Sheep will initially exhibit a nodding of the head and tremoring. The condition is often noticed when moving stock, and with forced exercise or fright, sheep may stagger, exhibit a stiff and bounding gait with jerky limb movements and may collapse with extended limbs. These signs may worsen with heat stress. Sheep will usually recover from this attack when left to recover. 90% of a flock may be affected and sheep of all ages and breeds are equally susceptible.
This staggers is caused by an endophyte, a microscopic fungus (neotyphodium lolii) which infects 90% of perennial ryegrass (Lolium perenne) pastures. Endophytes produce alkaloids which are beneficial to seedling production and vigour, insect resistance, drought resistance and persistence in the paddock. However, alkaloids also protect the plant from predation through its toxic properties. The major alkaloid, Lolitrem B, disrupts the nervous system of the sheep, leading to hyperactivity, while ergovaline acts to constrict blood vessels, leading to increased susceptibility to heat stress.
Staggers most commonly occurs during summer and autumn after a dry period, when sheep pick green shoots at the base of the plant where toxin is concentrated. Toxin levels are greatest when the soil is high in nitrogen and low in moisture.
Diagnosis is based on clinical signs and a prevalence of ryegrass in the pasture. Laboratory tests can identify endophyte and alkaloid levels. A post-mortem may typically reveal a microscopic swelling of neurons within the brain and haemorrhage of the heart, rumen and lymph nodes.
Sheep should be moved off the infected pasture to avoid permanent neurological damage. Recovery may take 1-2 weeks. Prevention involves avoiding overgrazing, restricting spread of infected grass and seed, incorporating other grass species into the pasture and sowing novel endophyte cultivars which lack toxic alkaloids yet maintain a satisfactory persistence in pasture.
A fatal disorder more uncommon in NSW, occurring in spring and resulting from ingestion of bacterial (Clavibacter toxicus) toxin introduced to ryegrass by the nematode Anguina funesta. Diagnosis is made with evidence of staggering, prevalence of annual ryegrass and detection of nematode and bacteria. Postmortem may reveal oedema of the brain and widespread haemorrhage. Affected sheep should be moved off the pasture and infected seed heads should be destroyed.