Acute poisoning with organophosphate based pesticides is a notable cause of morbidity and mortality in the developing world, but much less common in the United Kingdom. However, the potential for frequent low level exposures to organophosphates exists for many occupational groups including agricultural workers, sheep dippers, and pesticide sprayers. Over the past decade an increasing number of people began to suspect that repeated low level exposure to organophosphate compounds was causing adverse effects on their health. The worst affected were sheep farmers who dipped their flocks in organophosphate based dips, although other circumstances of exposure were also implicated. A broad range of symptoms was associated with exposure but none of these were sufficiently specific to indicate a possible physiological or toxic mechanism. Few patients presented with robust clinical signs so that clinical investigation was difficult and a diagnostic marker was not identified.

Some studies did suggest that long term effects on the central and peripheral nervous systems might be associated with frequent but low level exposure to organophosphate.1 2 These neurological effects were different from those associated with the delayed neuropathy known to follow acute poisoning with those organophosphate compounds that were already acknowledged to be neurotoxic and consequently had been banned from use. The new neurotoxic effects in humans ranged from neurobehavioural and electroencephalographic changes to increases in the variability of action potential latencies in skeletal muscles (the “jitter” of neuromuscular transmission measured in humans by single fibre electromyogram (SFEMG)). Neuropsychiatric …