An investigation into the possible chronic neuropsychological and neurological effects of occupational exposure to organophosphates in sheep farmers

Stephens et al, Institute of Occupational Health, Birmingham, HSE Contract Research Report No 74/1995

Introduction

It is well-established that acute over-exposure to OPs can lead to adverse short term health effects of varying severity (mediated by cholinergic excitation), but recently concerns have arisen that OP exposure may also give rise to chronic health effects in users. There is currently little scientific evidence available either to support or refute the existence of these chronic health effects in OP exposed individuals, however, and there have been a number of calls for research of this problem in the literature (Mearns et al 1994; Davies, 1990). In addition there has been an absence of studies focusing in particular on sheep-dippers - most studies of occupational OP exposure having been restricted to spray applicators. As a result of anecdotal reports of chronic ill health and media coverage this study was conceived with the principal objective of investigating the possible occurrence of chronic ill health effects in sheep dippers occupationally exposed to OPs.

The aim of the study was to investigate whether chronic effects occur in sheep farmers who experience repeated exposure to organophosphates (OPs) through sheep-dipping. A cross-sectional design was employed in which sheep farmers exposed to OPs were compared with non-exposed quarry workers on a range of outcome measures which assessed mood and symptom reporting after acute exposure, and cognitive functioning, psychiatric state and neurological symptoms in the absence of recent exposure.

Summary

1. The performance of the sheep-dippers was significantly poorer than that of the quarry worker controls on three of the neuropsychological tests, namely Simple Reaction Time, Symbol Digit Substitution and Syntactic Reasoning.
2. These results indicate the presence of chronic effects in certain areas of cognitive functioning occurring in the sheep-dippers. These effects are subtle in nature and relate to the ability to sustain attention and the speed of higher level information processing, particularly in areas where a linguistic component is involved. There are several reasons to support the view that, although these effects are subtle, they are real effects, and are likely to be associated with exposure to organophosphate pesticides.
1. The pattern of performance in the two groups (exposed and controls) was similar, and conformed to that which would be expected in normal subjects. However, the speed of performance was affected in the exposed group. This indicated that there was no difference between the groups in terms of their understanding of what was required to do the tests, or in their motivational level.
2. Differences between the groups remained after control for factors which might be expected to influence test performance namely age, educational level, previous computer experience, lifetime alcohol consumption, time of day of testing and first language.
3. The exposed group consisted of a randomly selected group of sheep-dippers which was judged to be representative of the farming community as a whole. It was not composed of self-selected individuals who had previously judged themselves to be suffering from the effects of organophosphates. In addition those with medical or lifestyle factors which might result in effects on the nervous system were excluded from the investigation.
4. The control group was well matched in terms of geographical, social and cultural factors and also in terms of the type of manual work undertaken. Those included in the control group did not have exposure to pesticides as indicated by questionnaire results and by the results of the urine samples taken from a random subset of controls.
5. Testing of the sheep-dippers was undertaken at a time when they had not experienced any recent exposure. This was confirmed by the results of the urine analysis. It is safe to assume therefore that the effects observed were of a chronic and not an acute nature.
6. The fact that the groups did not differ in performance on all psychological tests adds strength to the argument that these are real effects on specific aspects of cognitive functioning and do not represent a consistent, stable difference between the groups.
3. In case of one test, the syntactic reasoning test, the effects were clearly dose-related when a dose surrogate composed of both the frequency and duration of exposure was used. It is difficult to explain why a dose-effect relationship occurred in respect of only one test. However, this may be related to the fact that this was th emost complex test in the battery, involving the highest level of information processing.
4. The mental health of this group of sheep-dippers was clearly poorer than that of the controls in that they appeared to be significantly more vulnerable to developing a psychiatric disorder of clinical significance. It is not possible to state unequivocally that this was related to organophosphate exposure. It has previously been established that farmers of all types suffer from poor mental health and that this has been linked to current social and economic factors. Nevertheless, it should be noted that the results of the mental health questionnaire are consistent with those of the cognitive tests in indicating some effects on the nervous system occurring in this group.
5. Few of this group of sheep-dippers reported experiencing acute or sub-acute (delayed) effects associated with the dipping process and the level of symptom reporting in the exposed group as a whole was not significantly higher than that of the controls. There was also no association between the experience of acute symptoms and performance on the neuropsychological tests. The neuropsychological data suggested that chronic effects occur independently of acute effects.
6. There were some indications of neurological signs, specifically in relation to two point discrimination, in those dippers who reported the highest level of symptoms. These examinations were carried out on an extremely small sample. The results are currently difficult to interpret in terms of a consistent pattern of neurological damage. However, they strongly suggest the need for further more detailed investigation of this aspect, which in the present study was included on a largely exploratory basis.
7. It is not possible from the results of this investigation to draw any firm conclusions about the level or type of exposure likely to produce the observed effects. It should be noted, however, that the majority of sheep-dippers studied here did not use the type of clothing or equipment likely to protect them from exposure. The levels of dialkylphosphates in the urine do provide some indication of the levels of exposure and since, for most sheep-dippers, dipping practices appear to have remained relatively constant, it may perhaps be assumed that the levels measured at this dip were fairly typical. However, it was not an objective of this study to attempt to link the results of biological monitoring to neuropsychological outcomes. Urine analysis was carried out with the objective of confirming the presence or absence of recent exposure at the time of testing.

Recommendations

The present study has confirmed the occurrence of subtle chronic effects on the nervous system in individuals exposed to organophosphates through sheep-dipping, identified by performance on sensitive neuropsychological tests. Analysis of dose-effect relationships in respect of one test indicated that these effects occurred in the 'lowest exposure' group, which included individuals with only two years of exposure.

The results suggest that further work is required to investigate the suggestion that those reporting a high level of acute symptoms immediately following exposure may experience effects identifiable by the presence of neurological signs.

In addition, it is suggested that further work might be carried out to define more precisely the levels of exposure at which the identified effects occur. Dipping practices appear to have remained relatively unchanged during the period that organophosphates have been in use. This suggests that current biological and biological effect measurements may be of relevance in developing a more detailed retrospective assessment of exposure than was undertaken in the present study. Finally, although the effects identified are not severe, the results of the investigation suggest that further efforts should be made to reduce exposure to organophosphates in terms of identifying the most appropriate protective clothing and dipping equipment and encouraging its use.