is recommended at housing. These products are effective against encysted stage larvae acquired in the latter stages of the grazing season. If untreated, heavy burdens of encysted larvae can cause type-2 ostertagiosis in later winter or the following year due to triggered mass emergence.
    • Encysted worm burdens cannot be assessed by worm egg count.
  • Whilst housing effectively eliminates risk of infection with pasture associated parasites such as intestinal roundworms and flukes, risk of louse infestations may increase at this time since these parasites can spread directly from animal to animal very easily once they are penned in close proximity to one another in what are relatively dry, warm conditions. Group 3-ML pour-on preparations have the added advantage they are also effective against both sucking and chewing lice (Figure 7).
  • Cattle exposed to liver fluke infection can either be dosed with a product containing triclabendazole  at housing, or use an alternative product at a delayed interval post-housing. Provided levels of infection are not too severe, the latter option is more desirable in the context of drug resistance, since it allows triclabendazole to be used more sparingly, preserving its efficacy on farm for when it is most needed (i.e. in acute clinical disease).
    • If using a product other than triclabendazole it will be necessary to either repeat or delay treatment so that all flukes are of a sufficient age for the drugs to be fully effective. For example, if treating with closantel COWS recommend delaying treatment for 6-7 weeks post-housing before treating, or treating at housing then testing 6-7 weeks later to determine whether a second treatment is needed.
    • Some of these alternative products, such as albendazole,  oxyclozanide also have the added benefit of being licenced for use in lactating animals provided milk withhold periods are observed. It is important to check labelling of individual products.
    • Whichever product is ultimately used, cattle should be tested later in the housing period to assess treatment efficacy.

For more information please speak to your vet, or visit the COWS website.


Figure 7: Pour-on preparations of 3-ML wormers are also effective in controlling sucking and chewing lice.

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


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