Annual Ryegrass Toxicity

Annual ryegrass toxicity (ARGT) is a poisoning of livestock caused by ingestion of annual ryegrass (Lolium rigidum) that has been infected by a specific toxic bacterium. The bacteria are carried into the plant by a parasitic nematode and bacterial proliferation then creates the toxin. As plants mature and dry off (around hay making time), toxin levels may rise and peak.

ARGT occurs in the extensive sheep-wheat belts of Western Australia and South Australia, where many export sheep are sourced. The feeding of contaminated hay may cause disease during the export process.

Signs include high stepping gait (hypermetria), staggering, collapse, trembling, terminal convulsions and death within one to two days. Affected animals may be found dead without observation of previous signs.

Signs appear from 4 days to several weeks after exposure and can persist intermittently for weeks after removal from the source. Mortality rates of 50 % in animals with clinical signs are typical.

At necropsy there is variable congestion, oedema and haemorrhage of many organs, and usually more obvious in the lungs, kidneys, liver and brain.

In suspect pastures, a yellow slime may be observed on infected ryegrass heads.

Laboratory confirmation requires formalin-fixed liver and suspect grass or grass seed showing bacterial galls.

Differential diagnoses include enterotoxaemia and polioencephalomalacia. Other stagger syndromes such as phalaris, perennial ryegrass and paspalum staggers can look similar but don’t have high numbers of acute deaths.

No drug treatment is effective. Mildly affected sheep should be left undisturbed with access to water, feed and shade. Sheep in lateral recumbency should be euthanised.

Deaths can be reduced by early recognition and removal from the source of the toxin.