Corynetoxins:
Corynetoxins are toxins produced by the bacteria Rathayibacter toxicus. This bacterium normally lives in the soil but it can sometimes infect the seed heads of certain grasses. The bacterium gains entry to the seed head by adhering to a parasitic nematode that infects the seed head.
The nematode, Anguina funesta, forms a gall that takes the place of the developing grass seed. The bacterium multiplies inside the nematode gall and a toxic bacterial gall is formed. Seed heads may look normal or they may be somewhat twisted and deformed and some may exude a yellow slime.
Grasses containing corynetoxins are poisonous to all livestock species. Animals are at risk if they ingest the infected grasses, the stubble after hay has been cut, or the hay of infected grasses. Hay cut from infected grasses can remain toxic for years. The grasses that have been known to accumulate corynetoxins in Australia are annual ryegrass, blown grass and annual beardgrass.
On ingestion of infected grass or hay, the corynetoxins are absorbed into the bloodstream and are transported to tissues where they accumulate.
The toxins cause damage to small blood vessels in the brain and this can lead to a disruption in the supply of oxygen to parts of the brain, resulting in brain damage. The corynetoxins also inhibit an important enzyme leading to the disruption of several metabolic processes.
Most cases of poisoning occur in late spring to early summer. This corresponds to the end of flowering and the period that toxins are highest in grasses infected with Rathayibacter toxicus. Cases can occur at other times of the year when animals graze infected stubble or ingest infected hay or grain.
Cases of corynetoxin poisoning in Australian horses are not common. One report describes poisoning in several horses from three farms in Western Australia. The horses had eaten meadow hay containing annual ryegrass infected with Rathyayibacter toxicus. Many of the affected horses died. Corynetoxin poisoning of horses has also been reported in South Australia and in northern New South Wales.
Signs of corynetoxin poisoning:
The signs of poisoning occur abruptly and include:
- Muscle tremors and shivering.
- Incoordination, stumbling, stifflegged gait
- Wide stance
- Convulsions and death
Affected animals should be moved to a quiet area and should be supplied with water and good quality food. Veterinary attention should be sought immediately if corynetoxin poisoning is suspected in horses. Horses that survive corynetoxin poisoning can take up to a month to make a full recovery.
Annual ryegrass:
Lolium rigidum [Poaceae]
Animals affected by corynetoxins in annual ryegrass are said to be suffering ‘annual ryegrass toxicosis’. Annual ryegrass is native to the Mediterranean region. It is highly valued as a pasture grass as it provides good early season grazing. On the downside is the potential for annual ryegrass toxicosis that can occur after the grass has flowered and set seed.
Growth and identification:
Annual ryegrass is an upright annual that can grow to almost 1 m in height. The grass produces large numbers of seeds and a large seed bank can accumulate in areas where the grass grows. Seeds are small and light and can be easily distributed by wind or water run-off. The stems of annual ryegrass are hairless and are reddish-purple towards the base, particularly on young plants. The leaf blades are shiny and hairless. The flowering stems are flat and grow to 30 cm in length.
Blown grass:
Lachnagrostis filiformis (formerly Agrostis avenacea) [Poaceae]
Blown grass is an Australian native grass.
Growth and identification:
Blown grass is an upright or semi-upright annual or perennial grass that grows to 70 cm in height. The lower parts of the leaves are sheathed around the stem. The leaf blades have shallow grooves along their length. The flower spike can be up to 30 cm long and the flower spikelets are spreading when mature. Once the seed drops the flower spike usually breaks off.
Annual beardgrass:
Polypogon monspeliensis [Poaceae]
Annual beardgrass is native to the Mediterranean region. It is often found in pastures but it is not valued as a forage species and is considered a weed in some areas.
Growth and identification:
Annual beardgrass is a loosely tufted annual that grows to 80 cm in height. The lower part of the leaves is sheathed around the stem. The leaf blades are flat and have a prominent midvein. The flower spike is up to 11 cm long. It is soft, compact and bristly.
Management notes:
- Avoid introducing the nematode and the bacteria that cause corynetoxin poisoning onto farms that are free of the problem. Do not feed hay or grain in paddocks if the feed has come from areas where the organisms causing corynetoxin poisoning are known to occur. Only use certified seed when improving paddocks.
- Graze permanent pasture through late winter and spring to remove the early flower heads of annual ryegrass, blown grass and annual beardgrass. This prevents the grasses from setting seed and thus prevents nematode infestation and subsequent bacterial infection of the nematode galls.
- Annual ryegrass hay should be cut between the time that the grass starts to flower and the end of flowering. Seed heads are most toxic when seed is set and this occurs just after flowering has ended.
- Hay with a high percentage of annual ryegrass seed heads should not be fed to horses.
- An integrated weed management approach is necessary to control annual ryegrass, blown grass or beard grass growing as weeds in pastures.
- A management plan that includes grazing, cultivation and herbicide use should be devised to control these grasses on farms where corynetoxin poisoning may be an issue. Seek advice from a local agronomist or weed management agency to select the most appropriate management program for your property.
Ornamental plants:
Many ornamental plants contain toxins that could cause poisoning in horses. It is important to select non-toxic species when planting out hedges or windbreaks alongside paddocks, when planting trees to improve the look of horse properties and to provide shade, or when landscaping stable and arena areas.
Toxic ornamental plants can spread seed or suckers and can sometimes encroach upon areas where horses are kept and this is especially so in urban and semirural areas. An awareness of toxic ornamentals will reduce the chance of accidental exposure and the potential for poisoning in horses.
Contain cardiac glycosides:
Lily of the valley
Convallaria majalis [Liliaceae]
Oleander
Nerium oleander [Apocynaceae]
Yellow oleander
Thevetia peruviana [Apocynaceae]
Mother of millions
Bryophyllum spp [Crassulaceae]
Cotton bushes
Gomphocarpus spp and Asclepias spp
[Apocynaceae]
Foxglove
Digitalis purpurea [Scrophulariaceae
Star of Bethlehem
Ornithogalum spp [Hyacinthaceae]
Blue periwinkle
Vinca major [Apocynaceae]
Contain grayanotoxins. Signs (usually appear with 6 hours of ingestion): depression, salivation, colic, seizures, heart irregularities and incoordination. Usually not fatal unless large amounts of the plant have been eaten.
Rhododendrons and azaleas
Rhododendron spp [Ericaceae]
Sheep laurel (lambkill)
Kalmia angustifolia [Ericaceae]
Mountain laurel (calico bush)
Kalmia latifolia [Ericaceae]
Pieris (lily of the valley shrub)
Pieris spp [Ericaceae]
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Hydrangea
Hydrangea macrophylla [Hydrangeaceae]
Contains potentially toxic alkaloids, saponins, resins and the plant can accumulate cyanogenic glycosides (see pages 30-34). Signs: Severe gastrointestinal irritation characterised by colic and diarrhoea that may contain blood. Seek immediate veterinary attention if hydrangea poisoning is suspected. Hydrangeas have been responsible for a small number of horse poisoning cases overseas and only a very small amount of ingested leaves can prove fatal.
Common elder:
Sambucus nigra [Caprifoliaceae]
Plums, peaches, apricots,
cherries, nectarines, almonds
Prunus spp [Amygdalaceae]
Spotted fuchsia
Eremophila maculata [Myoporaceae]
Wattles
Acacia spp [Fabaceae]
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These plants contain cells known as idioblasts. Inside the cells are sharp, needle-like crystals of calcium oxalate that are packed in a gelatinous material. When the plant material is chewed, saliva or plant sap can enter the idioblasts. The gelatinous material swells and this causes the calcium oxalate crystals to shoot out of the idioblasts. The crystals can penetrate the tongue, throat and the inside of the mouth to cause severe irritation. Some of these plants also contain chemicals that aggravate the symptoms by causing an inflammatory response. Signs (can appear immediately or within hours of ingestion): head shaking, salivation, mouth and tongue swelling, depression.
In very rare cases the swelling can cause airway obstruction. Affected animals may take days to fully recover. If poisoning is suspected, rinse the horse’s mouth with water and seek veterinary attention.
Dumb cane
Dieffenbachia seguine [Araceae]
Caladium
Caladium bicolor [Araceae]
Philodendrons
Philodendron spp [Araceae]
Arum lily
(calla lily, pig lily)
Zantedeschia aethiopica [Araceae]
Peace lily
Spathiphyllum spp [Araceae]
Elephant’s ear (cunjevoi)
Alocasia spp [Araceae]
Elephant’s ear (cocoyam, taro)
Colocasia spp [Araceae]
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Christmas rose
(stinking hellebore, Corsican hellebore, winter rose)
Helleborus niger [Ranunculaceae]
Contains cardiac glycosides (see pages 35-43), protoanemonin (see page 44) and other toxins. Signs (usually occur soon after ingestion): colic and other gastrointestinal problems. Neurological problems such as incoordination might be seen. Seek veterinary attention if Christmas rose poisoning is suspected.
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Green cestrum
Cestrum parqui [Solanaceae]
Night scented jasmine
Cestrum nocturnum [Solanaceae]
Contains carboxyatractyloside
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Larkspur
Consolida ambigua [Ranunculaceae]
Delphiniums
Delphinium spp [Ranunculaceae]
Monkshood
Aconitum spp [Ranunculaceae]
Contain diterpene alkaloids. Signs (usually appear within hours of ingestion): excitement, confusion, muscle tremors, weakness, collapse, respiratory paralysis and death. Very rare in horses in Australia. Important cause of poisoning in the northern hemisphere where the plants are commonly found in pastures, although cattle are the species most commonly affected. Seek immediate veterinary attention if diterpene alkaloid poisoning is suspected.
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