Parasite Forecast

Issue: January

Weather report

Average UK temperature in November 2018 was 7.3oC, which is 1.1oC above the long-term national average (1981-2010). This was the case across all regions of the UK, with temperatures also above average for the previous 3 months (September-November) for all regions except Northern Ireland.

Overall, rainfall was 101% of the long-term average for the UK in November, but varied greatly between regions: rainfall was above average in southwest and east Scotland, southwest and southeast England, south Wales and Northern Ireland, and below average in north Wales, northeast and northwest England, East Anglia. Regional rainfall in November also bore little resemblance to overall rainfall from September-November, with below average rainfall observed across England, Wales and Northern Ireland, and above average rainfall for this period in Scotland.

With average temperatures dropping, it is likely that development of eggs being shed onto pasture will begin to slow considerably. However, it is important to remember that already infective pastures may continue to be a source of disease, with the some roundworm species able to survive the colder conditions present on pastures over winter. It is therefore important to continue monitoring at risk groups of grazing animals for signs of infection and disease.

Parasite Control Planning

Winter provides a good opportunity to review and plan on-farm parasite control strategies ahead of coming grazing season. Preparation will help you to develop a robust yet practical programme, which should ultimately help reduce disease burdens, costs, and selection for anthelmintic resistance. Please speak to your vet or SQP about devising a parasite control plan to work for your farm. More information on sustainable parasite control is sheep and cattle can be found on the “SCOPS” and “COWS” websites, and cattle and sheep-specific parasite control planners are available through NADIS (Figure 1), which can help devise and visualise your plan for the coming year. Important things to bear in mind include:

  • Seasonal risk and farm history. Different parasites cause problems at different times of year and under different conditions. When planning for the future bear in mind what specific issues you have encountered in the past and when these have occurred. The NADIS parasite forecast is published monthly and can be a useful resource to highlight specific seasonal and regional risks, including the Nematodirus and fluke forecasts.
  • Identify at-risk animals. Generally, younger animals are more susceptible to parasitic disease, particularly calves and lambs entering their first (and potentially second) grazing season, although other groups may also be at risk, such as adult cattle brought on to farms with a history of lungworm.
  • Choice and rotation of anthelmintics. Make sure you are familiar with the products available to you, their active ingredients and the particular indications for their use. Appropriate choice of wormers and product rotation will help reduce selection for anthelmintic resistance on your farm.
  • Bio-security and quarantine treatments. Appropriate control measures surrounding holding and quarantine treatment of purchased animals prior to turnout on to your pastures will help prevent introduction of anthelmintic resistance on to your farm.
  • Diagnostic and performance testing. Diagnostic tests such as worm egg counts provide key information when deciding between treatment and pasture management options. Performance testing can also help provide a more targeted approach to worm control through identification of those animals worst affected by parasite burdens. Routine diagnostic testing should ideally be at the centre of any sustainable parasite control scheme. If used correctly, diagnostics can identify issues before they become severe, reduce the amount of anthelmintics required and improve timing of their administration, ultimately reducing costs and boosting productivity.
  • Identify “safe” and “contaminated” grazing. Planning of pasture management and rotation should be done in conjunction with diagnostic testing and treatment regimens to help reduce exposure, infectious burden and disease in at risk groups of animals, and selection for anthelmintic resistance. Contaminated pastures are those grazed previously by infected animals, including pastures grazed the previous season in some instances. Safe pastures are those which have not been grazed previously, such as freshly seeded leys, and silage and hays aftermaths as they become available later in the season. Pastures previously grazed by sheep are generally safe for cattle and vice versa with respect to roundworms, although it is important to note this is not necessarily the case with liver fluke.
  • Other work planned through the year. Incorporating parasite control into other seasonal activities such as spring turnout, shearing, winter housing etc. can help with implementation. An important aspect of for any successful parasite control plan is that it is adhered to. Simplifying and doubling up with other activities where possible will help facilitate this.

Figure 1: Cattle and sheep specific parasite control planners are available through NADIS and can help develop a sustainable, practical on-farm strategy.

Liver Fluke

Whilst the Autumn fluke forecast for 2018 predicted only Medium risk  in southwest Scotland and Low risk in all other regions (Figure 2), it is important to continue to be vigilant for signs of disease where this parasite has been known to be a problem previously, particularly since the milder than usual weather experienced in November may have allowed prolonged development of the infective stages of fluke later into the season.

Figure 2: Regional risk for liver fluke in autumn 2018. It is important to remember this forecast is a guide, and local conditions should be considered when evaluating on-farm disease risk.

It is also important to be on the lookout for chronic infection at this time of year. Chronically infected animals may show no obvious signs of disease, but can have significant reductions in productivity (Figure 3). Furthermore, chronically infected sheep and cattle can remain infected for months or even years if untreated, making them an important source of pasture contamination in the coming season.

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
  • In the absence of any obvious signs, chronic infection should still be considered in animals that have previously grazed at risk pastures (Figure 3). Such animals can be identified through diagnostic testing.
    • Worm egg counts can be used to diagnose chronic infection using faecal samples from either individual animals, or to determine infection status in groups of animals a pooled sample from ten animals representative of the overall group.
      • It is important to note egg counts cannot be used to diagnose acute disease.
    • For more information on diagnostic options and sampling, please speak to your vet.
  • Routine clostridial vaccination to prevent Black disease and should be considered if not already in place.

Figure 3: Chronic fluke infection in sheep caused by adult flukes living in the bile ducts. Such animals may show no obvious signs of disease so it is important to check for infection through diagnostic testing.

 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.
  • In housed animals consider use of a product other than triclabendazole to treat chronic infection due to concerns over emerging drug resistance.
    • If treating cattle with an adulticide product like closantel, the “COWS” group recommend repeated or delayed treatment at 6-7 weeks post-housing (
    • For sheep, the “SCOPS” group suggest using closantel or nitroxynil at 3 weeks post housing, with a further treatment to kill any residual adult parasites in the spring (
  • Due to concerns over emerging drug resistance, testing for treatment efficacy through pre- and post-treatment diagnostics is also advised.
  • For more information about how best to implement the various treatment and control options and conduct efficacy testing on your farm, please speak to your vet.


Parasitic Gastroenteritis (PGE)

The larvae of many disease-causing roundworms, particularly Trichostrongylus, may continue to be a risk in store and replacement lambs and, sometimes, yearlings into the winter (Figure 4). It is therefore important to maintain vigilance for signs of disease in at risk animals grazing potentially contaminated pastures.

It is also important to consider that acquired larval infections of abomasal roundworms (including Haemonchus and Teladorsagia) will undergo encystment or “hypobiosis” during the winter months, resuming development in the spring. Such populations can be an important source of pastures contamination the following spring, and in yearling lambs with heavy infections may cause scours in a similar manner to type-2 ostertagiosis in cattle. In such infections FECs can be negative, so it is important to consider grazing history and previous management when identifying at risk animals.  These hypobiotic infections can be targeted with treatments effective against arrested larval stages, with products available in most major worming groups. For more information on anthelmintic selection and treatment options please speak to your vet or SQP.

Figure 4: Trichostrongylosis, characterised by black scour, is a common problem in store and replacement lambs in the autumn and winter months

Advised actions include:

  • Monitoring for signs of disease:
    • Loss of appetite
    • Diarrhoea (black scour)
    • Dehydration
    • Rapid weight loss
  • Consider worm egg counts and weight gain in lambs to determine infection status and need for treatment, but remember this will not identify hypobiotic larval infections.

Where anthelmintic treatments are required:

  • Move to safe pasture 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.

 Scab and Lice

Scab (mite) and louse infestations can become a significant problem in sheep flocks over the autumn and winter months, typically September-April. Whilst the signs of scab and louse infestations (pediculosis) are similar, treatment options vary due to scab mites living beneath the surface of the skin, whilst lice reside within the fleece. This makes diagnosis an important first step towards treatment.

Sheep scab is caused by psoroptic mites (Psoroptes ovis; Figure 5). Infestations cause loss of condition, secondary skin infections and eventually death if untreated. Signs include severe itching, wool loss, restlessness, biting and scratching of affected areas and weight loss or reduced weight gain. When examined, the fleece may be wet, sticky and yellow due to serum discharge and the skin may become thickened and corrugated (Figure 6). Studies show scab mites can remain infective in the environment for up to 17 days. Consequently, fields, sheds and pens where infected sheep have been kept and handled should be considered a potential source of infection to other sheep during this period.

Figure 5: Psoroptic mites (left) can be identified from skin scrapings, whilst louse infestations can be confirmed in affected fleece (right). Photos courtesy of Dr Joseph Angell.

Louse infestations in the UK are caused by chewing lice (e.g. Bovicola ovis) and may present in a similar way to scab. Lean or emaciated sheep tend to be most severely affected, with widespread louse infestations often indicative of an underlying problem with flock management.

To reach a diagnosis:

  • For scab, diagnosis can be made through microscopic examination of skin scrapings from suspected animals to detect the mites (Figure 5), or by detection of antibodies in blood samples with an ELISA test.
  • Chewing lice reside in the fleece and can identified from wool samples taken from affected areas. Lice can sometimes be seen with the naked eye and confirmed by microscopy (Figure 5).
  • For more information concerning diagnosis, please speak to your vet.
  • It is important to remember sheep scab is notifiable in Scotland.

Figure 7: Severe case of sheep scab characterised by wool loss, serous exudate and thickening of the skin.

 Where treatment is required:

  • First, ensure the correct diagnosis has been reached.
  • Injectable macrocytic lactone (3-ML) products are effective against sheep scab with varying periods of protection. For more information concerning treatment with 3-MLs please speak to your vet or SQP.
    • It is important to remember that 3-MLs are also an important class of anthelmintics and, if used, should be factored into your roundworm control strategy.
    • There is new evidence suggesting emergence of resistance in scab mite populations to treatment with 3-MLs in the UK. It is therefore vitally important to ensure correct diagnosis and treatment protocols are adhered to, and that veterinary advice sought if treatment failure is suspected.


Parasitic gastroenteritis (PGE)

For growing cattle housed after their first or second season, treatment with a product containing either a 3-ML or 1-BZ anthelmintic is recommended as these are effective against encysted larval infections which may have been acquired in the latter stages of the grazing season. If untreated, heavy burdens of encysted larvae can cause type-2 ostertagiosis due to triggered mass emergence in late winter/ early spring. Encysted larval Ostertagia infections cannot be assessed by worm egg count.


Louse and mite infestations in cattle are not uncommon during winter housing. Generally low level infections are not of major concern, but heavy louse infestations may indicate an underlying management or health issue and, where sucking lice are present, may cause anaemia compounding existing problems (Figure 7).

A range of pour-on or spot-on synthetic pyrethroid products are available with efficacy against lice, whilst group 3-ML pour-on worming preparations are also effective. Injectible group 3 ML preparations are also effective against sucking lice.

A relatively small number of injectable and pour-on group 3-ML based products are available in cattle against mange mites, with some pour-on synthetic pyrethroids preparations effective against sarcoptic and chorioptic mange. Where mange infestations are a suspected cause for concern, please seek veterinary advice concerning further diagnosis. For more information on ectoparasite control in cattle please speak to your vet or SQP, and see the “COWS” guidelines.


Figure 7: Louse infestations are not uncommon in cattle housed over winter. Heavier infestations may indicate, and contribute to, underlying health and management issues.

John Graham-Brown BVSc (hons) MSc (hons) PhD MRCVS
January 2019


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