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Editorial Information

Professor Mike Taylor BVMS PhD MRCVS DipEVPC DipECSRHM CBiol MRSB

Published 2018

Parasite Forecast - June 2018


The cold weather in March continued into early April with snow for some, mainly on high ground. The first half of the month continued unsettled and generally cloudy, although generally less cold becoming hot after the middle of the month followed by cooler, unsettled weather towards the end of the month. The UK mean temperature was 8.4 °C, which is 1.0 °C above the long-term average, with maximum temperatures 0 - 1 °C above average in most areas. Mean minimum temperatures were also 0 - 1 °C above average in Scotland and Northern Ireland, but more than 2 °C above average in parts of the Midlands and Southern England.  The mean minimum temperature for England was the second highest since 1910 and temperatures in mid-April were the highest since 1949.  Rainfall was 119% of average, with most places near, or above average, apart from northern Scotland where it was drier than average.

June Parasite Forecast/Update


Parasitic gastroenteritis (PGE)

  • A key component of the farm's sustainable parasite control programme is forward planning for the provision of "safe grazing" from the start of the grazing season.
  • One of the benefits is that lambs grazing safe pastures with their dams, shouldn't need worming until after they are weaned.
  • Ideally, wean the lambs onto silage or hay aftermaths that have not been grazed by sheep earlier in the year, as by mid-summer any over-wintering larvae will have died off and the fields can then be considered "safe".

18-06 PFF1

Planning ahead - silage aftermath should be grazed by weaned lambs as part of the sustainable parasite control programme on your farm.
  • Lambs grazing permanent pastures will usually require worming to limit the build-up of infective larvae later in the season ('mid-summer rise').
  • Weaned lambs can be moved to aftermaths. When lambs are dosed, then delay the move to allow the treated flock to become "lightly" re-infected to reduce the selection pressure for resistance development.
  • The timing of the move and need for worming treatment(s) for lambs will depend upon grazing history, levels of contamination by periparturient ewes, stocking density, and prevailing weather conditions.
  • Persistent moxidectin formulations should not be used in delayed "dose and move" strategies and partial flock treatments are the only option.
  • Where aftermaths are not available, then performance monitoring, or worm faecal egg counts (FECs) on lambs every 2-4 weeks from June onwards, can be used to guide anthelmintic treatments.
  • Pooled faecal samples from approximately 10-12 lambs in a group can be used for FEC and will help decide the need for treatment.
  • Treatment is generally recommended when FECs exceed 500-700 epg.
  • Follow SCOPS recommendations by leaving some lambs untreated and monitor treatment efficacy by performing a drench test post-treatment (see targeted selective treatments below).
  • Prolonged local dry weather conditions during summer can delay larval challenge to lambs grazing contaminated pastures with a return to infectivity when wet weather arrives.
  • Remember to include rams in the farm's parasite control programme because they are fully susceptible to PGE.
  • It will take at least two months of good grazing for a ram to gain one unit of condition score with a target of 3.5 at mating in October/November.
  • Rams typically graze the same field every year with significant build-up of infection on this pasture.

18-06 PFF2

Include rams in the farm's parasite control programme as they are fully susceptible to PGE; these rams need to start gaining condition before mating from September onwards.

Targeted Selective Treatments (TSTs)

  • Some lambs in the flock in good body condition and performing well can be left unwormed. In general, only 40-60 per cent of lambs require worming.
  • Target anthelmintic treatments for those lambs that are failing to meet expected growth rates.
  • Regular weighing also identifies poor growth which may be caused by overstocking, trace element deficiencies etc. and prompt management review.
  • Electronic identification and automated weight recording simplifies TST.
  • This strategy greatly reduces the likelihood of selecting for resistant strains of worms by allowing a pool of unselected parasites to pass out eggs onto the pasture helping to maintain wormer efficiency in the longer term.
  • Accurate record keeping also allows selection of the best genetics in the flock with breeding stock kept based on performance not appearance.

18-06 PFF3

TST selects only those lambs for treatment that are failing to meet expected growth rates.

Drench Testing

  • Testing for the presence of wormer resistance is an increasingly important part of maintaining an effective worm control strategy.
  • The cheapest and simplest way is to perform a drench test to check on anthelmintic efficacy.
  • Collect faecal samples as previously described, both prior to worming, and again at either 14 days post-treatment for benzimidazole (1-BZ) and macrocyclic lactone (3-ML) wormers; or 7 days post-treatment for levamisole (2-LV) wormers.
  • If mean FECs have been reduced by <95% then resistance is suspected.

Fly Strike

Blowfly Alerts, updated monthly through June and July predict the risk of blowfly strike throughout the UK using met office data and a detailed understanding of fly biology.

  • Blowfly strike affects around 80% of UK sheep flocks each year.
  • Female flies are attracted by the odour of decomposing matter such as wounds or soiled fleece.
  • Shearing reduces the risk of blowfly strike in adult sheep.
  • Treat all shearing cuts immediately to prevent fly strike and further self-inflicted injury by the sheep.
  • Preventing diarrhoea caused by worm infections will greatly reduce the risk of blowfly strike on the breech.
  • Active maggot infestations can be treated using pour-on products containing (alpha) cypermethrin; spot-on products containing deltamethrin; or by dipping in diazinon dip baths.

18-06 PFF4

Treat all shearing cuts immediately to prevent fly strike and further self-inflicted injury by the sheep.


  • Strategic worm control in cattle is usually applied to autumn/winter-born weaned calves in their first grazing season, and in spring-born beef suckler calves in their second grazing season.
  • Wormers should always be administered following the COWS 5 R's principles - Right product; Right animal; Right time; Right dose; given in the Right way (see for further details).
  • Cattle receiving strategic anthelmintic treatments in the early part of the grazing season should remain on the same pasture during the entire grazing season or moved to safe pastures (aftermaths) when these become available.
  • Strategic regimes based on products with prolonged persistence against the stomach worm, Ostertagia ostertagi, may allow the build-up of other worm species against which they have less efficacy, or little persistent effect (for example Cooperia species), which may appear later in the year.
  • Worm infections can reduce growth rate by around 30% in beef calves and replacement dairy heifers, even with low levels of worm challenge.
  • In dairy cows, worm infections can cause a drop in milk yield of 1kg per day.
  • Incidents of clinical disease due to ostertagiosis in cattle peak during August/September.
  • In severe infections, there is ill-thrift, loss of body condition and diarrhoea.

18-06 PFF5

Strategic regimes are commonly used to control Ostertagia ostertagi infestation in autumn-born beef cattle during their second grazing season.


  • Lungworm disease ('Husk') can appear from June onwards in unvaccinated calves, those cattle without an effective anthelmintic programme, and naïve adult cattle.
  • Adult dairy cattle which have not built up immunity through natural challenge in previous years are susceptible to lungworm.
  • Early signs include coughing, initially after exercise then at rest, increased respiratory rate and difficulty in breathing.
  • Affected cattle rapidly lose weight and body condition and should be removed from infected pasture and treated as quickly as possible.
  • Adult dairy cows may show a sudden and dramatic drop in milk yield
  • Supportive therapy may be required depending on clinical presentation.

18-06 PFF6

Severe lungworm infestation in a susceptible dairy cow from an organic farm where vaccination was not used.

18-06 PFF7

Large numbers of lungworms in the airways can result in severe respiratory distress and death.




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