Parasitic gastroenteritis (worms)

NADIS is a network of 40 veterinary practices and six veterinary colleges monitoring diseases of cattle, sheep and pigs in the UK.

NADIS data can highlight potential livestock disease and parasite incidences before they peak, providing a valuable early warning for the month ahead.

NADIS disease bulletins are written specifically for farmers, to increase awareness of prevalent conditions and promote disease prevention and control, in order to benefit animal health and welfare. Farmers are advised to discuss their individual farm circumstances with their veterinary surgeon.

NADIS Sheep Disease Focus – August 2005

Parasitic gastroenteritis is probably the most important production limiting disease of sheep. 


The aim of effective worm control during the spring and early summer months was to limit the exposure of susceptible lambs to significant burdens of infective worm larvae on pasture.  The options for worm control now are:

  • finish lambs quickly before pasture larval burdens become production limiting

  • only graze susceptible sheep on ‘safe’ pasture

  • suppress pasture larval contamination using wormers

  • a combination of the above – integrated control

Worm eggs shed in the faeces of sheep hatch and develop to infective third stage larvae on pasture.  Disease is caused following ingestion of these larvae with pasture.  The rate of egg hatching and development to third stage larvae is highest during warm and damp weather, while survival of infective larvae on the pasture is optimum during cool and damp conditions.  Thus, recent weather conditions could result in significant worm problems during the next few weeks.

Under typical UK summer conditions, few infective larvae survive on pasture for more than 9 months.  In general, disease is caused in sheep and cattle by different worm species. Thus, pastures which have been cropped, or grazed by cattle, and not grazed by sheep for the previous 9 months, only carry very small numbers of infective sheep worm larvae. These pastures are often referred to as ‘clean’. Furthermore, grazing with cattle for a shorter period of time, or hay/silage cropping enables the physical removal of a high proportion of the infective sheep worm larval population, producing ‘safe’ pasture. 

When ‘safe’ or ‘clean’ grazing is available for lambs at weaning, the traditional advice has always been to dose the lambs before moving.  Further anthelmintic treatments should not then be necessary until late autumn, by which time most of the lambs may have been sold.  While this practice ensures good worm control in the short term, it is now recognised that it can also select for anthelmintic resistance. 

When lambs graze on contaminated pasture, most of the total worm population is present on the pasture as eggs, developing, and infective larvae, with only a small proportion in the lambs as fourth stage larvae and adults. 

Thus, when the lambs are dosed with an anthelmintic, only a small proportion of the total worm population is exposed and any surviving resistant worms only make up a tiny proportion of the remaining worm population.  However, if lambs are dosed with an anthelmintic before moving to ‘safe’ or ‘clean’ pasture, then a larger proportion of the remaining worm population is made up of surviving resistant worms. 

While these are initially present in insufficient numbers to cause production loss, with time they are multiplied through susceptible sheep, giving rise to production limiting burdens of worms which cannot be controlled using anthelmintics belonging to the same action group as that used before the move.

This concern may be overcome by:

  • Not dosing lambs before moving.

    • Production loss due to Teladorsagia circumcincta, the main worm present during the summer months, is caused mostly by the effects of recently ingested larvae burrowing into the lining of the abomasum (stomach).  Therefore, even if lambs have significant worm burdens before moving to clean pasture, they will thrive following the move because they are no longer exposed to incoming larvae.

    • The potential problems with this strategy are:

      • Pastures don’t remain clean, so if lambs aren’t finished quickly, satisfactory growth rates may become difficult to achieve later in the year.

      • While most problems are caused by Teladorsagia, less common worm genera such as Haemonchus and Trichostrongylus can cause severe disease in some flocks.   In addition to the effects of incoming larvae, the feeding activities of adult worms of these genera can cause ill thrift.  Therefore lambs may fail to thrive after a move to clean pasture if these genera are present.

  • Dosing 90% of the lambs before weaning and leaving about 10% untreated.

    • Within any group of lambs, individual susceptibility to worms varies, with the most resilient animals thriving despite the presence of significant worm burdens.  If the heaviest 10% of lambs are left untreated before moving, they should continue to thrive, but will ensure that the previously clean pasture becomes contaminated with anthelmintic susceptible worms, as well as those resistant worms which survived dosing of the remaining 90% of the group.  Thus the sub-population of diluting resistant worms on the pasture is small compared to that of susceptible worms and the rate of development of resistance is slow.

      • The disadvantage of this strategy is that the pasture does not remain clean, although it is likely to remain safe for longer than it would have if none of the lambs had been dosed before moving.

  • Returning lambs to contaminated pasture after dosing, before moving to clean pasture a few days later.

    • The rationale for this strategy is that both resistant worms which survived anthelmintic dosing and susceptible worms acquired from the contaminated pasture between dosing and moving will be carried onto the clean pasture, thus slowing the rate of development of anthelmintic resistance.

When ‘safe’ or ‘clean’ grazing is not available, anthelmintic treatments will probably need to be repeated throughout the summer months.  The frequency of summer anthelmintic treatments will depend on the persistence of the product used.  Conventional wormers (white drenches, levamisole, ivermectin and doramectin) should be used at about 4 week intervals, while the dosing interval can be extended to 6 – 8 weeks if the persistent wormer, moxidectin is used. 

Depending on the level of pasture larval contamination, it may be possible to maintain satisfactory production using fewer suppressive anthelmintic treatments.  However, while reducing reliance on anthelmintics reduces the rate of selection for resistance, the strategy can result in poor production if it enables pasture larval contamination to develop. 

Once pastures are heavily contaminated, satisfactory production may be difficult to achieve despite anthelmintic treatments. This problem may be overcome by regularly performing worm egg counts on faecal samples from 7 – 10 lambs. However, trends rather than absolute counts must be closely monitored, otherwise pastures may already be heavily contaminated before high counts are identified and anthelmintic treatment instigated.

Treatment of clinical parasitic gastroenteritis. There are numerous NADIS reports every year of scour, ill thrift and lamb deaths caused by worms. 

The objective of worm control should be to prevent pasture contamination by worm larvae.  However, once pastures have become heavily contaminated with infective worm larvae, it is difficult to achieve satisfactory weight gains in lambs that graze them.  It is often reported that conventional anthelmintic drenching improved performance for about a week, after which lambs became open fleeced and started to scour again. 

This problem can be overcome by moving lambs to clean pasture after dosing with an effective conventional wormer, although in the longer term this selects for resistance. Alternatively if clean pasture is unavailable, a persistent wormer (moxidectin) should be used so that infective parasite larvae are killed before they can cause production losses. The meat withdrawal periods for oral and injectable moxidectin (Cydectin) are 14 and 70 days respectively. Furthermore, the persistence depends on the parasite species present. 

There are no simple recipes for the control of parasitic gastroenteritis.  Your vet can provide advice about sustainable worm control in your flock.   
Copyright © NADIS 2005

While every effort is made to ensure that the content of this forecast is accurate at the time of publication, NADIS cannot accept responsibility for errors or omissions. All information is general and will need to be adapted in the light of individual farm circumstances in consultation with your veterinary surgeon.


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