Motor neurone disease in molybdenum-deficient sheep fed the endogenous purine xanthosine: possible mechanism for Tribulus staggers

BACKGROUND

The occurrence of Tribulus terrestris motor neurone disease (MND) in sheep is linked with grazing Tribulus growing on cultivation paddocks. A previous survey found that the molybdenum (Mo) content of Tribulus growing on uncultivated soils in the Coonabarabran district of New South Wales was 3.03 ppm, but on cultivated soils it was <0.04 ppm. Tribulus contains the purine, xanthosine, which functions as a neuromodulator, and the catabolism of xanthosine is Mo-dependent.

DESIGN

To investigate the relationship between xanthosine ingestion and low Mo concentration, eight sheep were fed Mo-deficient lucerne chaff (<0.10 ppm), the Mo antagonist, sodium tungstate, and xanthosine (25 mg/kg/day) over 18 weeks and then returned to pasture.

RESULTS

Signs of MND developed in two sheep 30 months later and astrocyte degeneration occurred in all sheep.

CONCLUSION

The findings were similar to those observed in sheep with T. terrestris MND, suggesting that the combination of xanthosine ingestion and Mo deficiency may be the cause of this disorder.

Abnormal turning behaviour, GABAergic inhibition and the degeneration of astrocytes in ovine Tribulus terrestris motor neuron disease

OBJECTIVE

To observe the clinical signs of sheep affected by Tribulus terrestris motor neuron disease, to ascertain their response to striatal dopamine reducing drugs, and to examine their brains and spinal cords for microscopic changes.

PROCEDURES

Twenty-eight sheep displaying well developed clinical signs of the disorder were observed. Twenty-two of these and 22 normal sheep were then randomly allocated to three groups and treated with diazepam, chlorpromazine, or xylazine. The time that it took an animal to return to a standing position following drug administration was recorded. The brain and complete spinal cord were removed from each of the other six affected sheep and fixed in formalin. Brains were sectioned throughout at 5 mm intervals and spinal cords at 10 mm intervals. All tissues were paraffin embedded and examined by light microscopy. A few samples were examined by electron microscopy.

RESULTS

Clinical signs included postural asymmetry with a right:left body-side dominance within the group of 50:50, unequal flaccid paresis in the pelvic limbs, extensor muscle atrophy and adduction of the weaker pelvic limb, and concurrent abduction of the stronger. Forward motion followed either a fixed left or right hand curved trajectory, the sheep no longer being able to choose which. Twelve animals intermittently displayed rotational behaviour that involved loss of postural balance without locomotor activation. The administration of diazepam, chlorpromazine, or xylazine caused limb paresis and sedation, with affected sheep being slower than normal sheep by factors of 8, 3 and 2 respectively, to return to a standing position. There were scattered areas of mild Wallerian degeneration throughout the spinal cord, and in both the brain and the cord there were small numbers of degenerate astrocytes containing novel cytoplasmic pigment granules.

CONCLUSIONS

Affected sheep had a dysfunction in the control of directional change and this provides a new insight into the normal mechanism for 'turning' in quadrupeds. Directional change requires a functional asymmetry or lateralisation within the upper motor neuron to accommodate a difference in the rate of forward progression of each body side and, simultaneously, a lateral shift of the centre of gravity. The sensitivity of affected sheep to diazepam is consistent with a pre-existing elevation in GABAergic neuronal inhibition, probably as a result of a reduction in glutamatergic neuronal excitation. The cytoplasmic pigment found in degenerate astrocytes was novel and its presence in the brain nuclei known to contribute to turning behaviour could have aetiological significance. The motor output of the basal ganglia in Tribulus neurotoxicity appeared to be excessively inhibitory to the pelvic limb extensor muscles and was asymmetric, causing fixation of the turning posture but not locomotor activation. An intoxication of specific purine sensitive, glutamate releasing astrocytes, located in nuclei controlling turning, was suspected.

Reassessment of the toxicity of Hypericum perforatum (St John's wort) for cattle

OBJECTIVE

To investigate the clinical effect of administering sufficient Hypericum perforatum to cattle to deliver quadruple the reported oral toxic dose.

ANIMALS

Thirty-six yearling Hereford (n = 18) and Angus (n = 18) steers.

DESIGN

A series of six experiments was conducted, each using 12 animals in a 2 x 2 factorial design, with two breeds of cattle (Hereford, Angus) and two dose levels of hypericin, 1.5 mg/kg (treated group) and 0 mg/kg (control group). Each set of 12 steers was used in duplicate experiments, with all animals alternated between treated and control groups.

PROCEDURES

Treated groups received finely milled H. perforatum administered orally in gelatin capsules to provide 1.5 mg hypericin/kg body weight. All cattle were then exposed to direct sunlight for 5 h per day for 5 successive days. Rectal temperatures were measured immediately before and at the end of each sunlight exposure session. Rectal temperature above 40 degrees C, together with some other clinical sign of hypericin poisoning, was considered indicative of intoxication.

RESULTS

No animals developed a rectal temperature above 40 degrees C or other clinical signs of hypericin poisoning.

CONCLUSIONS

While the reported bovine oral toxic dose of 3 g dried plant/kg body weight, for flowering stage, presumed narrow leaved biotype, H. perforatum, is probably correct, the corresponding dose for hypericin of 0.37 mg/kg is incorrect. Based on its known concentration in this plant the toxic dose of hypericin for partially pigmented Hereford-cross cattle is estimated at about 10.5 mg/kg body weight and more than this for fully pigmented cattle. This would imply that cattle of the former type should be about three and a half times better protected against H. perforatum toxicity than are unpigmented, wool protected, Merino sheep. Cattle, particularly if fully pigmented, may have a role in grazing management to control H. perforatum.

The effect of shade, shearing and wool type in the protection of Merino sheep from Hypericum perforatum (St John's wort) poisoning

OBJECTIVE

To investigate the roles of shade, fleece length and wool type in the protection of sheep from Hypericum perforatum poisoning.

ANIMALS

Adult Merino ewes of superfine, fine and medium wool type.

DESIGN

Seventy sheep were divided into seven equal groups. During late spring and summer a series of successive, replicate experiments was conducted, each using one group and lasting 5 days. The sheep carried 14 to 24 weeks wool growth. In each experiment the treatments tested were Hypericum +, sunlight + (n = 7); Hypericum +, sun - (n = 1); Hypericum -, sun + (n = 1); Hypericum -, sun - (n = 1). Next, 24 sheep in two equal groups were used in experiments of similar design to the above. Each group consisted of nine recently (1 to 3 weeks previously) shorn and three wool covered (25 to 26 weeks growth) sheep. The treatments tested were Hypericum +, sunlight +, fleece - (n = 9); Hypericum +, sun -, fleece + (n = 1); Hypericum -, sun +, fleece + (n = 1); Hypericum -, sun -, fleece + (n = 1).

PROCEDURES

Finely milled Hypericum was administered by gavage to provide 3 mg hypericin / kg body weight. Sheep were sheltered from direct sunlight or were exposed for 5 h per day for 4 successive post-treatment days. Rectal temperatures were measured immediately before and at the end of each sunlight exposure session. Rectal temperature above 40 degrees C was considered indicative of hypericin poisoning.

RESULTS

After Hypericum treatment hypericin poisoning was displayed by 26.5% of woolled sheep that were exposed to sunlight, but by none of those that were fully shaded. In similarly treated but recently shorn sheep 94% displayed hypericin poisoning when exposed to sunlight. In the wool covered group the percentages of poisoned animals based on wool type were: superfine 14%, fine 28.5%, medium 33.3%. In the recently shorn group the percentage for all three approached 100%.

CONCLUSIONS

A majority of Merinos with at least 14 weeks wool growth will not be poisoned by a single oral dose of 3 mg hypericin /kg, but because hypericin persists in the blood circulation for several days this safe dose will be lowered by continuous daily ingestion. Sheep with access to substantial areas of shade could safely ingest much greater amounts of hypericin. Wool removal greatly increases the risk of poisoning. Superfine Merinos with a wool cover should be able to ingest more hypericin than comparable, medium wool types, without any increased risk of poisoning. The ability of ruminant livestock to safely ingest Hypericum is probably determined more by the amount of skin protection they have against incident sunlight than by differences in hypericin metabolism and excretion capacity.

Evidence that enforced sunlight exposure can cause hyperthermia in cattle ingesting low levels of ergot of rye (Claviceps purpurea), when air temperature and humidity conditions are only moderate

OBJECTIVE

To determine the rectal temperature response of cattle, following the oral administration of ergot of rye (Claviceps purpurea), under pen conditions of enforced sunlight compared with those of enforced shade.

DESIGN AND PROCEDURE

Hereford cross steers were divided into two groups of 18. One group was dosed once, on a Monday morning, with finely ground rye grass ergots at a rate of 180 mg/kg body weight and held in the sun for 7 h each day until Friday afternoon. The other group was not dosed but was similarly held in the sun during the same period. Their rectal temperatures were measured early morning and mid afternoon, from Monday to Friday inclusive. The process was repeated for each group, but this time they were held in the shade. The four treatment options were run concurrently by conducting the experiments over 6 weeks and using 3 animals in each treatment group, each week.

RESULTS

The thermic response over all weeks, of the ergot treated, sunlight exposed cattle, was deemed greater than for the other groups, based on the following four parameters. The increase in rectal temperature between early morning and mid afternoon, the size of the mid afternoon rectal temperature rise, the difference between the maximum mid afternoon rectal temperature recorded by an animal in the sun compared with that recorded by the same animal in the shade, and finally the number of animals in a treatment group that recorded rectal temperatures > 40.00 degrees C. The difference in the daily increase in body temperature between the ergot treated, sun exposed cattle and the ergot treated, shaded cattle, was greater than that observed between the sun exposed and shade restricted control cattle. Nine of 18 ergot treated and sun exposed cattle developed hyperthermia; no cattle in the other three groups did.

CONCLUSIONS

Some sunlight exposed cattle, dosed with a low amount of ergot of rye, can experience a body temperature elevation above the normal range, even under mild ambient temperature and humidity conditions. Sunlight exposure causes an increase in body temperature in normal cattle that is probably attributable to solar radiant heat. Ergot ingestion causes an increase in body temperature in shaded cattle that is probably attributable to ambient temperature and humidity effects. This response can be increased by sun exposure and this increase is attributable to solar radiant heat effects and possibly also solar radiation. A response to radiant heat is consistent with the known physiological effects of ingested ergot alkaloids, but a response to solar radiation is not.

Seasonal variation in hypericin content of Hypericum perforatum L. (St. John's Wort)

Hypericin and pseudohypericin, bioactive constituents in St. John's Wort (Hypericum perforatum), have been determined in the soft tops of the plant that are most likely to be browsed by foraging livestock. In two consecutive seasons, the hypericin/pseudohypericin concentration in a broad leaf biotype varied from a winter minimum of less than 100 ppm to a summer maximum approaching 3000 ppm. In contrast the narrow leaf biotype increased from similar winter values to summer maxima approaching 5000 ppm. The latter biotype was slower in returning to low levels of hypericin/pseudohypericin.

Sunlight associated hyperthermia as a consistent and rapidly developing clinical sign in sheep intoxicated by St John's wort (Hypericum perforatum)

OBJECTIVE

To assess the usefulness of rectal temperature responses in Australian bred Merino sheep, following the oral administration of Hypericum perforatum (St John's wort), as an early indicator of Hypericum intolerance.

DESIGN

Thirty-three Merino ewes were divided into three groups of 11. Each group was dosed with finely ground, dried, flowering growth stage H perforatum plant material at either 5.7, 4.0, or 2.85 g dry plant per kg live weight. This corresponded to 5.3, 3.7 and 2.65 mg hypericin per kg live weight, respectively.

PROCEDURE

The sheep were dosed with a plant slurry by stomach tube and then exposed to bright sunlight for up to 5 h per day over successive days. Their clinical responses were observed and rectal temperature measured.

RESULTS

Ingestion of H perforatum followed by exposure to bright sunlight frequently resulted in clinical signs attributable to skin irritation and central nervous effects, including an inappropriate increase in body temperature. A decrease in H perforatum ingestion from 5.7 to 2.85 g dry plant per kg live weight and a corresponding decrease in hypericin ingestion from 5.3 to 2.65 mg per kg live weight, was associated with a decrease in the severity of the clinical signs, including the severity of the hyperthermia.

CONCLUSIONS

The rectal temperature rise in affected sheep is a reliable indicator of the early development of an adverse clinical effect. There appears to be an absolute requirement for exposure to bright sunlight before any effects of H perforatum will develop. A single dose of H perforatum remains potentially effective for up to 4 days. In the small group of Merino sheep tested a tolerance level for H perforatum, eaten at the flowering stage, of < 1% (plant wet weight) of body weight and a tolerance level for hypericin of < 2.65 mg per kg live weight, were demonstrated.

Chronic gypsum fertiliser ingestion as a significant contributor to a multifactorial cattle mortality

OBJECTIVE

To assess the validity of claims that heavy metal contamination from an open-cut mine caused the death of 226 cattle on a nearby farm over a period of 18 months, and to investigate other possible contributing factors.

PROCEDURE

A retrospective assessment of previous investigations combined with additional chemical analyses.

RESULTS

Extensive chemical analyses produced no evidence of heavy metal contamination associated with the mine. Analysis of bones indicated exposure to fluoride in greater than normal amounts. The main source of fluoride seems to have been gypsum that was included in a feed supplement and also ingested from fertiliser dumps on paddocks. The gypsum itself may have contributed significantly to the ill health. Other factors probably affected some classes of animals, notably the young calves.

CONCLUSIONS

What originally seemed to be a disease problem of single aetiology probably was an expression of interacting multifactorial causes. This investigation has highlighted the potential toxicity of gypsum to livestock and the need for further studies to establish its basis.

Bovine asymmetric hind limb paresis, a presumptive in-utero plant poisoning

A new locomotory disturbance of cattle is described. The condition has occurred sporadically since the mid-1980s. Affected herds had all grazed flood plain pastures in a restricted area of north-western New South Wales. Calves were either born with clinical signs or developed them by 4 months of age. The disease was characterised by a slowly progressive, irreversible, asymmetrical, paresis of the hind limbs. Affected cattle experienced persistent hyperextension of the hip and stifle joints. Macroscopic and microscopic examination of the nervous and musculoskeletal system failed to demonstrate abnormalities that would account for the clinical signs. The disorder shares many similarities with bovine spastic paresis. It is suggested that the pathogenesis of the disorder is nervous, and probably involves nigro-striatal, medulla oblongata, and spinal dysfunctional inputs. An in-utero plant poisoning was suspected but no specific plant association was determined.

The clinical differentiation of nervous and muscular locomotor disorders of sheep in Australia

Many of the nervous and muscular locomotor disorders that affect sheep throughout Australia are commonly referred to as "staggers" syndromes. The range of clinical signs displayed by sheep suffering these disorders is sufficiently diverse to enable each syndrome to be graded into one of 5 progressive clinical groups. The first group, the limb paresis syndromes, includes the primary myopathies associated with the ingestion of Ixiolaena brevicompta, Malva parviflora, and Trachymene ochracea, as well as selenium and Vitamin E disorders, Paroo virus staggers, congenital progressive muscular dystrophy, humpy back, hypocalcaemic muscle weakness, Tribulus terrestris staggers and tetanus. The second group is characterised by limb paresis with knuckling of the fetlocks, and includes the plant-associated toxicities of Romulea rosea, Stachys arvensis, Trachyandra divaricata, and Tribulus micrococcus, together with haloxon toxicity, enzootic ataxia (copper deficiency), and the probably genetic disorders of segmental axonopathy, neuroaxonal dystrophy, and degenerative thoracic myelopathy. Other locomotor disorders that fit more loosely into this group are listerial myelitis (post-dipping staggers), vitamin A deficiency, cervico-thoracic vertebral subluxation Stypandra glauca toxicity, Ipomoea spp toxicity, ivermectin toxicity, and botulism. The third group, the falling syndromes, includes the probably genetic disorders of thalamic cerebellar neuropathy, cerebellar abiotrophy, and globoid cell leucodystrophy, together with Swainsona spp toxicity. The fourth group, the falling syndromes, includes the plant associated toxicities of phalaris staggers, perennial rye grass staggers and nervous ergotism (Claviceps paspali).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms underlying Phalaris aquatica "sudden death" syndrome in sheep

Twenty outbreaks of Phalaris aquatica "sudden death" syndrome in sheep were investigated between 1981 and 1991. Four were confirmed and one was suspected, to be a cardiac disorder; 5 were confirmed and 3 were suspected, to be a polioencephalomalacic disorder; the aetiology of the remaining 7 outbreaks could not be determined. Potentially toxic levels of hydrocyanic acid (20 to 36 mg/100 g) were measured in the 3 toxic phalaris pastures tested. The measurement of potentially toxic levels of nitrate nitrogen (2920 micrograms/g) in toxic phalaris pastures by others, was noted. It is suggested that phalaris "sudden death" syndrome could have as many as 4 different underlying mechanisms, and that these might reflect the presence in the plant of a cardio-respiratory toxin, a thiaminase and amine co-substate, cyanogenic compounds, and nitrate compounds.

Locomotor effects in sheep of alkaloids identified in Australian Tribulus terrestris

Fresh, mature, ungrazed Tribulus terrestris plant material was subjected to a standard alkaloid extraction procedure. The extract was fractionated by thin layer chromatography (TLC) and high performance liquid chromatography (HPLC). Two major alkaloid fractions were demonstrated. These fractions were identified by means of TLC, ultraviolet spectrofluorimetry (UVS) and HPLC, as the beta-carboline indoleamines harmane and norharmane. The extractable alkaloid content was determined to be 44 mg/kg dry matter. Synthetic harmane and norharmane were administered subcutaneously to sheep at a dose rate of 54 mg/kg. Both compounds caused similar nervous effects. The main effect observed was limb paresis, which in some sheep was body side blased. The clinical signs observed in the experimental sheep were consistent with those described for naturally occurring cases of Tribulus terrestris staggers. It was proposed that harmane and norharmane accumulate in tryptamine-associated neurones of the central nervous system, during months of tribulus ingestion, and gradually interact irreversibly with a specific neuronal gene DNA sequence.

The pathogenesis of the nervous syndrome of Phalaris aquatica toxicity in sheep

The clinical signs displayed by 96 sheep affected by the nervous syndrome of Phalaris aquatica toxicity and 10 normal sheep injected intravenously with the phalaris alkaloid, 5-methoxy dimethyltryptamine (dose range 0.01 to 5.0 mg/kg), were observed. The distributions of phalaris indole-like cytoplasmic pigments in nuclei of the brains and spinal cords of 9 naturally affected sheep were determined microscopically. Based on the relationship between clinical signs and the central nervous system nuclei involved in their production, the distribution of phalaris indole-like pigments, and the pharmacology of dimethylated tryptamines, it is suggested that the nervous syndrome induced by Phalaris aquatica results from a direct action of phalaris alkaloids upon serotonergic receptors in specific brain and spinal cord nuclei.

Upper motor neurone effects in sheep of some beta-carboline alkaloids identified in zygophyllaceous plants

The beta-carbolines harmane, norharmane, tetrahydronorharmane, harmine, harmaline and harmol were administered to sheep to assess their effects on upper motor neurone function. Harmane at a dose rate of 54 mg/kg induced hypomotility, head tremors, pelvic limb paresis, hypermetria and a wide based stance. A range of similar effects were observed with norharmane at the same dose rate. Tetrahydronorharmane at a dose rate of 54 mg/kg induced hypermotility followed by hypomotility, asymmetrical pelvic limb paresis, hypermetria, a wide based stance, and stereotyped eating behaviour. Harmine and harmaline at 6 mg/kg induced mild head and body tremors, and at 18 mg/kg induced hypomotility, intense head and body tremors, pelvic limb paresis, crossing over of limbs, neck extension and head swaying. Harmol was not effective at 54 mg/kg by either the subcutaneous or intraperitoneal routes, but at an intravenous dose of 27 mg/kg it induced hypermotility followed by hypomotility, body tremors, limb paresis, muscle asynergy, a wide based stance and jumping behaviour. Harmane, tetrahydronorharmane, harmaline and harmol were convulsive in some sheep at high dose rates.

Experimental evidence that tryptamine alkaloids do not cause Phalaris aquatica sudden death syndrome in sheep

The acute toxicity for sheep of 3 alkaloids that occur in Phalaris acquatica was examined by intravenous and oral administration. The lowest tested dose rates that produced clinically observed signs were, for 5-methoxy dimethyltryptamine, 0.1 mg/kg body weight intravenously and 40 mg/kg orally; for gramine, 10 mg/kg intravenously and 500 mg/kg orally; and for hordenine, 20 mg/kg intravenously and 800 mg/kg orally. All induced the clinical signs observed in the nervous form of phalaris toxicity, but none induced the cardiac, sudden death, syndrome.

Toxic tunicaminyluracil antibiotics identified in water-damaged wheat responsible for the death of pigs

A unique mixture of toxic tunicaininyluracil antibiotics, closely related to the corynetoxins which causc annual ryegrass toxicity and to the tunicamycins, has been identified in rain-damaged, stored wheat implicated in a fatal intoxication of pigs. The toxins, present at a level of approximately 4.5 mg per kg, were isolated by preparative t.1.c. They displayed specific inhibition of uridine diphospho-N-acetylglucosamine : dolichol-phosphate N-acetylglucosamine-1-phosphate transferase and bacterial inhibition consistent with this type of antibiotic, and produced symptoms in rats identical with those associated with the tunicamycin and corynetoxin complexes. Chemical identification, based on t.l.c., h.p.l.c., co-chromatography with authentic toxins and catalytic hydrogenation, was confirmed by fast atom bombardment mass spectrometry. The origin of these toxins is unknown, but the unique mixture of components detected suggests a previously unreported tunicaminyluracil antibiotic-producing microbial source.

A novel nigrostriatal dopaminergic disorder in sheep affected by Tribulus terrestris staggers

An investigation was carried out into the pathogenesis of a unique locomotory disorder of sheep. Thirty sheep which had exhibited clinical signs for more than 15 months were examined for the presence of muscle atrophy, limb paresis, and spontaneous rotational behaviour. A single large dose of levodopa was administered to 12 normal and 12 affected sheep, and the drug-related effects of sedation and excitation were monitored continuously for three hours. The striatum was removed from eight normal and eight affected sheep and assayed for dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC). The findings of asymmetrical atrophy of pelvic limb extensor muscles, asymmetrical paresis of the limbs, a left:right dominance ratio of 50:50, spontaneous rotational behaviour, decreased response to levodopa loading, and decreased levels of DA (19 per cent reduction) and DOPAC (30 per cent reduction) in the striatum of affected sheep, were interpreted as indicative of a primary nigrostriatal dopaminergic disorder. It is suggested that a striatal presynaptic receptor malfunction has occurred.

Staggers in sheep associated with the ingestion of Tribulus terrestris

The history of an unusual locomotory disturbance of sheep is traced from its first recognition in 1937 through to the most recent outbreak in 1981-83. The condition occurred only at certain times and in restricted areas of the central and northern slopes districts of New South Wales. Outbreaks were repeatedly associated with drought periods during which sheep grazed large areas of Tribulus terrestris for many months at a time. Many thousands of sheep were affected, but the prevalence varied greatly between flocks. The course of the disease was characterised by a slowly developing, irreversible, asymmetrical, weakness of the hindlimbs. The clinical signs suggested that a lesion of the thoraco-lumbar spinal cord region was present. The macroscopic and microscopic examination of the nervous and musculoskeletal systems failed to demonstrate abnormalities which would account for the clinical signs. Haematological, biochemical and toxicological test results supported the concept of a neuromuscular disease process being present, but failed to indicate its aetiology.