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Heliotrope Toxicity In Sheep and Cattle

Heliotropium europaeum, often referred to as potato weed, blue weed, or common or wild heliotrope, was introduced into Australia last century and has established in northern Victoria, the Riverina of New South Wales and southern parts of South Australia.

It is a summer sowing annual herb that thrives on disturbed, bare or open, cultivated ground. Heliotrope has branched stems, is up to 300 mm high and is covered with coarse white hairs. The leaves, which are grey-green in colour and oval shaped, are arranged alternately on the stems. It has small white flowers.

Seeds germinate in the warm, moist conditions after each late spring or summer rain, and growth is prolific where seedlings are not shaded and there is no competition from perennial plants. Fallow ground is ideal-, hence it is more of a problem in wheat/sheep areas than pastoral areas. Heliotrope has enormous seeding potential and the seeds are viable for many years.

Heliotrope contains poisons or toxins known as pyrrolizidine alkaloids. The toxins are present at all stages of growth and in all parts of the plant, including the seed.


Nature of the problem   Top
All animals are susceptible to poisoning. The most susceptible are monogastric species such as pigs, poultry, horses and humans. Poisoning in these species is usually caused by the consumption of cereal grains contaminated by heliotrope seed. Ruminants including sheep and cattle are less susceptible because some of the alkaloids are destroyed by microbes in the rumen. However because sheep are the dominant species grazed in areas where heliotrope growth is most prolific, they contribute most to heliotrope poisoning losses that are worth about $45 million, each year in Australia.

When heliotrope is eaten, the absorbed pyrrolizidine alkaloids form secondary compounds that remain in and damage the liver. Liver damage increases progressively with additional intakes of heliotrope. The disease is usually chronic in nature with signs of sickness and death often delayed for weeks, months or years after the consumption of the heliotrope has ceased.

Heliotrope toxicity in sheep   Top
Heliotrope poisoning problems are more common in British breeds and crossbreeds than Merinos. Merinos tend to avoid the plant when other feed is available, whereas British breeds and crossbreeds will eat it more readily and sometimes preferentially. In general, symptoms are not seen until exposure to heliotrope has occurred over more than one season. The death rate increases with age.

The effects of breed and age were shown in a recent survey of sheep losses in the Victorian Mallee. The average loss in ewe flocks was 7%. The average for Merino ewe flocks was 6%; increasing from 3% in flocks up to one year old to 9% in flocks three or more years old. In non-Merino ewe flocks the average loss was 11% (7% in young flocks, and 13% in old flocks). In this survey, about half of the total losses were attributed to heliotrope poisoning.

Affected sheep can live without obvious sickness or symptoms and death can occur. Severe poisoning episodes are common, characterised either by some sheep losing weight and dying over a period of time, often while they are grazing lush medic or clover pastures, or by the onset of deaths associated with some stressful event such as lambing, mustering or shearing. In the latter case, deaths usually follow one or two days of depression and separation from the mob, with jaundice (yellowing of the fat), but no loss of condition. Photosensitisation (reddening or scabs on the ears, muzzle or other wool-free areas) is common if affected sheep are grazing green pasture. Less obvious but important losses are caused by reductions in appetite, body weight, wool growth and quality, fertility and productive life span. The sheep "age" prematurely.

The secondary effects of heliotrope poisoning include pregnancy toxaemia, copper poisoning and ammonia poisoning. Pregnancy toxaemia occurs when sheep within the last six weeks of pregnancy are unable to provide sufficient energy for the developing foetus (often twins). As the provision of energy is a function of the liver, those cwcs with chronic liver damage are more likely to develop pregnancy toxaemia. This condition is a major cause of ewe deaths in the Mallee.

Copper poisoning causes sudden death, with jaundice, when the sheep are stressed. Liver damage caused by heliotrope leads to an abnormally high uptake of copper by the liver, especially when the diet contains clovers or medics that have a high copper content. Under stressful conditions (transport, yarding, lactation, malnutrition, etc) the stored copper may be released suddenly from the liver. This causes massive destruction of red blood cells that result in jaundice, red-brown discolouration in the urine (red-water) and, often death.

Ammonia poisoning causes the sudden death (without loss of condition) of sheep grazing lush clover or medic pastures that have a high protein content. Normally the ammonia produced in the rumen from dietary protein is converted in the liver to urea and excreted in the urine. The capacity of the liver to do this is reduced by heliotrope damage, and as ammonia is toxic in large amounts, death can result.

Photosensitisation is the result of an excessive sensitivity to sunlight caused by an accumulation of phylloerythrin in the blood. This chemical is a breakdown product of chlorophyll, the pigment present in green plants. Normally phylloerythrin is removed from the body in bile, produced in the liver. Liver damage due to heliotrope poisoning retards the rate of excretion of phylloerythrin, causing a build-up of this compound in the blood and tissues (including the skin) when the intake of green plants is high. The interaction of phylloerythrin and sunlight results in skin damage resembling severe sunburn.

Heliotrope toxicity in cattle   Top
Cattle are more susceptible to the toxic effects of heliotrope than are sheep and deaths can occur within 10 days of commencing to eat heliotrope. Usually however, deaths are spread over several months, with affected cattle showing depression, reduced appetite, loss of condition, diarrhoea, restlessness and persistent aimless walking. As with sheep, cattle can suffer low lethal effects from heliotrope poisoning and the resulting chronic liver damage can be responsible for serious production losses. Cattle will avoid eating heliotrope when other feed is available, but introduced cattle are more likely to graze it.

Prevention   Top
  • Where heliotrope poisoning is suspected, confirmation should be obtained through a veterinary surgeon or animal health adviser. Other diseases can cause similar symptoms. For example, pyrrolizidine alkaloids occur in plants other than heliotrope. In the Riverina of New South Wales, Echium plantagineum (Paterson's Curse, or Salvation Jane) is one such plant that is often abundant in pastures. More losses in cattle in these areas are attributed to this plant than to heliotrope because it is more palatable. In sheep, also, the possibility of involvement of plants other than heliotrope should be considered. Similar symptoms can result from other causes of liver damage.
  • The feeding of a high energy - low protein grain diet (e.g. barley, wheat or oats, but not peas or lupins) and a reduction in stress, will minimise problems with pregnancy toxaemia and ammonia poisoning. Consideration should be given to culling flocks or mobs in which exposure to heliotrope and losses have been high, as the maintenance of a suitable diet for long periods will be difficult and costly.
  • Minimise the exposure of stock, especially valuable stock, to heliotrope-dominant grazing. Sheep are often used to graze fallows or stubbles as an aid to weed control. The risk associated with this practice should be appreciated.
  • Use minimum tillage cultivation and stubble retention. Both will reduce the amount of heliotrope that grows.
  • Grow lucerne or other perennials. Heliotrope seedlings are very susceptible to shading and competition. In the Riverina, dense stands of lucerne (21 to 77 plants per square metre) have been shown to completely control heliotrope.
  • Combined weed control and livestock management. Seedlings less than 70 nun high can be controlled with non-selective herbicides, but larger plants are quite resistant to herbicides. Also, repeated applications of herbicide are necessary to control later germinations, limiting the usefulness of this method. Sheep of low value and low susceptibility are commonly used to clean up the sprayed weeds.
  • Where cattle are to be grazed, heliotrope should not be dominant in the pasture.

  • Research   Top
    Heliotrope has continued to spread, slowly but relentlessly, in defiance of current control practices. Some scientists consider that biological control methods, still to be developed, offer the only practical long-term solution.

    Several approaches being investigated by the CSIRO are:

  • Biological control is being examined by the CSIRO Division of Entomology. The most promising programs involve two European plant pathogens, the rust fimgus Uromyces helio~rqpii and the blight fungus Cerospora heliotropii-bocconii.
  • Inactivation or breakdown of pyrrolizidine alkaloids in the rumen. The CSIRO Division of Animal Health is developing controlled-release capsules to be inserted into the rumen for administering a substance that stimulates the activity of naturally-occurring microbes that break down pyrrolizidine alkaloids. The presence of these microbes explains why some sheep are naturally resistant to heliotrope poisoning. This work is considerably advanced and field trials could begin soon.
  • Vaccines, similar to those likely to become available in the near future against the natural plant toxins responsible for lupinosis and animal rye grass toxicity, are difficult to produce against pyrrolizidine alkaloids because the toxins are too small to be recognised by animal immune systems. These difficulties may be overcome in future.

    Assays to measure toxin exposure. Research is being directed at detecting the presence of pyrrolizidine alkaloid toxins in animal feeds, animal tissues, and in products such as eggs, milk and honey, all of which can become contaminated. This would enable the checking of cereal grain for heliotrope seed contamination, and checking the level of previous exposure to alkaloids of a sheep flock for which a decision to either buy, sell, or keep, was to be made.
     

    Doug Harris, Swan Hill
    © The State of Victoria, Department of Primary Industries, 1996-2008.
    This material may be of assistance to you but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or consequences which may arise from your relying on any information contained in this material.



     
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