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Editorial Information

Professor Mike Taylor BVMS PhD MRCVS DipEVPC DipECSRHM CBiol MRSB

Published 2018

Parasite Forecast - April 2018

18-04 Capture

February started off cold, becoming less cold in the middle of the month, but with snow towards the end of the month. Maximum and minimum temperatures were between 1.0 and 1.5 °C below average in most areas, with the UK mean monthly temperature 2.4 °C. which was 1.3 °C below than the 30-yr long-term average. Rainfall was 73% of average, and a few places had less than half the average rainfall total for the month, with only a few areas near or above average.

April Parasite Forecast/Update


Worming ewes at lambing time

With lambing now well under way, attention will be focusing on controlling parasite burdens in ewes.

The principle aim of worming ewes at lambing time is to minimise future pasture contamination with worm eggs during the 'peri-parturient rise" (PPR).

Eggs passed out in the faeces of infected ewes hatch and develop into infective larvae under favourable conditions, which can then lead to disease in lambs later in the season as larval numbers build up on the herbage.

Worm faecal egg output is much reduced in well fed ewes in good condition.

Provision of 'safe grazing' for ewes and lambs available at turnout will also help avoid the risk of worm infections in lambs later in the season.

  • 'Safe' pastures should not have been grazed by lambs the previous year and include fields grazed by cattle last year; and re-seeded pastures.

If only limited safe grazing is available, then this should be reserved for ewes with twin lambs whilst those with single lambs can graze the more contaminated pastures.

As wormer resistance is becoming more common, advice on worming ewes is changing, influencing both the choice and frequency of ewe treatment pre- and post-lambing.

The timing and choice of wormer are both important in controlling the PPR, as the ewes can quickly become re-infected, particularly when grazing heavily infected pastures post-turnout.

Current worming advice recommends leaving a proportion of the ewes untreated by targeting treatments to include;

  • Gimmers and young ewes
  • Ewes nursing twins and triplets
  • Ewes in low body condition

Ewes with single lambs or those in good body condition can be left undosed unless there is a risk from fluke or haemonchosis.

Persistent, or long-acting wormers, which provide a prolonged period of protection, if given later in the lactation period, before ewes become re-infected from the in refugia population, can be highly selective for resistance.

Worms in refugia include the population of worms present in untreated sheep and the free-living population of eggs and larvae not exposed to wormers.

The recommendation for long-acting formulations of moxidectin, is to use these products prior to lambing, or at turnout.

Further details can be found on the SCOPS website at

18-04 PFF1

Ewe worming treatments should aim to reduce pasture contamination during the periparturient rise whilst at the same time not selecting for anthelmintic-resistant strains of parasites.

18-04 PFF2

Some ewes nursing singles could be left untreated; seek veterinary advice for your farm.


Severe outbreaks of nematodirosis can occur in 6 to 12-week-old lambs usually from April to June in some years, depending on prevailing weather conditions.

Cold late springs followed by sudden changes in temperature can trigger a mass synchronised hatch of infective larvae leading to severe production losses and even death in lambs grazing contaminated pastures.

As February has been cold, and weather conditions during March and April can significantly alter early season predictions of nematodirosis for flocks lambing during March/April, an updated disease risk will be included with the NADIS May parasite forecast.

PF Map 16-04

Prediction of Nematodirus Peak Hatch for the week of 16th April.  To learn more about the full Nematodirus alert, click here

Disease outbreaks can be prevented by grazing lambs on pasture not grazed by lambs the previous year ('safe pasture').

Where this is not possible, and local weather conditions are such that an early hatch occurs, then late January/February-born lambs may need prophylactic anthelmintic drenching.

March/April-born lambs may require prophylactic anthelmintic drenching in May if cold weather prevails during March/April and hatching is delayed.

Continue to monitor the SCOPS ( and NADIS websites for regular updates on risk of disease in your area this year.

While incidents of wormer resistance have been reported with Nematodirus, white drench (1-BZ) wormers are still generally recommended to control this parasite.

As disease is primarily caused by developing larvae, faecal egg count (FEC) monitoring is unreliable in determining risk and the need to treat.

When a white drench (1-BZ) wormer is used in outbreaks of nematodirosis, the FEC of several lambs should be checked 10 days later for the presence of other worm species, which if present, would require treatment with a wormer from another group.

18-04 PFF3

Nematodirus infection in lambs.  These lambs suffered a serious and costly check in growth rate.


Outbreaks of coccidiosis may be encountered during April in lambs between 4-8 weeks of age, particularly in twin lambs grazing contaminated pastures.

Coccidiosis is a disease of intensive husbandry with stress a major factor in triggering outbreaks of disease.

Adverse weather conditions, poor colostrum supply, overcrowding, wet muddy paddocks previously grazed by sheep, and/or extended housing periods all predispose.

Reduction of stocking densities, batch rearing of lambs, creep-feeding and avoidance of heavily contaminated pastures/premises are measures that can be taken to reduce the risk of disease outbreaks.

Disease prevention can also include strategic dosing lambs on contaminated pastures with diclazuril or toltrazuril at around 3-4 weeks of age, or administration of medicated creep feed containing decoquinate for 28 days.

Liver fluke

Sheep on premises with known fluke populations, or in high-risk areas, should already have been dosed in the autumn and early winter but may need to be dosed again this spring.

Cases of chronic liver fluke disease are being reported this winter from the predicted high-risk areas of the country and may still be encountered in sheep flocks this spring.

The disease can be confirmed by checking for the presence of fluke eggs in faeces or by use of abattoir feedback and post mortems to determine on-farm control measures.

On positive farms, the presence of fluke eggs in faecal samples reflects inadequate control of liver fluke disease and control should be reviewed.

Limiting pasture contamination with fluke eggs from patent infections will help reduce subsequent fluke challenge later in the year.

Flukicides containing closantel, nitroxynil, oxyclozanide or albendazole (at the fluke dose rate), are all effective against adult flukes present during the spring and should be used to reduce reliance on triclabendazole, especially where resistance has been reported.

Sheep should always be moved to clean pastures after treatment; and supplementary feeding may be necessary to maintain condition.


Type II Ostertagiosis

Housed, yearling cattle not dosed in the autumn, may be at risk from type II ostertagiosis towards the end of the housing period.

Disease may occur due to the resumption in development of large numbers of immature larvae acquired the previous autumn and which entered a state of arrested development (hypobiosis) over the winter months.

Prevalence of clinical disease is usually comparatively low and only a proportion of animals in a group may be affected.

The disease presents as intermittent diarrhoea with loss of appetite and rapid loss of body weight.

Mortality in affected cattle can be high unless prompt treatment with a wormer effective against both arrested and developing larval stages is given.

Worm Control

Decisions should have been made on the parasite control plan for the forthcoming grazing season.

Prevention of parasitic gastroenteritis (PGE) in growing cattle on a sustainable basis is best achieved by annual rotational grazing (cattle/sheep/crops) but this is not often possible on many farms.

Parasite control plans based on anthelmintic use may be strategic (early season dosing) or "wait-see" (monitor/treat in the latter part of the grazing season).

18-04 PFF4

Unless safe grazing is available, dairy calves and weaned beef calves born during the previous autumn require preventive treatment in their first full grazing season to control PGE.

To be effective, strategic worm dosing needs to be initiated at, or around turnout, to limit pasture contamination up to mid-July by which time the over-wintered larval population should have declined to insignificant levels.

Strategic treatments include administration of a bolus at turnout. or administration of pour-on, or injectable macrocyclic lactone (ML) at defined intervals.

Cattle treated strategically should remain set-stocked or moved to safe pastures (aftermaths) when these become available.

If a "wait and see" strategy is to be adopted then ensure that effective, regular monitoring and diagnostic procedures are in place to act quickly if required.

Where lungworm is a problem, there is still time to discuss control, including vaccination, with your veterinary surgeon before turnout in most areas.

Vaccination of calves over two months-old requires two doses of lungworm vaccine four weeks apart with a second dose at least two weeks before turnout.

The lungworm vaccine is a live attenuated vaccine with a short shelf-life, so ordering and administration needs to be planned well in advance of turnout.

For more information see the COWS ( website.











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