Epidemiological study of the relationship between exposure to organophosphate pesticides and indices of chronic peripheral neuropathy, and neuropsychological abnormalities in sheep farmers and dippers. Phase 2. Cross-sectional exposu

Overview of the aims: In 1994, the then Minister of Agriculture, Fisheries and Food announced that the Government had accepted advice from the Medical and Scientific Panel of the Veterinary Products Committee that there should be further research into the effects on health of organophosphate (OP) sheep dips. Subsequently, the Health and Safety Executive (HSE), the Department of Health and the Ministry of Agriculture, Fisheries and Food (MAFF) jointly commissioned a major epidemiological study into the effects of long-term exposure to OP sheep dips. This study was carried out between November 1995 and April 1999 by the Institute of Occupational Medicine in Edinburgh, in collaboration with the Institute of Neurological Sciences in Glasgow. The conduct and results of the research are being reported in three companion volumes, of which this report is the second.The broad aim of the study as a whole was to investigate whether cumulative exposure to sheep dip OPs is related to clinically detectable measures of neuropathy. The specific objective of Phase 2 was by means of a cross-sectional field study of sheep farmers and dippers to study the relations between cumulative exposure to OPs, and clinically relevant indices of peripheral neuropathy. Phase 1 of the study developed the exposure model applied in Phase 2, and Phase 3 studied the clinical features of a sub-group of subjects from Phase 2.Study design: The study consisted of a cross-sectional comparison of exposure to sheep dip OPs and chronic peripheral neuropathy throughout the UK. For practical reasons it was decided to base the study on two areas of the UK where there is a relatively high density of sheep farming. The inclusion of two areas was designed to give some indication of consistency across location, and consequently of ability to generalise the results, without overly complicating the conduct of the study. The areas chosen in England were Hereford and Worcester, and the Borders, Lothians and Ayrshire in Scotland.Two groups of low-exposed workers from the same broad geographical areas were included to augment the numbers of low-exposed sheep dippers: the first a group of pig and chicken farmers and farm workers, the second a group of other unrelated workers. It was planned to survey 600 sheep dippers and 200 low-exposed workers.Suitable farms were identified from a sampling frame, and contact was made with the farm owner to help recruit all potentially exposed persons employed at the farm. The sampling frame for recruitment of farms into the study was constructed from databases of annual census data for farms. The databases are maintained by the Ministry of Agriculture, Fisheries and Food (MAFF) for farms in England and Wales and by the Scottish Office for farms in Scotland.We opted for a design based on clustering of farms within parishes, but with a restricted number of farms selected from any one parish. Of the farms sampled 56% were Scottish farms and 44% English farms.Census data is also kept by MAFF and the Scottish Office on pig farms, in a similar manner to that kept for sheep farmers. MAFF also sought and obtained approval from the Office for National Statistics and the Scottish Office for the release of pig farm census data for the same geographical areas covered by the sheep farm data.Chicken farmers were recruited in Scotland on the basis of local knowledge of the location of farms, and the IOM having performed previous occupational health and hygiene work at some of these locations. The ceramics workers were recruited with the assistance of the British Ceramics Confederation. A number of member ceramics companies in the same broad geographical areas as the farms were approached concerning their willingness to participate in the survey. Two companies were selected and visited, one in south east Scotland and the other in the English MidlandsExposure history questionnaire: Retrospective exposure information was obtained for the period of common usage of OPs (1970 onwards), using a questionnaire developed during the first phase of the study. Phase 1 of the study involved careful observations of task and working practice in relation to uptake of OPs as assessed by urinary OP metabolites. The exposure history questionnaire was developed on the basis of relatively stable and easily identifiable features of the sheep dipping roles, i.e. features which were related to uptake as shown in Phase 1 of the study, and were also considered amenable to recall at survey in Phase 2. The main features included were flock size; concentrate handling; and principal task/job. Information was also gathered on the use of gloves and other personal protective equipment (PPE), although Phase 1 had shown little benefit from their use, possibly because the gloves and PPE were usually in poor condition.The questionnaire was administered by trained clerks. A pilot study established that the recall of farmers and farm workers was better when questioned by job rather than by pre-determined time periods and this underpinned the questionnaire design. Aspects of the work histories were later summarised into various surrogate indices of cumulative exposure such as total number of dipping days, total number of concentrate handling events, exposure to dilute dip (in the form of splash score) and a combined index based on the Phase 1 uptake model. Based on the Phase 1 findings, the use of gloves or PPE was not formally included in these indices.Neuropathy symptoms and sensory tests: Neurological assessments were conducted using a symptoms questionnaire in conjunction with a series of quantitative sensory tests (QST) based on the Mayo Clinic Methodology (Dyck et al, 1980). For this study, the design of the questionnaire was modified, as the original was designed for clinical use, while in this case the questionnaire was to be administered to farm workers in the field by a trained technician. Also, the full battery of neurophysiological tests, particularly the EMG and nerve conduction studies, was not appropriate for a field study carried out by a non-specialist. However, two automated tests for thermal sensation (hot and cold) and another for vibration sensation were suitable for application by a trained technician using portable equipment. All three tests had demonstrated high sensitivity and specificity in a laboratory setting with controlled ambient temperature.The Mayo Clinic methodology however supports the use of sensory testing and questionnaire as a diagnostic screening test for neuropathy, and an integral part of the overall study design included a clinical follow up a subgroup of field participants with a complete battery of test in the third phase of the survey. It is therefore considered that the choice of tests used was fit for the intended purpose.Reproducibility of field medical measurements: The study was designed from the outset to include a detailed clinical examination of selected subjects who had earlier participated in the field survey. Phase 3 was included in order to assess the reliability of case identification, to characterise the nature and severity of any abnormalities, and to include a neuropsychological assessment of subjects attended. Subjects selected for Phase 3 were a sample of Phase 2 subjects stratified on the basis of the likelihood of neuropathy using field measurements. Further details are contained within the Phase 3 report (Pilkington et al, 1999c).In an agreed change to the original protocol, the analysis of case identification in the field relative to the clinic was carried out prior to the exposure-response analysis of the field survey data in Phase 2. This was done due to the unexpectedly high number scoring positive for abnormal sensory thresholds in relation to clinical reference values among both ceramics workers and farmers.For the modified version of the Mayo Clinic neuropathy scoring system that had originally been proposed as a method of scoring likelihood of neuropathy status in the field, reproducibility between field and clinic was found not to be sufficiently better than chance to warrant use as the principal response variable in exposure-response analyses. When the specific component scores that were combined in the scoring system were analysed separately, it revealed that it was the scoring of the sensory test thresholds relative to a clinical reference values that were least reproducible. This lent weight to earlier suspicions that the high proportion of ceramics workers in the control group who had scored ‘abnormal’ for cold and vibration thresholds, in particular, could not be plausible.Since, the use of the clinical reference values to detect abnormality in the sensory test thresholds measured in the field could not be justified, it was decided that the symptoms score and the three sensory test thresholds, as they were measured in the field, would be analysed separately in relation to exposure in the field study. The symptom score had proved reasonably reproducible between field and clinic, particularly taking account of the time lag (up to 18 months) between the two assessments. The actual measured thresholds showed biases between the field and the clinic, in particular, the hot (factor of two lower in the field) and vibration (factor of two higher in the field) could be explained by the generally low ambient field temperatures during the field survey relative to the controlled temperature in the clinic. However, all three thresholds showed significant linear correlation between field and clinic and no evidence of any additional inter-individual variation in the field.Main findings in relation to the study group: Of the 995 sheep farm owners sent letters of invitation, 611 (61%) initially agreed to participate in the study. The most common reasons cited for non-participation were that the farmer was not interested, or was too busy. Of those agreeing who were suitable and practicable to visit, 293 (88%) were actually visited by the survey team. Two contract sheep dippers out of five approached also agreed to participate. The majority of workers in both ceramics factories visited were surveyed to reach the target for non-exposed controls. After exclusions due to reporting having a disease or taking medication that may have confounded a diagnosis of neuropathy, the study group consisted of 612 farmers with sheep-dipping experience (SD farmers), 53 farmers with no sheep dipping experience (NSD farmersanf farm workers) and 107 ceramics workers. SD farmers were six years older, on average, than the other groups, and included the highest proportion of females (14%), who were often additional family members. Alcohol consumption was highest among the ceramics workers.Among SD farmers, cumulative exposure was found to be highly skewed. Most subjects had experienced fewer than 100 days dipped (median 54 days), although a small number of individuals had experienced over 1000 dipping days. Total dipping days was highly correlated with the model-based exposure index (r=0.92) together with the cumulative concentrate handling and splash score components. Age at survey was not correlated with any of the exposure indices.Main findings in relation to comparison of occupational groups: In all groups autonomic symptoms were more reported than sensory or motor symptoms. The crude prevalence of reported symptoms overall was highest among SD farmers (19%), followed by NSD farmers (11%) and ceramics workers (5%). Symptoms prevalence was found to be positively associated with age and higher in English farmers than in Scottish farmers. Adjusting for age and country, the prevalence of symptoms among SD farmers remained high compared to ceramics workers (OR=4.3), but was similar to NSD farmers (OR=1.3).Age was also found to be positively related to all three sensory test thresholds. In addition, males had higher thresholds, on average, than females. There was no effect on thresholds, or on symptoms, of current reported alcohol consumption. Adjusting for age and sex there were inconsistent differences among the occupational groups between the two countries for both hot and vibration thresholds. This was partly due to differences in both thresholds between the two ceramics factories . However, for neither threshold was there a clear difference between SD farmers and ceramics workers across the two countries.Adjusting for age and sex, cold thresholds among SD farmers were, on average 1.35x higher than among ceramics workers, and 1.65x higher than among NSD farmers.Main findings in relation to exposure-response analyses: Adjusting for the important confounding variables, only for symptoms among the four neurological response variables was there evidence of a positive relationship with cumulative exposure. The model-based cumulative exposure index did not provide a better fit to the neurological response variables than the total days dipped. The estimated effect predicted a 13% increase in the odds of symptoms per 74 days dipped (inter-quartile range across all SD farmers) and was consistent across the full range of exposures and consistent between the two countries. However, the statistical significance of the gradient depended on the inclusion of the small number of very highly exposed individuals. Restricting the cumulative exposure indices to the period 1984-91, when OP sheep dips were at peak usage, did not improve the fit to the symptoms response. However, the effect of cumulative exposure did not explain fully the higher prevalence of symptoms among SD farmers compared to ceramics workers.Further analysis of exposure effects revealed that the average concentrate handling intensity, independent of duration of exposure, could explain the difference between SD farmers and ceramics workers in relation to both symptoms reporting and cold threshold. For symptoms, those had ever acted as principal concentrate handler reported more symptoms than those who had not (OR=3.4; 95% CI 1.6??7.2). An effect of duration of exposure, in addition to this effect, was not statistically significant at the 5% level.Adjusting for concentrate handling also revealed a lower rate of symptoms reporting among males compared to females of the same age, country and exposure.There remained a much higher prevalence of symptoms among English subjects compared to Scottish subjects (OR=2.0). An analysis of field versus clinic reproducibility among farmers attending the clinic showed that symptoms reporting among English subjects was more reproducible than among Scottish subjects, and indicated that the higher symptoms rates among English farmers were the more reliable result. Subsequent comparisons of the OP sheep dip products recalled by farmers did not reveal any marked difference in product useOn the basis these results, the prevalence of symptoms was predicted to be highest among concentrate handlers in the English regions, for example, 21% (95% CI: 15-27%) at age 40 years, rising to 35% (27-43%) at age 60 years. The corresponding prevalence among ceramics workers was predicted to only rise above 10% for those in their sixties. It is acknowledged that in some cases reported symptoms could be associated with other medical conditions and not necessarily be associated with exposure.There was a similar effect of concentrate handling intensity on all three sensory test thresholds that was more marked for cold and vibration thresholds. This effect rose from zero intensity and peaked at around the mid-point of the intensity range (4 handling events per day). Adjusting for the other important confounders, this effect could explain the difference in cold thresholds between SD farmers and ceramics workers. There was evidence that the use of gloves while handling concentrate, although estimated to have occurred on less than half of occasions, was increased with increased concentrate handling intensity.Key findings from Phase 2: Results showed higher rates of symptoms between OP exposed sheep dippers as a group compared with non-exposed workers. The associations between symptom score and various indices of cumulative exposure to OPs suggest that in at least some of the sheep farmers and farm workers reported symptoms are due to exposure to sheep dip chemicals. Sensorysymptoms were more commonly reported than motor symptoms by sheep dippers in the field study. The critical exposure factor seems to be contact with concentrate in that markedly higher rates of reported symptoms (adjusted for other factors) were reported among those who had at some time been principal concentrate handlers. These differences generally disappeared when non-exposed groups were contrasted with dippers who had not principally handled concentrate. There was no evidence that cumulative exposure to OPs was associated with impairment of measured sensory thresholds. The results suggest a relationship between QST measurements and exposure to concentrate but these are difficult to interpret. The possibility of an associated sensory neurophysiological component to the suggested symptom effect should therefore notbe discounted.The implications of these findings are considered in more detail in a summary of all three Phases which can be found in the Phase 3 report. “”