By Meg Smart, University of Saskatchewan, Saskatoon
Polioencephalomalasia, or PEM, is a sporadic nervous system disease caused primarily by a deficiency of thiamine (Vitamin B1) in cattle.
Thiamine plays a key role in the sugar metabolism of the brain. A deficiency results in swelling of the brain tissue, and eventually death of brain cells in the areas crushed against the bones of the skull. Clinically, the animal suddenly becomes blind and unco-ordinated. If untreated, head pressing (pushing its head into a solid wall), opisthotonos ("star-gazing"), and convulsions occur, eventually followed by death. If treated early, the animal responds rapidly to intravenous thiamine, but the blindness can take up to six weeks to clear up.
Similar clinical signs and brain lesions are seen with cobalt deficiency, water intoxication, and selenium, molasses and lead toxicities.
The exact role that sulphate plays in PEM is controversial. The dietary sulphur requirement for beef cattle is 0.08 to 0.15% of dietary dry matter intake. Intakes above 0.4% may be toxic. In the rumen, sulphate is converted to sulphide, which can kill the rumen bacteria that produce thiamine or be directly toxic to the brain.
Research and case studies suggest a link between high dietary sulphate and thiamine. In a survey of Saskatchewan cow/calf operations, if water sulphate concentrations were 1000 mg/L, 20% of the cattle had low blood thiamine concentrations. When the water was low in sulphates, only 2.5% of the cattle had low thiamine levels. Forty-three percent of the farms tested in the southern and central parts of Saskatchewan had water sulphates 1000 mg/L. The significance of these findings in relation to the incidence of PEM was not investigated. Although the exact relationship is unknown, thiamine may play a protective role in sulphur toxicity.
A case report, describing an outbreak of PEM in pastured cattle, illustrates the complexity of the problem. In late July, a group of cattle were moved to a new pasture. The ambient temperature was 37oC. During the move, many of the cattle were deprived of water for up to two days. The initial well water contained 745 mg of sulphate per litre, but the new well contained 7200 mg/L. This was a sudden and dramatic increase in sulphate intake (the safe upper intake is 1000 mg/L). These sudden changes in diet, water quality and intake resulted in 11 of 110 cattle developing PEM (4 died). In another case, some mature beef cows developed PEM on a native pasture late in the grazing season. The affected cows were in poor condition (reflecting an inadequate nutrient intake). The sulphate content of the slough water at the time of the problem was 23,000 mg/L.
Fortunately, these problems occur sporadically and some of the risk factors can be controlled:
avoid sudden dietary and water quality changes, especially during extreme weather conditions
monitor pasture conditions and water quality, particularly under drought conditions and where alkali soils exist
ensure an adequate macro and trace mineral intake
control the intake of Kochia, especially if the drinking water is high in sulphate
periodically evaluate the body condition of pastured cattle; if they are losing condition, supplementary feed will be required.
To contact the author, write to the Dept. of Vet Internal Medicine, WCVM, 52 Campus Dr., Saskatoon, SK S7N 5B4.
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