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.

Associations between nematode larval challenge and gastrointestinal tract size that affect carcass productivity in sheep.

Effects of gastrointestinal parasitism on sheep productivity are usually described using live weight change, however carcass productivity is more accurately described using dressing percentage (carcass weight as a proportion of live weight). This experiment had a 2x2x2 factorial design whereby 10-month-old Merino wethers were fed lucerne (Medicago sativa) diets (fresh lucerne or lucerne chaff) with 2 levels of carboxymethycellulose (CMC) inclusion (0% or 8% CMC) and nematode larval challenge (no larval challenge or 10,000 Teladorsagia circumcincta and 10,000 Trichostrongylus colubriformis per week). Sheep were weighed and euthanased 50 or 51 days after larval challenge and CMC supplementation commenced. Weight of the carcass (hot standard carcass weight) and gastrointestinal organs (full and empty) were recorded and expressed as a proportion of live weight. Larval challenged sheep had a worm egg count (mean+/-standard error) of 173+/-38 eggs per gram of faeces and total worm count of 30,237+/-2013 at slaughter. Larval challenged sheep had 1.3% lower dressing percentage (p=0.048), and 2% heavier full (p=0.007) and 1.2% heavier empty gastrointestinal tracts (p=0.012) compared to unchallenged sheep. There was no effect of CMC inclusion or lucerne type (fresh or chaff) on gastrointestinal tract weight or dressing percentage. Larval challenged sheep had 1.1% heavier full (p<0.001) and 0.6% heavier empty (p<0.001) small intestines, and 0.6% heavier full (p=0.005) and 0.3% heavier empty (p=0.026) large intestines compared to unchallenged sheep. Use of live weight change or other measures based on live weight (e.g. feed conversion efficiency) to assess the impact of nematode challenge in sheep may underestimate carcass productivity losses associated with larval challenge in sheep even at moderate levels of larval intake and without overt clinical signs of parasitism. Measurement of carcass weight and/or lean meat yield may better reflect the true economic effects of parasitism in sheep.