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Editorial Information


Published 2016

Parasite Forecast - December 2016

16-12 PF Capture

On the first day of the month a shallow depression tracked east across Wales and then England, bringing some rain, but after that all parts of the UK became predominantly dry, as high pressure took charge. For much of the time the high was centred over Scandinavia, bringing easterly winds, and restricting temperatures to near the seasonal average but with plenty of sunshine, especially in western areas. As the second week of the month progressed, increasing numbers of showers encroached from the east, and around 15th-17th the weather was more generally unsettled with a depression making its influence felt. Thereafter there was again a build of pressure, giving mostly fine weather, though fog affected parts of England and Wales, particularly in the south, while frontal systems affected north-west Scotland as the month approached its end. The last four days of the month were notably mild over much of England & Wales. (

December Parasite Forecast/Update



  • Continue to monitor worm egg counts of pooled faecal samples where there is a risk of trichostrongylosis. Outbreaks of trichostrongylosis in store and replacement lambs, and even gimmers, are a risk during mild wet weather in November and December. Heavy infestations cause black foetid diarrhoea (black scour) and rapid weight loss.
  • As the weather becomes colder, most acquired Haemonchus larvae will arrest their development in the abomasal wall of the host sheep resuming development again in the spring if sheep are left untreated.  Treatment will be indicated in those flocks with a history of haemonchosis; note that arrested larvae will not produce eggs so faecal examination may yield a negative result.
  • It would be prudent to test faecal worm eggs counts of rams after the mating period as considerable loss of body condition may render them more susceptible to worm larval challenge/development.

16-12 PFF1

Trichostrongylosis in store and replacement lambs is a risk if mild wet weather continues through November.


Consider testing faecal worm eggs counts of rams after the mating period as weight loss may render them more susceptible to larval challenge.


Weight loss over the mating period may also render rams more susceptible to development of severe louse infestation.  Lice can be eliminated from the flock by various methods including correct plunge dipping.


Supplementary feeding after the mating period will help restore lost condition and resilience to gut parasites.


Excessive weight loss in a purchased ram (note remnants of sale number) due to poor parasite control (both liver fluke and gut parasites).

Liver Fluke disease

  • Where a second triclabendazole treatment has been required in the higher-risk, western regions of the country, the next fluke treatment can probably be delayed until January when closantel or nitroxynil should be used.
  • Albendazole and oxyclosanide are effective from 10-14 weeks post infestation and can be used when treatment is recommended to remove adult flukes in late spring (often in May).
  • Treatment in May will remove any surviving adult flukes and prevent pasture contamination by fluke eggs reducing the risk of disease later in 2017.


Lice in sheep

  • Louse populations are highest in sheep during late winter.
  • Spread occurs by direct contact between sheep.
  • Louse burdens are most severe in lean/emaciated sheep.
  • Louse infestations on sheep and cattle are highly visible and indicate/highlight poor stock management.
  • The welfare implications of heavy louse infestations must not be forgotten.
  • Use of plunge dipping for other reasons, such as control of sheep scab, cutaneous myiasis and headfly problems, eliminates lice.
  • Louse infestations can also be controlled with topical application of high cis cypermethrin or deltamethrin but these are best used soon after shearing.


Louse populations are highest during late winter and may cause disrupted feeding patterns, fleece damage/loss, and self-inflicted trauma.
Louse burdens are most severe in ewes in poor condition rather than lice causing marked weight loss. Note the numerous wool plucks over the ewe's flanks.  This sheep should also be checked for sheep scab.

Liver Fluke in Cattle

  • Out-wintered cattle considered at risk should be dosed for fluke with a product with activity against immature fluke.  Whilst triclabendazole is the most effective against early immature fluke, because acute fluke is rare in cattle,  consideration can also be given to using either closantel or nitroxynil.
  • Housed cattle, if not already dosed after housing, may be dosed with closantel or nitroxynil (when housed for 6-7 weeks or more) or a benzimidazole such as albendazole (when housed for 12 weeks or more), thereby avoiding the use of triclabendazole and reducing the selection pressure for resistance.
  • There are benefits associated with treating cattle at housing as this will immediately remove the impact of developing and adult flukes on growth and feed efficiency. Where this approach is practised, cattle should be tested later in the housing period for the presence of adult fluke, or given a second treatment to ensure all fluke are removed. The interval between housing and testing or re-treatment will depend on the product used.
  • Faecal egg counting, serum or bulk milk ELISA testing, and slaughterhouse liver reports, are practical methods of detecting fluke-infected herds.


Slaughterhouse liver reports are a practical means of detecting fluke-infected herds and flocks.


  • Autumn-born beef calves should not require anthelmintic treatment at housing.
  • Ostertagia larvae ingested by susceptible yearling stock during the autumn may have halted development and over-winter (as early L4) in the abomasal (stomach) wall.
  • These arrested larvae resume their development in late winter/spring and can cause outbreaks of acute scour with subsequent death (type 2 ostertagiosis).
  • Susceptible animals exposed to contaminated pastures during the later grazing season will be at risk from type 2 ostertagiosis unless they have been dosed with a anthelmintic effective against inhibited larvae,at housing, or at least before late winter.
  • Lungworm outbreaks decrease significantly from November onwards. Coughing in unvaccinated susceptible cattle at pasture before housing, or in un-dosed stock after housing, should be investigated for the possibility of lungworm larvae in faecal samples.


Autumn-born beef calves should not require anthelmintic treatment at housing.  Treatment of all stock for lice is strongly recommended.

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

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