Diarrhoea associated with gastrointestinal parasites in grazing sheep

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.

Merino ewes bred for parasite resistance reduce larval contamination onto pasture during the peri-parturient period

The peri-parturient period is crucial for controlling worms as the acquired immunity of ewes is disrupted, resulting in an increase in faecal worm egg counts. Two hypotheses were tested in this experiment - that ewes bred for worm resistance would have lower faecal worm egg counts than unselected control ewes, during late pregnancy and lactation, under similar but separate grazing areas; and also that numbers of infective nematode larvae would be lower on pastures grazed by resistant ewes than pastures grazed by unselected control ewes. Faecal samples were collected from resistant and unselected ewes in late pregnancy and early lactation, during the winter rainfall season, and analysed for numbers of Trichostrongylus colubriformis and Teladorsagia circumcincta. Pasture samples were taken 1 week before and 7 weeks after lambing started and analysed for infective larvae. In all sheep, worm egg counts rose 2 weeks prior to lambing and continued into lactation. Worm egg counts were significantly lower in the resistant ewes from 1 week before lambing to 2 weeks after lambing. There were no differences in egg counts between single- and twin-bearing ewes in the resistant line. However, twin-bearing control ewes had significantly higher egg counts than single-bearing control ewes. Following lactation, plots grazed by resistant ewes had substantially less contamination with T. colubriformis larvae, but there were no differences in numbers of T. circumcincta larvae. Our results demonstrate that sheep bred for worm resistance has lower worm burdens during the peri-parturient phase and that lambs born to resistant ewes face a lower larval challenge during their introduction to grazing. In our environment, selection for low worm egg counts has produced sheep highly resistant to T. colubriformis, but has had less impact on resistance towards T. circumcincta.

Genetic aspects of sheep parasitic diseases

There is evidence of genetically determined host resistance mechanisms for most of the sheep parasites evaluated. The mechanisms vary; from no or reduced establishment, early expulsion, to suppression of parasites resulting in reduced size and fecundity. There is a need to integrate breeding for parasite resistance with the genetic improvement of production traits in farm animals, aiming for optimum solutions for potentially conflicting responses. Sustainable parasite control must be based on Integrated Parasite Management utilising an interdisciplinary approach.

Immune-mediated pathology of nematode infection in sheep--is immunity beneficial to the animal?

Nematode parasitism is a severe impediment to sustainable and profitable sheep production in many countries in the world. Parasite resistance to anthelmintic treatment and consumer demand for organic agricultural products has led to much research into harnessing natural immunity as a long-term control measure. However, there is evidence that many of the clinical signs of nematode infection in sheep are due to immune-mediated pathology rather than direct effects of the parasite. Therefore, the desirability of promoting a strong natural immunity in sheep has been questioned. This review attempts to clarify some of the arguments for and against promoting strong natural immunity, particularly through selective breeding of parasite-resistant animals. It is concluded that the detrimental effects of immune-mediated pathology are outweighed by epidemiological and welfare benefits. Thus, control of nematode parasites through selection of naturally resistant sheep is a sustainable and desirable objective.

Relationships between faecal dry matter, worm burdens and inflammatory mediators and cells in parasite-resistant Merino rams

Immune-mediated scouring due to ingested parasite larvae is a major concern for sheep producers in Mediterranean climates. We investigated immune-mediated scouring in parasite-resistant Merino sheep in Australia. Forty-adult, parasite-resistant Merino rams were judged to be either susceptible or non-susceptible to immune-mediated scouring on the basis of dag scores taken under field conditions. We hypothesised that the susceptible rams would have lower faecal dry matter during larval challenge than non-susceptible rams and that, at post-mortem examination, inflammatory mediators and granulocytes would be negatively correlated with both faecal dry matter and worm numbers. In pens, the rams received a dose of 500 Teladorsagia circumcincta L(3) and 500 Trichostrongylus colubriformis L(3) each day for 6 weeks before euthanasia. Ten rams acted as unchallenged controls. Challenging sheep with larvae reduced faecal dry matter at 2, 3 and 4 weeks after challenge began and the greatest reductions were with the sheep susceptible to scouring. The sheep showed good resistance to the parasite challenge as evidenced by low faecal worm egg counts and low total worm counts at post-mortem, with the numbers of T. colubriformis particularly low. Sheep with low faecal dry matter had significantly higher numbers of eosinophils in small intestine tissue. Sheep with low total worm counts had significantly higher levels of bradykinin in abomasum mucus. Sheep with more granulocytes in tissue and inflammatory mediators in mucus tended to have fewer numbers of T. circumcincta but there was little relationship with numbers of T. colubriformis. Our results show that dag scores are correlated to a reduction in faecal dry matter, which can be attributed to the challenge with infective parasite larvae. Inflammation during worm infection is associated with rejection of the worm challenge and may result in more fluid faeces and consequently diarrhoea. Therefore, sheep breeders should focus on breeding for both low worm egg counts and also low dag scores.

Relationships between immune indicators of parasitic gastroenteritis, nematode burdens and faecal dry matter in sheep

Immune-mediated scouring in sheep is a mucosal hypersensitivity response caused by ingesting infective parasite larvae. In this experiment we tested the hypothesis that levels of parasite-specific antibodies and also interleukin-5 (IL-5) would be negatively correlated with worm numbers, but also faecal dry matter (FDM), in parasite-resistant Merino sheep. Forty Merino rams were challenged with Trichostrongylus colubriformis and Teladorsagia circumcincta larvae every day for 6 weeks, after which they were euthanised and total worm burdens determined. Faecal samples were taken for measurement of worm egg counts and FDM. Serum was collected from the rams before and after the larval challenge commenced, and levels of ovine immunoglobulin light chain, IgG, IgM, IgA and IgE specific for T. colubriformis and T. circumcincta as well as IL-5 were determined by sandwich enzyme-linked immunosorbent assay. IL-5 and all serum antibodies apart from T. colubriformis-specific light chain were significantly increased by the larval challenge. However, none of the antibodies, or IL-5, was correlated with FDM. Negative correlations were observed between the number of adult T. circumcincta and antibody levels; however, there was little relationship between antibodies and numbers of T. colubriformis. It was concluded that serum antibody levels are a poor indicator of the susceptibility of sheep to immune-mediated scouring. Because of these results, sheep breeders should continue to select for low worm egg count and focus on phenotypic indicators of scouring, such as dags, as a means to reduce diarrhoea.

Nematode parasites and faecal soiling of sheep in lairage: evidence of widespread potential production losses for the sheep industry

Diarrhoea (scouring) and subsequent faecal soiling of fleece are important economic and welfare issues for the sheep industry. Nematode worm infections are commonly implicated as a cause of scouring. This study aimed to investigate the extent of strongyle parasite infections, and identify any association with faecal worm egg count (WEC) and scouring in sheep from winter rainfall environments in Western Australia consigned to an abattoir. Faeces were collected from sheep with evidence of scouring and normal sheep (firm faecal pellets and no evidence of fresh diarrhoea on breech). A total of 4430 sheep from 113 lines of lambs (<12 months old), 10 lines of yearlings (12–24 months old) and 235 lines of adult sheep (>24 months old) were sampled between September and January. Mean WEC in lamb lines was 1525 eggs per gram (epg) of faeces with mean WEC >1000 epg in 42% of lines and >2000 in 22% of lines. Mean WEC in adult lines was 486 epg, with 13% lines having mean WEC >1000 epg. There was a trend (P = 0.099) to higher WEC in scouring lambs (2289 epg) compared with normal lambs (1523 epg). The scouring adult sheep had lower WEC (417 epg) compared with normal adults (482 epg, P = 0.021). The findings suggest that large strongyle infections were common in lambs consigned for slaughter. The low WEC in scouring adult sheep was consistent with the suggestion that a hypersensitivity to ingested nematode larvae, rather than large worm burdens, may be responsible for scouring in mature sheep.