Phalaris (Phalaris aquatica) toxicity can cause both a sudden death syndrome and a staggers syndrome. Phalaris sudden death syndrome can be further differentiated into 'cardiac-sudden death' and 'polioencephalomalacia-like sudden death.'
'Phalaris staggers' is an incoordination syndrome that is associated with the ingestion of phalaris at a time when it contains toxic alkaloids. 'Staggers' is a term used to describe a brain disorder characterised by an unsteady stumbling gait, sheep may be unable to stand.
Sheep and cattle may be affected although cattle are less susceptible than sheep. In cattle the nerves that control the tongue and swallowing are affected, so they are unable to eat normally and can suffer severe loss of condition.
What causes phalaris staggers?
Phalaris staggers is caused by ingesting green, actively growing phalaris at a time when the grass contains a high concentration of toxic alkaloids, generally in autumn during the first re-growth or after periods of pasture stress following frost or water stress.
It usually takes a minimum of 10 days grazing on phalaris for staggers syndrome to develop. Most cases occur after one to two months of grazing.
'Phalaris sudden death' is thought to be a completely separate syndrome to 'Phalaris staggers'. The sudden death syndrome also occurs in autumn to early winter, but occurs within 36 hours of introducing stock onto phalaris pastures. The sudden death syndrome is poorly understood. It is proposed to occur as a result of ingesting a compound in phalaris that decreases nitrogen metabolism, resulting in peracute ammonia poisoning.
The polioencephalomalacia-like (PE) sudden death syndrome occurs in autumn to early winter within 12-36 hours of introducing stock onto pastures with new phalaris growth.
The first six weeks post autumn break after a dry season is the most risky period for phalaris PE like sudden death. The toxins in the grass result in animals presenting with high levels of ammonia in their system. It takes stock 24 hours to develop effective nitrogen metabolism on phalaris.
Phalaris cardiac-sudden death outbreaks may occur as soon as 24 hours following introduction to the pasture, however in some reports sheep had been grazing the toxic pastures for 2 weeks before outbreaks occurred.
Clinical signs and diagnosis of phalaris toxicity
Phalaris is well known to cause delayed onset clinical signs, nervous signs can persist for 2 months and have been known to arise up to five months after stock are moved off phalaris. In some animals signs may persist for life but some mildly affected animals will recover. The chance of recovery is higher the more rapid the onset of signs.
Signs of phalaris staggers include head nodding and bunny hopping with a wide based gait. Some cases walk on their knees, or knuckle at the fetlocks. Symptoms of staggers can be induced by normal mustering stress.
Tongue and swallowing paresis particularly in cattle can lead to "manic" attempts to eat with frenzied tongue stabbing at grass.
Phalaris staggers can progress to death due to the stress and alkaloid affects on the heart and respiratory system. Deaths are reported to continue for 1 week after removal from the pasture.
Characteristic pigmentation in the brain can be seen on microscopic examination at post mortem in most affected animals, and can often be seen grossly.
Phalaris polioencephalomalacia (PE) sudden death
Affected stock may be found staggering, wandering aimlessly, blind or convulsing. Some cases walk on their knees, or knuckle at the fetlocks.
Ammonia levels may be elevated in affected animals and some signs of damage to brain cells with a characteristic pigmentation in the brain can be seen on microscopic examination at post mortem in most affected animals, and can often be seen grossly.
Phalaris cardiac sudden death
Cardiac disturbances cause the animal to suffer from respiratory distress, their mucous membranes becoming blue. Signs may be induced by flock disturbance or when the animals are forced to exert themselves.
No nervous signs are seen with this form of phalaris poisoning, nor are there any obvious gross or histopathological lesions on post-mortem.
Most affected sheep die, however some may spontaneously recover.
Assessing risk of phalaris staggers
It is not possible to assess the phalaris staggers risk by taking samples from the plants, animals or soil. However known higher risk pastures (from a past history of problems) could be monitored more carefully for seasonal effects and animals grazing these pastures routinely protected by using a supplement.
Some producers may choose not to stock phalaris paddocks until July or in the six weeks after the break to avoid the risk of phalaris PE like sudden death. While this is an effective decision from an animal health perspective it may restrict feed options for stock at this time of year.
Other options to reduce the risk of phalaris poisoning include:
- limiting the intake of phalaris in the first 24-48 hours by restricting access to a few hours at a time
- feeding stock hay prior to introducing them to the at risk pasture
- set stocking of high risk paddocks at the autumn break
- introducing a small number of sentinel stock 48 hours prior to the rest of the mob to see if they are affected.
These preventative steps will limit deaths, but it is probably impossible to eliminate deaths altogether if you graze stock on phalaris paddocks.
The incidence of cardiac sudden death due to phalaris is greatest during the first few months of new growth, typically autumn to early winter thus it is wise avoid grazing phalaris dominant pastures during this period as described above.
The risk of stock developing phalaris staggers is a function of soil cobalt levels, levels of soil ingestion and levels of phalaris dominance and palatability. Cases can occur at all times of the year but peak in late autumn and winter. Phalaris has been found to be most toxic during rapid growth (autumn) and then increase again when the plant is placed under environmental stress (such as frost or water stress). The highest concentrations of alkaloids are in the first re-growth and in the leaf blades. Under field conditions, studies have found that the hazard from phalaris decreases as the plant matures. However, this may be because other species dilute the phalaris and because clovers and other legumes contain more cobalt.
Treatment and prevention of phalaris staggers
There is currently no cure. Severely affected animals should be euthanised.
Changing the pasture composition of the paddock and introducing less toxic cultivars of phalaris may assist in long term management of this condition on some properties.
Provision of oral cobalt protects against phalaris staggers. Cobalt appears to stimulate the proliferation in the rumen of microorganisms which are capable of destroying the causative agent/toxic alkaloid that causes the staggers syndrome. Cobalt foliar sprays and cobalt bullets are both effective at preventing phalaris staggers.
Intraruminal cobalt pellets: given using a 'bulleting gun', cobalt pellets are best for long term prevention of staggers. In sheep, pelleting is best done when lambs are weaned. Pellets are not suitable for lambs less than eight weeks old. One pellet may be enough in some cases, but two are recommended especially in higher risk situations. Sheep should be treated once every three years, and cattle once a year.
Foliar spraying: Annual foliar spraying of phalaris pastures with cobalt soon after the autumn break has reportedly been effective at preventing phalaris staggers. Mix 100 grams of cobalt sulphate per ha with water and apply to a quarter of the paddock. The foliar spray is only absorbed by green leaves, so producers may need to wait until a few weeks after the autumn break for phalaris to shoot before they spray. Rain within two to three days of application will wash the cobalt spray off before it can be fully absorbed, so spraying should be carefully timed. Livestock can be introduced into the paddock as soon as spraying is completed.
Cobalt licks: Cobalt sulphate or Cobalt carbonate can be added to drinking water, incorporated into salt licks or mixed with feed. The main problem with intake from licks and blocks is the variation in uptake/usage by individual animals. To prevent phalaris staggers the rate of cobalt recommended is much higher than usual so many of the nutritional supplement licks would need to be fortified to make them work and cobalt can also be toxic in excess.
Cobalt drenches: Cobalt sulphate drenches are available, however the protective effect only lasts about 1 week, demanding weekly re-dosing.
Injection of VitB 12 does not offer any protection against the staggers syndrome.
Treatment and prevention of phalaris sudden death
There is no treatment for the phalaris sudden death syndromes and stock should be removed immediately from the paddock with as little stress as possible to avoid further mortalities. Once moved, there should be no more new cases.
Cobalt supplementation does not prevent phalaris sudden death syndromes.
Each year Australia requires eligible sheep and cattle brains to be tested as part of the National Transmissible Spongiform Encephalopathies Surveillance Program (NTSESP). This helps demonstrate that Australia is free from TSEs including scrapie in sheep and bovine spongioform encephalopathy or mad cow disease, which is imperative for trade. If sheep or cattle show neurological signs, contact your local District Veterinarian or private veterinarian and ask them to check if the case is eligible for a department-subsidised veterinary investigation and an incentive payment.
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