Annual ryegrass toxicity (ARGT) is a potentially fatal poisoning of livestock after consumption of annual ryegrass infected by the bacterium Rathayibacter toxicus (formerly known as Clavibacter toxicus).
The cycle starts with a migratory nematode called Anguina funesta. These nematodes can travel into the flowering parts of developing ryegrass and colonise them, eventually taking over the seed head to form nematode galls.
ARGT problems can start where the bacterium is also present in the same soil. The bacterium is carried by the nematode as it moves into the developing ryegrass flowering parts. Once inside the plant, the bacterium also develops, often out competing the nematode to take over the seed heads as they develop. The bacterium produces toxins from the end of flowering, through seed set, to seed maturity. Toxicity develops at flowering and seed set.
Infected ryegrass remains toxic even when it has senesced and dried off because it still contains the toxin. Hay made from toxic ryegrass will also be toxic. All grazing animals are susceptible, including horses and pigs.
The toxins that cause ARGT (corynetoxins) are cumulative which means that the more the animal consumes the greater the impact. It takes up to six months to clear all effects of the toxin. If the animal consumes toxins up to nine weeks apart, those toxins will accumulate until they cause clinical disease.
Signs of ARGT may appear as soon as four days or as late as several weeks after animals are introduced to toxic feed (pasture, hay, grain). If clinical signs appear sooner than four days after stock are introduced to a paddock, stock have ingested toxin before being moved to the new paddock.
The toxins that cause ARGT (corynetoxins) are cumulative. Intakes up to nine weeks apart will accumulate until the animal consumes enough toxin to cause clinical disease. It takes up to six months to clear all effects of the toxin.
Signs of ARGT may appear as soon as four days or as late as several weeks after animals are introduced to toxic feed (pasture, hay, grain). If clinical signs appear sooner than four days after stock are introduced to a paddock, stock have ingested toxin before being moved to the new paddock.
What are the signs of ARGT?
ARGT causes neurological signs. These are brought on by stress or activity, and the disease is often first suspected when animals are being moved for yarding or to change paddocks. Horses that are observed closely may stop eating and show muscle tremors in the early stages.
Animals affected by ARGT will fall behind the mob, look uncoordinated, stop and often fall over. Animals that fall over will convulse, typically throwing the head back with stiff legs. Convulsions are always strong, unlike stock that fall due to weakness. If the animal is left undisturbed it may appear to recover, regain its feet and rejoin the mob with a stiff gait. Animals remaining down for long periods and exhibiting severe convulsions may die. Intermittent appearance of clinical signs is a feature of this disease. Pregnant stock may abort.
How is ARGT diagnosed?
A history of neurological signs, first appearing when animals are being driven, is strongly suggestive of ARGT.
There are a range of laboratory tests to assist with diagnosis of ARGT. The rumen contents can be tested for the presence of the toxigenic bacterium and the prevalence of the bacterium in pasture, hay and grain samples can be determined.
Other diseases that may resemble ARGT include:
- thiamine deficiency [1]
- pregnancy toxaemia [2]
- grass tetany [3]
- pulpy kidney (enterotoxaemia) of sheep [4]
- scrapie [5]
- mad cow disease [6] (bovine spongiform encephalopathy).
How should I treat animals affected by ARGT?
There is no specific treatment for ARGT, but less severely affected animals may recover given time. When affected animals are found, the mob should be immediately moved quietly to a 'safe' paddock with good water, safe feed and shade. Badly affected stock should be protected from the weather and predation. Stock unable to rise within 12 hours should be humanely euthanased.
A safe paddock is one without annual ryegrass or which has tested negative for ARGT during the current season. Safe feed is feed free of ryegrass seed or which has tested negative for ARGT.
Animals may continue to show clinical signs for up to 10 days after being moved off the toxic paddock. Peak deaths in ruminants occur four days after removal from affected feed source. Not all animals that develop clinical signs will die.
How do I prevent ARGT from occurring?
The keys to preventing ARGT occurring in livestock are:
- daily inspection
- ARGT testing
- vendor declarations for bought-in hay, grain and chaff
- good biosecurity to minimise introduction of the ARGT causative organisms
- paddock management.
Daily inspection
In districts in which ARGT is known to occur, stock grazing paddocks containing annual ryegrass should be inspected at least once daily from October through to December.
When stock are placed onto stubbles containing ryegrass in summer, they should be inspected daily for the first two weeks.
The signs of ARGT are brought on by stress or activity, so inspection should involve moving the mob briskly over 100 to 200 metres. Early detection of the disease will minimise losses.
Ryegrass testing
Testing of ryegrass in paddocks can be used to identify safe paddocks and to detect the bacteria early so it can be managed.
When buying hay for stock, request a vendor declaration that shows the hay has been tested for presence of the toxigenic bacterium and is safe to feed to livestock. Grain and chaff heaps can also be tested before being fed to stock.
Good biosecurity
Any means of spreading ryegrass seed can also spread the causative organisms of ARGT. These include movement of contaminated feed (hay, grain), infected ryegrass seed, uncleaned machinery or vehicles, or animals carrying galls in their fleece. The galls weigh less than the seed and can be moved by strong winds. Surface run-off water and creeks can move the organisms considerable distances. Producers in districts in which ARGT rarely occurs, or is unknown to occur, should take steps to minimise the risk of introduction of the causative organisms.
Paddock management
Once the organisms are introduced, paddock management that incorporates regular cropping or closing up of paddocks for hay is generally required for the organisms to reach levels sufficient to present a risk to livestock. It may take 5-15 years from the time of introduction of the organisms to clinical disease first being seen, depending on the environment and the paddock management. ARGT rarely, if ever, occurs in paddocks that are grazed every year, even if the organisms are present.
Report sheep and cattle with neurological signs to a veterinarian
Producers who identify cattle or sheep with neurological signs should contact their private veterinarian or Department of Primary Industries and Regional Development veterinary officer [7] to request an inspection. These animals may be eligible for the National Transmissible Spongiform Encephalopathy Surveillance Program (NTSESP) [6] and if eligible, the cost of the veterinary investigation will be subsidised.
To be eligible for the NTSESP subsidy, a veterinarian must observe the clinical signs of affected stock and post-mortem the sheep or cattle, then submit the brain and other samples for further testing. Cattle must be between 30 months and nine years of age and sheep must be over 18 months old to be included. Younger stock affected by neurological signs may be eligible for the Significant Disease Investigation program [8]. Ask your veterinarian about these programs.
Laboratory examination may diagnose the cause of the neurological condition at significantly reduced cost to the producer at the same time as providing evidence to support Australia's freedom from TSEs.
Rebates available
Producers who have suitable animals autopsied for the NTSESP are entitled to claim $330 for cattle and $110 for sheep (GST inclusive). A maximum rebate of two animals per disease outbreak per property may be claimed.
Private veterinarians who examine cases of nervous disease in cattle and sheep and submit the appropriate samples and paperwork to a government laboratory for TSE exclusion are also entitled to claim a rebate. Veterinarians are eligible for the following rebates:
- cattle: $330 per animal (GST inclusive) for a maximum of two cattle per outbreak per property
- sheep: the national TSE rebate is $220 per sheep (GST inclusive) for a maximum of two sheep per outbreak per property.
- travel rebate for a maximum total travel distance of 200 kilometres
The program also covers the cost of laboratory fees where they would normally be charged and subsidises the cost of freighting samples to the laboratory.
Report unusual disease signs, behaviour or deaths in livestock
If you see unusual disease signs in your stock, call your private veterinarian, DPIRD veterinarian [7] or the Emergency Animal Disease hotline on 1800 675 888.