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NADIS Parasite Forecast - May

Use of meteorological data to predict the prevalence of parasitic diseases

PF Capture

At the start of the month, the UK was in an unsettled Atlantic weather type bringing rain and strong wind at times. A succession of active depressions affected mainly northern and western areas from 5th to 12th, after which pressure built and the weather was more settled for most. The final week was rather unsettled with rain or showers generally and some very strong winds. It was generally driest in southern areas (

May Parasite Forecast/Update

The most recent version of this monthly parasite forecast may be accessed at


Control of PGE in adult sheep and lambs after lambing

  • Ewes and lambs remaining on safe grazing should not require anthelmintic treatment until lambs are around four months-old but this situation will be guided by faecal egg counts of lambs.
  • Ewes grazing contaminated pastures should have been treated with a persistent anthelmintic before turnout which will prevent re-infection from the pasture for several months and help to reduce subsequent larval challenge of lambs.
  • The warm weather during April will have promoted hatching of Nematodirus battus larvae.  February and early March lambs grazing contaminated pastures will require anthelmintic treatment during the second half of April to prevent/control nematodirosis. Late March/April lambs should not face a significant challenge because they are yet to start consuming significant amounts of grass (and larvae).
  • Local conditions will affect hatching so consult your veterinary practice for farm-specific advice on Nematodirus battus.
  • Monitor faecal eggs counts from rams regularly because they are more susceptible than ewes to PGE and often graze the same contaminated field every year.
  • Mean UK monthly temperatures from December 2014 to April 2015 have been above the 1961-1990 monthly averages suggesting an early hatch of Nematodirus battus eggs present on contaminated pastures which will require action now for flocks with February/early March lambs in England.  Hatching may be up to two weeks later in Scotland.
  • An average risk of disease this spring is expected for late March/April-born lambs because  there will be infective larvae  present on pastures the lambs are yet to start grazing.
  • Adult sheep are not affected by Nematodirus battus larvae.
  • Benzimidazole (Group 1) wormers are usually recommended for Nematodirus control but will be largely ineffective against other gut parasite species.
  • Faeces samples should be collected from lambs after prophylactic anthelmintic treatment to ensure that any adult Nematodirus parasites have been killed and that there are no eggs present from other gut parasites such as Teladorsagia spp.

Nematodirus control on contaminated pasture

Fig 1

Excellent farm planning - these lambs on safe grazing will not require anthelmintic treatment for Nematodirus parasites.


Fig 2

Lambs grazing contaminated pasture.  It can prove difficult to differentiate diarrhoea caused by nematodirosis from coccidiosis and your veterinary surgeon should always be consulted.

Liver fluke

While autumn/winter 2014 was a low to moderate risk year for liver fluke in all regions, sheep on farms with a known fluke problem should be dosed in April/May with a flukicide active against mature flukes. Do not use triclabendazole at this time as it may hasten the selection for resistance.

Blowfly strike

  • Almost every flock experiences blowfly strike - plan now to limit its impact.
  • Even very small fly strike lesions cause disrupted grazing and rapid weight loss.
  • Topical preparations to prevent blowfly strike are most effective when applied before the risk period, usually from mid-May onwards.
  • Cyromazine and dicyclanil preparations do not treat blowfly lesions; these products are designed to prevent establishment of infestations.
  • Some products have long meat withdrawal periods so careful planning is essential (see below) when treating lambs.
  • Preventing diarrhoea caused by parasitic gastroenteritis will greatly reduce the risk of blowfly strike on the breech.
  • Footrot lesions with granulation tissue, dermatophilosis (lumpy wool) lesions on the skin, and urine scalding around the prepuce also attract egg-laying adult flies.
  • High cis cypermethrin pour-on preparations provide protection against fly strike for up to 6 to 8 weeks but may require re-application. (Meat withdrawal period 8 days).
  • Cyromazine applied before the risk period provides protection against blowfly strike for up to 10 weeks after topical application. (Meat withdrawal period 28 days).
  • Dicyclanil affords 8-16 weeks' full body protection against blowfly strike. (Meat withdrawal period 7 and 40 days; 1.25% w/v suspension and 5% w/v suspension, respectively).
  • Organophosphorus dips provide good protection against blowfly strike for up to six weeks (Meat withdrawal period 49-70 days).


Fig 3

Flystrike problem waiting to happen - this lamb must be dagged immediately and a suitable preparation applied topically to prevent myiasis. A review of PGE control measures on the farm should be undertaken with the farmer's veterinary surgeon.


Fig 4

Effective dagging and topical application to prevent blowfly strike have been undertaken.


Fig 5

Even small fly strike lesions cause disrupted grazing and rapid weight loss.  Financial losses can be considerable.


Fig 6

Blowfly strike attracted by neglected footrot lesion.


Fig 7

Some topical preparations are only effective when correctly applied before blowfly lesions develop. Do not wait for flystrike to happen before taking preventive action.


Fig 8

Dispose of all animal carcases. This farmer's son shot rabbits for sport and left the carcases on pasture - note the large number of blowflies enjoying this free food source.


  • To control ostertagiosis later this year, dairy calves and suckled calves require preventive treatment for their first full grazing season unless they are on safe grazing.
  • If pasture egg contamination is suppressed by pulse release or continuous release bolus, repeated or long-acting anthelmintic injection until at least mid-summer, most pasture larvae should have died off by that time and the pasture should remain safe for the rest of the season.
  • Preventive strategies only work when cattle are set-stocked; movement of these cattle during the autumn onto contaminated pasture risks disease.


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