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Monday, 11 May 2015

Gastrointestinal Nematode Infestations in Sheep

Gastrointestinal nematode infestations are one of the most important groups of conditions limiting intensive sheep enterprises. 3 of the most important infestations are…
1.       Nematodirosis in young lambs during late spring/early summer.
2.       Parasitic gastro-enteritis of growing lambs from mid-summer onwards.
3.       Older sheep when control measures fail to be implemented correctly

Nematodirosis
·         In the UK, this disease affects young lambs during the late spring/early summer months when losses can be high.
·         Lambs of the previous seasons deposit large numbers of larvae on the pastures which are then grazed by young lambs, leading to outbreaks of diarrhoea and sudden death. This disease is limited to lambs; ewes are not affected by this disease.
·         There is acute onset of profuse watery diarrhoea and noticeable faecal staining of the wool of the tail and perineum. Lambs are dull and depressed and rapidly develop a gaunt appearance with obvious dehydration and condition loss. If left untreated in the early stages, death occurs due to dehydration; remaining lambs suffer considerable weight loss.
·      
   Treatment: remove lambs from infested pasture. Anthelmintic resistance is not a problem with N. battus.

·         Prevention/Control: This is based on avoiding pastures grazed by lambs during previous grazing seasons. Adult sheep are highly resistant to infection and only lambs produce significant number of eggs. Anthelmintic prophylaxis timing is guided upon environmental temperatures and disease forecasts. Typically, in normal risk situations, anthelmintic treatment should be given 3 weeks apart during May. In high risk situations, 3 anthelmintic treatments are given, extending the drenching period into June.


Parasitic Gastro-Entritis
Parasitic infestations of the gut can be attributed to a range of worms. Veterinary medicine is constantly evolving to offer more specific and more broadly scoped treatments.

·         Clinical presentation: infestations usually cause: profuse diarrhoea, reduced performance, weight loss, sometimes emaciation and anaemia.
·         The main worms that cause disease are:
Nematodirosis
Teladorsagia
Haemonochus contortus*
Trichostrongylus
*becoming a serious threat to intensive sheep production in the UK.
·         The severity of clinical signs of parasitism depends upon…
Age of sheep, nutritional intake, immune status, trace element intake and breed.


Teladorsagiosis
·         Typically seen in growing lambs with profuse watery diarrhoea during mid/late summer causing dehydration and reduced weight gain/condition loss.
·         Lambs ingest third stage larvae which lay eggs in the gut, which are then passed onto the pasture by excretion. The eggs then develop into larvae over 2 – 12 weeks, which are then ingested and the cycle continues.

Haemonchosis
·         This type of worm feeds on blood, therefore infested animals present with anaemia, submandibular oedema and increased heart rate and respiratory rates; diarrhoea is not a feature of this nematode disease.
·         Ingestion of large numbers of larvae over a short period of time causes acute disease with lethargy, weakness and rapid loss of condition.
·         Also seen in growing lambs. When lambs ingest smaller numbers of infective stages over several weeks to months, this usually causes a more gradual and general loss of condition progressing to emaciation.

Trichostrongylosis
·         Normally seen during early winter, affecting 8 – 10 month-old lambs, however yearlings and adult sheep are known to be affected in some cases.
·         Profuse dark-coloured, foul smelling diarrhoea and in severe cases there is much mucus in the excrement.

Diagnosis of the 4 Nematode Diseases
·         Teladosagiosi & Trichostrongylosis… are diagnosed using faecal egg counts. Generally this method has downfalls in that numerous factors can affect egg count. Pathology can be caused by developing larval stages before infestations start producing eggs. By only identifying strongyle eggs, it is safer to assume that the eggs are in direct proportion with the severity of the disease. Egg counts above 400epg are moderate, 1000epg is high and treatment is usually necessary.
·         Haemonchosis… identification of anaemia is taken as a reliable indicator. Egg counts are often very high in patent infestations with counts greater than 10,000epg being common. During necropsy, very large numbers of adult worms are visible on the surface of the abomasum (fourth stomach compartment) of untreated sheep.

Treatment
·         Best treatment involves the use of an effective anthelmintic. There are 4 major groups, defined by their active chemical:
1) 1-BZ benzimidazoles, probenzimidazoles
2) 2-LV imidazothiazoles, tetrahydropyrimidines
3) 3-ML avermectins, milbemycins
4) 4-AD monepantel
·         It is essential that: a representative number of sheep are weighed and dosage calculated, treatment is based on the heaviest sheep in the group, drenching equipment is accurately calibrated.

Management
·         In UK, where traditional management of sheep is done by sheep grazing permanent pasture, PGE in growing lambs results from ingestion of very large numbers of infective larvae from pastures during mid-summer.
T. circumcincta, and if warm, H. contortus larvae appear first with T. spp. During mid-summer.
·         Pasture larvae arise from two sources:
1) Eggs passed by ewes during the peri-parturient period. The reduction in immunity permits an increase in egg production during the last two weeks of gestation, which may continue for up to 8 week post lambing. Under suitable conditions, these eggs develop into larvae within 3 weeks, maximum levels seen up to 6 weeks. These larvae are the major source of infestations in lambs.
2) Young lambs may also ingest over-wintered larvae from pasture, which then develop into adults in the gut of the lamb. These adult nematodes produced large numbers of eggs which results in the appearance of a large number of infective larvae on pasture during mid-summer. Clinical parasitism results if action is not taken.
Control
·         Control is based upon not grazing potentially heavily-infested pastures with susceptible lambs.
·         Anthelmintics can be administered to both ewes and lambs to prevent the build-up of critical larval populations on continually-grazed pasture. However, this may not be sustainable because of resistance development.

Use of prophylactic anthelmintics
·         This kind of treatment, to prevent the peri-parturient rise in egg output by ewes, can be given at various times depending upon the farm system. For example, it can be given when housing ewes during mid-gestation, at the same time as vaccination against the clostridial diseases (4 – 6 weeks) prior to lambing or immediately prior to turn out to pasture when lambs are 1 – 2 days old.

·         It is important that the anthelmintic chosen to counter the egg increase is effective against arrested larval stages. To gain maximum benefit from the residual activity of Moxidectin, ewes should be drenched/injected at the latest possible stage (turnout) instead of earlier (during housing).


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