The pathogenesis and control of diarrhoea and breech soiling in adult Merino sheep

doi:10.1016/S0020-7519(99)00050-8

International Journal for Parasitology

Volume 29, Issue 6, 1 June 1999, Pages 893-902

https://doi.org/10.1016/S0020-7519(99)00050-8 Get rights and content

Abstract

Diarrhoea and soiling of the breech with faeces (`winter scours') is a serious problem in adult Merino sheep grazing improved pastures in south-eastern Australia during winter and spring. This occurs even on farms where gastro-intestinal nematodes are effectively controlled. It was shown that winter scours was associated with the ingestion of trichostrongylid larvae, and that host factors were important in determining susceptibility to this syndrome. No differences were detected in the protective immune response of affected and unaffected sheep to gut nematodes. However, affected sheep had a hypersensitive inflammatory reaction in the pylorus and upper jejunum, characterised by the infiltration of significantly more eosinophils and changed lymphocyte populations. The changes in the lymphocyte populations included a reduced number of CD8⁺ cells, increased CD4⁺:CD8⁺ T-cell ratio, and significantly reduced numbers of cells reacting to interferon-gamma. High doses of infective larvae (20 000/week of Ostertagia circumcincta and Trichostrongylus vitrinus) did not induce diarrhoea in sheep not susceptible to winter scours. In contrast, only low doses (2000/week) initiated scouring in sheep selected as being susceptible to winter scours. Therefore, even considerably improved worm control programmes, including the selection of sheep with increased resistance to gut nematodes, will not prevent winter scours. Rather, phenotypic culling and genetic selection, to remove sheep susceptible to the hypersensitivity inflammatory response, is proposed as the most suitable long-term control strategy.

Section snippets

Introduction—`winter scours' in Merino sheep

Persistent diarrhoea commonly affects adult Merino sheep grazing improved pastures in the high winter rainfall areas of Australia during winter and spring. The diarrhoea is present in sheep with low faecal worm egg counts (FECs) and occurs despite the use of strategic preventative control programmes, designed to minimise the production losses from gastro-intestinal nematode infections. Because it is associated with short, lush pasture dominated by improved pasture species, this syndrome is

Risk factors

An observational study was conducted on three farms over 2 years, involving 1300 2-year-old Merino ewes. It examined three risk factors associated with winter scours; exposure to trichostrongylid larvae, lactation and higher bodyweight at joining[2]. Initial comparisons were made among groups of ewes, with and without treatment with controlled-release capsules containing albendazole. Diarrhoea was assessed by the accumulation of faeces around the breech, on a scale of 0 (no dag) to 5 (heavy

Control of winter scours

Low doses of larvae can initiate diarrhoea in sheep susceptible to winter scours, and so worm control programmes based on the strategic use of anthelmintics and grazing management will not prevent the accumulation of severe dag[1].

Treatment with a controlled-release anthelmintic capsule was extremely effective in keeping sheep free from dag, but is unlikely to be cost-effective when used solely for this purpose2, 15. However, where controlled-release capsules are used as part of a strategic

Further work into winter scours

In the course of these studies, a number of areas requiring additional investigation were identified. Many overlap with areas of study currently underway into immunity to internal parasites and are summarised below.

What antigens are likely to induce winter scours?

This question was not directly addressed by the studies summarised here. However, there was no significant association of winter scours with FECs or adult worm burdens in these outbred Merino sheep. Additionally, controlled-release anthelmintic capsules, which effectively control winter scours, cause the death of most third- and fourth-stage larvae, and so very few infective larvae mature to adults in treated sheep[38]. Therefore, it is unlikely that adult-associated antigens initiate winter

Conclusions

The ingestion of trichostrongylid larvae by adult sheep was demonstrated to be a necessary, but not sufficient, cause of winter scours. That is, this must occur for the syndrome to arise, but other factors must be associated with the ingestion of larvae for the syndrome to inevitably develop. These include genetic susceptibility to the inflammatory response typical of winter scours, and the ingestion of pasture of a certain, as yet undefined, type.

A feature of the syndrome was a

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Pasture larval counts may be used to demonstrate exposure to strongyle infective larvae, but this is an insensitive test that is labour intensive and difficult to interpret, given that small numbers of larvae required to induce diarrhoea in susceptible sheep (Larsen and Anderson, 2000). Genetic selection appears the best long-term strategy currently available for control of hypersensitivity scouring, as it is repeatable and phenotypic culling of ewes with repeated dag has reduced the occurrence (Larsen et al., 1995b, 1999). The heritability for dag has been estimated at around 0.2 although this varies between studies (Larsen et al., 1999; Pollott et al., 2004; Pickering et al., 2012; Greeff et al., 2014), and as a visual trait, the dag score is easily recorded, cheap to measure and can be readily incorporated into breeding indices or used a secondary trait for culling.
Diarrhoea is a common, widespread and frustrating reality for sheep enterprises in most sheep producing regions globally and of particular concern in Australia as the major risk factor for breech flystrike. Parasitic disease has long been recognised as an important factor in diarrhoea in sheep, particularly the gastrointestinal nematodes (Trichostrongylus and Teladorsagia species). This review focuses on the role of parasitic infections in causing diarrhoea in sheep, with emphasis on the epidemiology of diarrhoea outbreaks related to worms and opportunities to manage the risk of diarrhoea outbreaks in sheep related to parasitic infections. Parasitic nematodes damage the gastrointestinal tract via a complex relationship between direct impacts from worms, such as physical changes to the gut mucosa, and indirect effects largely associated with the host response. Diarrhoea associated with large worm burdens is most efficiently managed through integrated parasite management programs. Despite some limitations, measuring faecal worm egg counts remains a mainstay for assessing the contribution of worms to outbreaks of diarrhoea in sheep. Larval hypersensitivity scouring is emerging as a significant cause of worm-related diarrhoea in sheep without large adult worm burdens in some geographic locations. The syndrome describes a heightened inflammatory response to the ingestion of trichostrongylid infective larvae seen in the gut of sheep with diarrhoea, and is most effectively addressed through selecting sheep for low breech soiling (‘dag scores’), as worm resistant sheep may show an increased propensity for diarrhoea, even with low rates of larval challenge. Importantly, dag should be considered as a separate trait to WEC in breeding indexes. Outbreaks of diarrhoea in young sheep are often multifactorial, and co-infections with nematodes and other infectious agents associated with diarrhoea are common. This presents challenges for the field investigation of diarrhoea in grazing sheep.
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However, our results showed that, amongst weaners with low worm egg counts (and hence low adult worm burdens), the probability of detecting Yersinia spp. would be increased, at most, only minimally by preferential selection of those with high faecal consistency scores for faecal culture. This was probably because many other factors, including hypersensitivity scouring in response to internal parasites and nutrition (Larsen et al., 1999; Jacobson, 2006), were affecting faecal consistency scores in the trial mobs. Likelihood ratios (and hence, the usefulness of preferential selection of sheep with high faecal consistency scores for faecal culture) would be expected to be greater in sheep less likely to be exposed to other causes of watery faeces.
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The pathogenesis and control of diarrhoea and breech soiling in adult Merino sheep

Abstract

Section snippets

Introduction—`winter scours' in Merino sheep

Risk factors

Control of winter scours

Further work into winter scours

What antigens are likely to induce winter scours?

Conclusions

Livestock Prod Sci

Parasitol Today

J Dairy Sci

Vet Immunol and Immunopathol

Int J Parasitol

Int J Parasitol

Vet Parasitol

Vet Parasitol

Drug Discov Today

Immunol Today

Toxicological letters

Int J Parasitol

Int J Parasitol

Diarrhoea in Merino ewes during winter: association with Trichostrongylid larvae

Aust Vet J

Diarrhoea due to trichostrongylid larvae in Merino sheep during winter: repeatability and differences between bloodlines

Aust Vet J

Relative contribution of diet and sunshine to the overall vitamin D status of the grazing ewe

Vet Rec

Seasonal vitamin D deficiency in sheep in south-eastern Australia

Proc Aust Soc Anim Prod