This condition is a disease of deer, sheep, cattle, alpaca and horse caused by the consumption of perennial ryegrass infected with certain fungi. The fungal spores are toxic and cause nerve and brain damage, which manifests itself as tremors and staggering.
The fungus spreads from one generation of ryegrass plant to the next. The risk periods are summer and autumn.
It can be difficult to notice initial signs of ryegrass staggers if the animals are at rest or quietly grazing. There may be a slight head tremor or trembling of the muscles of the neck, shoulder or back. If the deer are moved or yarded, the tremors and shaking become more exaggerated. Severally affected animals can fall over and thrash or have convulsions. They may stand with a very stiff legged stance and shake or nod.
Not all deer are affected to the same degree. Red and fallow deer appear only moderately susceptible to ryegrass staggers. Wapiti are highly susceptible. There are also differences within a breed.
Affected deer can have low growth rates, poor conception rates and low velvet production. They are also vulnerable to misadventure (e.g. getting tangled in fences) and can die of shock.
Observing clinical symptoms are the first step to a diagnosis. Autopsy will show torpedo-like lesions in parts of the brain.
There is no specific medicine to treat ryegrass staggers. The most effective treatment is to quietly move deer off the dangerous pastures and graze them on alternative safe pastures. These would be non-ryegrass pasture like clover, lucerne, fescue, short-term ryegrass or brassica. Holding deer in a yard and feeding them hay and concentrates is another option. It can take between one and three weeks for the signs to abate.
Long-term, the best option is to undertake pasture renewal, replacing high endophyte ryegrass with AR1 ryegrass or another pasture species. Although AR37 does not produce Lolitrem B it can cause ryegrass staggers. Trials have shown that on average the frequency, duration and severity of ryegrass staggers is less than for standard endophyte. However on occasions, sheep (and potentially other animals) grazing AR37 ryegrass may be severely affected for short periods. Due to the fact that no trial work has been undertaken on deer and horses, pastures with AR37 are not currently recommended for these livestock classes. Anecdotal comment is that deer do not find ryegrass with AR37 endophyte very palatable and it can cause staggers under certain conditions.
Some genetic lines of deer are less prone to ryegrass staggers than others. Select deer replacements based on susceptibility.