Gastrointestinal nematode infestations are perhaps the most
important group of conditions limiting intensive sheep
The important nematode infestations are:
Nematodirosis in young
lambs during the late spring /early summer
of growing lambs from mid-summer onwards
Older sheep when control
measures fail to be implemented correctly
gastro-enteritis affecting growing lambs from mid-summer
caused by failure to control PGE.
In the UK nematodirosis is an
important disease affecting young lambs during the late spring
/early summer months when losses can be high.
an important disease affecting young lambs during the late spring
/early summer months.
Sudden death and outbreaks of
diarrhoea in surviving lambs can occur in young lambs grazing
pastures contaminated with large numbers of larvae which develop
from eggs deposited by lambs during the previous grazing
of diarrhoea in lambs caused by nematodirosis occur in young lambs
grazing pastures contaminated with large numbers of infective
are not uncommon from nematodirosis in severely affected
Only lambs are
affected with nematodirosis, ewes do not show
Only lambs are affected, ewes
do not show disease. There is acute onset of profuse watery
diarrhoea in young lambs with faecal staining of the wool of the
tail and perineum. The lambs are dull and depressed and rapidly
develop a gaunt appearance with obvious dehydration and condition
loss. If left untreated during the early stages of disease, deaths
occur from dehydration and there is considerable weight loss in the
remaining lambs. It is not unusual with severe larval challenge for
5 per cent of lambs to die within a few days.
anthelmintic treatment is protracted with affected lambs taking
much longer to achieve market weight.
Sheep should be moved from
infested pastures whenever possible. Anthelmintic resistance is not
a problem with N. battus and Group 1 (BZ)
anthelmintics are commonly used.
Prevention is based upon avoidance of pastures grazed by lambs
during the previous grazing season because adult sheep are highly
resistant to infection and only lambs produce significant numbers
Anthelmintic prophylaxis timing is guided upon environmental
temperatures and disease forecasts (follows NADIS monthly parasite
forecasts and the SCOPS website). Typically, for lambs born
from mid-March onwards in "normal risk" years anthelmintic
treatments are given three weeks apart during May. In "high risk"
years, three anthelmintic treatments are given extending the
drenching period into June.
This section deals in general terms with parasite control in the
UK. This is a rapidly changing area of veterinary medicine,
especially following the introduction of monepantel (4-AD) and
derquantel/abamectin combination (5-SI). These products are
only available from a veterinary practice and consultation with
your veterinary surgeon is recommended.
emaciation in some
anaemia in some
important worms are:
is becoming a serious threat to intensive sheep production
throughout the UK and not just south-east England.
the most important clinical sign is
The severity of clinical
signs of parasitism depend upon
The classical signs of
parasitic gastro-enteritis are observed in growing lambs exposed to
large numbers of infective larvae during warm summer
Disease is typically seen in
growing lambs with profuse watery diarrhoea during mid/late summer
causing dehydration and reduced weight gain/condition
As a consequence of its blood feeding, haemonchosis presents
with anaemia, submandibular oedema, and increased heart and
respiratory rates; diarrhoea is not a feature of this nematode
infestation. Ingestion of large numbers of larvae over a short
period of time causes acute disease with lethargy, weakness, and
rapid loss of condition. This form of the disease is more commonly
seen in growing lambs. Ingestion of smaller numbers of infective
stages over several weeks to months causes a more general loss of
condition progressing to emaciation.
typically presents with anaemia and submandibular
a Suffolk shearling - ingestion of large numbers of larvae over a
short period of time causes acute disease with lethargy, weakness,
and rapid loss of condition.
Diarrhoea is not
a feature of haemonchosis.
Disease is normally seen
during early winter, usually affecting 8 to 10 month-old lambs but
also yearlings and adult sheep. The most prominent clinical feature
is profuse dark-coloured, foul-smelling diarrhoea with much mucus
present in the worst affected sheep.
Trichostrongylosis is normally seen during early
winter affecting 8 to 10 month-old lambs
Trichostrongylosis affecting a Scottish halfbred
hogg during early winter.
Faecal egg counts are routinely used to aid diagnosis of
nematode infestations but have certain inherent limitations.
Due to numerous factors the faecal egg count may not always
accurately indicate the adult nematode population present within
the gastrointestinal tract at that time because pathology can be
caused by developing larval stages before infestations become
patent and produce eggs.
By identifying only strongyle eggs, it is possible for less
pathogenic species to make a disproportionate contribution to the
total egg count. However, as a general rule strongyle egg
counts above 5-700 epg are considered high and treatment is
recommended. Your veterinary surgeon is the best person to
guide you on your own farm situation.
Identification of anaemia is
taken as a reliable indicator of haemonchosis in countries with
endemic disease. Egg counts are often very high in patent
infestations with counts greater than 10,000 epg not uncommon. At
necropsy very large numbers of adults are visible on the surface of
the abomasum (fourth stomach compartment) of untreated
Treatment involves the use of
an effective anthelmintic (please see later section detailing
The five major anthelmintic
groups, defined by the active chemical, comprise:
- 2-LV imidazothiazoles, tetrahydropyrimidines
- 5-SI derquantel and abamectin
Benzimidazoles such as
albendazole and fenbendazole have a similar mode of action.
Levamisole is an imidazothiazole, Preparations for sheep that
contain avermectins include doramectin and ivermectin, and
milbemycins such as moxidectin.
Closantel and nitroxynil can
be used in situations where H. contortus is the major
It is essential
a representative number of
sheep are weighed before treatment,
treatment is based upon the
heaviest sheep in the group,
drenching equipment is
These points are emphasised
in Sustainable Control of Parasites in Sheep (SCOPS).
Management/Prevention/Control measures for parasitic
With traditional management
of sheep on permanent pasture in the UK, parasitic gastro-enteritis
in growing lambs results from ingestion of very large numbers of
infective larvae from pasture during mid-summer. Teladorsagia
circumcincta, and in warmer areas Haemonchus
contortus, larvae appear first with Trichostrongylus
spp. during mid summer.
Pasture larvae arise from two
Eggs passed by ewes during
the periparturient period. The reduction in host immunity permits a
significant increase in egg production during the last two weeks of
gestation which may persist until eight weeks post lambing. Under
suitable environmental conditions these eggs develop to infective
larvae within three weeks but maximum levels may not be present on
pasture for up to six weeks. These larvae are the major source of
infestations in young lambs.
Young lambs may also ingest over-wintered infective larvae from
pasture. The large numbers of eggs produced by these adult
nematodes resident in the gastro-intestinal tract of young lambs
results in the appearance of significant numbers of infective
larvae on pasture during mid-summer. Clinical
parasitism results unless appropriate action is taken.
Control is based upon not grazing potentially heavily-infested
pastures with susceptible lambs. Avoidance of infested
pastures from July onwards can be integrated into some farm
management systems by moving weaned lambs onto hay or silage
aftermaths during mid June onwards. On some mixed farms, it
may be possible to rotate pastures annually between cattle and
sheep and operate a "modified" two year clean grazing system.
Anthelmintics can be administered to both ewes and lambs to
prevent the build-up of critical larval populations on
continually-grazed pasture but this may not be sustainable because
of selction for resistant strains.
Use of prophylactic anthelmintics
Anthelmintic treatment to
prevent the periparturient rise in egg output by ewes can be given
at various times depending upon farm management system such as when
housing ewes during mid gestation, at the same time as vaccination
against the clostridial diseases four to six weeks prior to
lambing, or immediately prior to turnout to pasture when lambs are
one to two days-old.
treatment can be used to prevent the periparturient rise in egg
output by ewes.
It is important that the anthelmintic chosen to counter the
peri-parturient rise is effective against arrested larval
stages. To gain maximum benefit from the residual activity of
moxidectin, ewes should be drenched/injected at turnout to pasture
with their lambs rather than at housing. The importance of
good ewe nutrition during early lactation should not be
underestimated with respect to parasitic gastro-enteritis.
The timing of early season prophylactic anthelmintic
administration to control nematodirosis has been discussed above
but essentially comprises a strategic anthelmintic drench(es)
depending upon disease forecasts. (see: www.nadis.org.uk and at www.SCOPS.org.uk) While
such forecasts are reasonably accurate, local factors may operate
such that two drenches are given two weeks apart from early May -
your veterinary surgeon will advise regarding local conditions.
early season prophylactic anthelmintic administration to control
nematodirosis will be given by your veterinary
Traditionally, where lambs
graze permanent grassland, anthelmintic treatments were repeated
every month until the autumn but the interval will depend upon the
persistence of the particular anthelmintic. Grazing heavily
contaminated pasture with reliance upon chemical
prophylaxis/treatment is unsustainable and lambs fail to grow to
their potential under such mis-management. A more integrated
pasture management policy is undoubtedly overdue on most intensive
sheep farms in the UK - again consult your own veterinary surgeon
as to how this goal can be best achieved on your farm.
integrated pasture management policy is overdue on most farms -
consult your own veterinary surgeon as to how this goal can be best
achieved on your farm.
Consult your own
veterinary surgeon regarding quarantine
What is the best
strategy for this farm - individual farm sustainable PGE control
measures will be drafted by your own veterinary
While segments of tapeworms
are often seen in the faeces of growing lambs in the UK they exert
no adverse effects on growth rate. Treatment is not usually
considered necessary because tapeworms are non-pathogenic. Only
members of the benzimidazole group (1-BZ) were previously effective
against adult tapeworms but praziquantel is effective and now
available though only in combination with LV