Timber milk-vetch-Astragalus miser Dougl. ex Hook.-Poisonous plant

Timber milk-vetch

General poisoning notes:

Timber milk-vetch (Astragalus miser) is a native herb that is found in southern British Columbia and southern Alberta. This plant causes acute and chronic toxicity in cattle and sheep. Experimental poisoning has been caused in other livestock. Honey bees that forage on the flowers of timber milk-vetch were also poisoned (Majak and Pass 1989).

Nomenclature:

Scientific Name: Astragalus miser Dougl. ex Hook.

Vernacular name(s): timber milk-vetch

Scientific family name: Leguminosae

Vernacular family name: pea

Geographic Information

Alberta, British Columbia.

Notes on Poisonous plant parts:

Timber milk-vetch contains miserotoxin. This toxic principle is found primarily in the leaves and reaches its highest concentration during the bud and mature-flower stages of growth. The levels drop rapidly when leaves dry. Herbicides bleach leaves and cause a reduction in the concentration of miserotoxin. Tests with fertilizer on range plants showed that use of urea (nitrogen at 200 kg/ha) increased the level of miserotoxin during the second year of fertilizer use on a clearcut site. Use of urea (nitrogen at 100 kg/ha) on grassland sites did not affect miserotoxin levels (Cheeke and Schull 1985; Majak and Wikeem 1986).

Toxic parts:

Leaves, stems.

Notes on Toxic plant chemicals:

The glycoside (3-nitro-1-propyl-beta-D-glucopyranoside), called miserotoxin, is the poisonous principle in timber milk-vetch. Other toxic chemicals have been found including the following:

3-nitro-1-propyl-beta-D-gentiobioside (called gentitoxin), 3-nitropropyl-beta-D-allolactoside.

Miserotoxin is rapidly hydrolysed by rumen organisms. The acute toxic effect is related to methemoglobinemia, where hemoglobin is oxidized by nitrite (Cheeke and Schull 1985, Majak et al. 1988).

Miserotoxin is not converted to NPA (3-nitro-1-propionic acid) in the digestive tract of nonruminants. Cattle and sheep can be intoxicated by feeding the chemical NPOH (3-nitro-1-propanal) at 20-60 mg/kg of body weight. NPOH is lethal at 30-35 mg/kg in cattle when fed intravenously:

LD₅₀; 77 mg/kg for oral acute toxicity in rats

LD₅₀; 25 mg/kg for chronic toxicity in rats fed twice daily for several days.

The acute and chronic syndromes were caused experimentally in rats, pigs, chicks, pigeons, rabbits, and mice (Majak and Pass 1989).

Toxic plant chemicals:

Miserotoxin, 3-nitropropanol.

Chemical diagram(s) are courtesy of Ruth McDiarmid, Biochemistry Technician, Kamloops Range Station, Agriculture and Agri-Food Canada, Kamploops, British Columbia, Canada.

Animals/Human Poisoning:

Note: When an animal is listed without additional information, the literature (as of 1993) contained no detailed explanation.

Cattle

General symptoms of poisoning:

breathing, labored

collapse

cyanosis

death

emphysema

incoordination

liver, congestion of

muscle, weakness of

weakness.

Notes on poisoning:

Cattle that have ingested timber milk-vetch can suffer from acute syndrome, in which a rapid onset with death occurs a few hours to a day after ingestion. Chronically affected animals have liver damage, emphysema, Wallerian degeneration of the spinal cord and peripheral nerves, and focal hemorrhages in the brain. Lactating animals are most susceptible to the toxin (Majak and Pass 1989).

Honey bees

General symptoms of poisoning:

Death, incoordination, weakness.

Notes on poisoning:

Honey bees were poisoned after ingesting the nectar of timber milk-vetch. Sickness and death occur with 0.1-0.8% daily mortality rate of the hive population. In experiments, 2.5% miserotoxin in a 50% sugar solution killed 100% of bees within 48 h. Poisoned bees were unable to fly, and dead bees were usually seen with the wings extended and the proboscis protruding (Majak et al. 1980).

Horses

Sheep

General symptoms of poisoning:

Breathing, labored, cyanosis, death, incoordination.

Notes on poisoning:

Sheep often collapse and die from acute miserotoxin poisoning after exhibiting few clinical signs. In sheep the respiratory signs of chronic poisoning are more prominent than the nervous system signs. The animals lose weight and develop respiratory distress, hind limb paresis, nasal discharge, and a roaring sound. Lactating sheep are more susceptible to intoxication than nonlactating ones (Majak and Pass 1989).