Effect of pesticide exposure on acetylcholinesterase activity in subsistence farmers from Campeche, Mexico

The authors surveyed agricultural production methods and pesticide use among subsistence farmers (campesinos) in 4 rural communities of Campeche, Mexico. Self-reports of symptoms of poisoning resulting from occupational pesticide exposure were elicited by questionnaire (N = 121), and acetylcholinesterase (AChE) activity during insecticide use was evaluated from blood samples (N = 127). In individuals from 2 of the 4 communities, AChE activity was significantly lower (p < 0.05) than the mean of activity determined for individuals in a reference group. Results of this study show that erythrocyte AChE inhibition provides a good biomarker of exposure to organophosphate pesticides in field studies with human populations. Carbamates, particularly carbofuran, seem to be more associated with exuberant and diversified symptomatology of pesticide exposure than organophosphates. Studies in field communities where both carbamates and organophosphates are suspected to exist should include blood AChE determinations, symptomatology surveys, and socioeconomic questionnaires. The authors recommend that the Mexican National Health Ministry authorities specify additional provisions regarding the use of protective equipment and the adoption of other safety practices during field work, increase information campaigns about the risks of pesticide use and the value of safety practices, and increase programs of medical monitoring and assistance for rural communities dealing with pesticides.     

Effect of Pesticide Exposure on Acetylcholinesterase Activity in Subsistence Farmers from Campeche,Mexico

The authors surveyed agricultural production methods and pesticide use among subsistence farmers (campesinos) in 4 rural communities of Campeche, Mexico. Self-reports of symptoms of poisoning resulting from occupational pesticide exposure were elicited by questionnaire (N = 121), and acetylcholinesterase (AChE) activity during insecticide use was evaluated from blood samples (N = 127). In individuals from 2 of the 4 communities, AChE activity was significantly lower (p < 0.05) than the mean of activity determined for individuals in a reference group. Results of this study show that erythrocyte AChE inhibition provides a good biomarker of exposure to organophosphate pesticides in field studies with human populations. Carbamates, particularly carbofuran, seem to be more associated with exuberant and diversified symptomatology of pesticide exposure than organophosphates. Studies in field communities where both carbamates and organophosphates are suspected to exist should include blood AChE determinations, symptomatology surveys, and socioeconomic questionnaires. The authors recommend that the Mexican National Health Ministry authorities specify additional provisions regarding the use of protective equipment and the adoption of other safety practices during field work, increase information campaigns about the risks of pesticide use and the value of safety practices, and increase programs of medical monitoring and assistance for rural communities dealing with pesticides.     

Response of biomarkers in amphibian larvae to in situ exposures in a fruit-producing region in North Patagonia, Argentina

The authors evaluated biomarker responses in caged larvae of the amphibian Rhinella arenarum in water channels during fruit production season and compared them with those elicited by a transient exposure to azinphos methyl (AzM) (0.02-2?mg/L; 4?h), the main pesticide applied in the Alto Valle region, Patagonia, Argentina, taking into account the maximum environmental concentration detected in superficial water (22.5?μg/L). The traditional biomarkers of organophosphate exposure, acetylcholinesterase (AChE) and carboxylesterase, were inhibited in tadpoles after one week of exposure in channels potentially receiving pesticide drift, whereas the antioxidant glutathione (GSH) and the detoxifying activity of GSH S-transferase (GST) were induced. In a two-week monitoring study, AChE activity was induced in larvae exposed at the agricultural site, and carboxylesterase showed an inhibition followed by return to control values, suggesting an exposure-recovery episode. Antioxidant glutathione levels were first depleted and then surpassed control levels, whereas GST activity was continuously induced. These responses were mimicked in the laboratory by 2?mg/L AzM-pulse exposure, which notably exceeds the expected environmental concentrations. The results draw attention to the complexity of responses after pesticide exposure, strongly depending on exposure time-concentration and recovery periods, among other possible factors, and support the necessity of the integrated use of biomarkers to assess exposure episodes in agricultural areas. Environ. Toxicol. Chem. 2012; 31: 2311-2317. ? 2012 SETAC.     

Effects of occupational exposure to organophosphate pesticides on nerve and neuromuscular function

This study aimed to investigate whether occupational exposure to organophosphate (OP) pesticides caused neurophysiological abnormalities. Thirty farmers who regularly spray OP pesticides and 30 fishermen (controls), living close by but not involved in pesticide spraying, were evaluated during and between cultivation seasons. The farmers had higher erythrocyte acetylcholinesterase levels than the controls during (P = 0.06) and between cultivation seasons (P = 0.09). During the cultivation season, there was a significant reduction in erythrocyte acetylcholinesterase activity in both groups (P < 0.01). Significant differences between the farmers and controls were found in sensory conduction velocity (P = 0.04) and motor conduction velocity (P = 0.04) between cultivation seasons. Sensory conduction velocity was reduced significantly in farmers (P < 0.01) and in controls (P = 0.04) during the cultivation season. Effects of OP poisoning were seen both in farmers and in controls, who had no history of spray activities. Evidence of sensory dysfunction after acute exposure and sensory and motor impairment after long-term low-level exposure to OP was seen.     

Rational use of cholinesterase activity testing in pesticide poisoning.

BACKGROUND: The use of acetylcholinesterase (AChE) activity testing in pesticide poisoning often falls on family physicians when evaluating a suspected poisoning or when monitoring the health of pesticide applicators. METHODS: A review of the literature and consideration of three illustrative cases shows misunderstandings in the pathophysiology of the enzyme and in procedures for effective testing and monitoring of AChE levels. RESULTS AND CONCLUSIONS: The physiologic characteristics of acetylcholine neurotransmission are described and related to carbamate and organophosphate poisoning. Pre-exposure monitoring is described using the California plan. A 23 percent variance in AChE levels exists among normal patients. It is necessary, therefore, to establish baseline levels to overcome individual variance. The practice of measuring of AChE levels in acute poisoning is limited. In employees who have been monitored and for whom baseline AChE levels have been established, a diagnosis of poisoning can be made by comparing postexposure AChE levels with baseline levels. If there is no baseline level recorded, and if the offending chemical is in question, the clinician must base treatment on the clinical signs and symptoms.     

抗虫灵对大 鼠全血和人红细胞膜乙酰胆碱酯酶的抑制作用

目的应用Ellman法或羟胺法检测乙酰胆碱酯酶(AChE)活性.方法研究有机磷杀虫剂抗虫灵对大鼠全血和人红细胞膜AchE活性的影响.结果雄性大鼠每天经口给以抗虫灵6.81mg/kg连续5d可引起全血AChE活性明显下降;动物亚慢性90d染毒后,雄性中剂量(0.25mg/kg)和高剂量(1.00mg/kg)组动物与雌性低剂量(0.05mg/kg)和高剂量(1.00mg/kg)组动物全血AChE活性明显下降;体外试验发现,抗虫灵在所试验的5×10-8～5×10-8mol/L的浓度范围内对人红细胞膜AChE活性都有明显抑制作用,并有剂量和时间依赖性,而且这种抑制为不可逆抑制.结论抗虫灵对AChE活性的抑制是其毒作用的重要机理,符合有机磷杀虫剂毒作用的一般规律.     

The recovery of plasma cholinesterase and erythrocyte acetylcholinesterase activity in workers after over-exposure to dichlorvos

Plasma cholinesterase and erythrocyte acetylcholinesterase activities have a long history of use in monitoring both workers at risk of organophosphorus pesticide (OP) exposure and in investigating accidental exposures to OPs. On account of wide inter-individual variation, the establishment of unexposed, baseline enzyme activities is necessary for accurate interpretation. This paper describes the rate of recovery of the two enzymes' activity after substantial over-exposure of eight subjects to the OP dichlorvos. Plasma cholinesterase activity, immediately after exposure, was substantially more inhibited than erythrocyte acetylcholinesterase activity. The plasma enzyme activity showed an exponential pattern of recovery with a half-life of around 12 days, so recovery was essentially complete after about 50 days. This reported half-life of recovery is consistent with the reported de novo synthesis rate of plasma cholinesterase. The mean recovery of erythrocyte acetylcholinesterase activity appeared linear over time, attaining unexposed activity after about 82 days, which is somewhat shorter than the life-span of erythrocytes. These indicate the sort of time period, after an OP incident, before a valid unexposed level can be established in an individual; and substantiate the guidance given in the Health & Safety Executive's document MS17 on a minimum period of 60 days without exposure in order to establish pre-exposure baseline levels.     

Prevalence of Abnormal Serum Cholinesterase and Associated Symptoms from Pesticide Exposure among Agricultural Workers in the South of Thailand

Objective: To determine prevalence of abnormal serum acetylcholinesterase (AChE) and behavioral risks associated behaviors from pesticide exposure.

Methods: Cross-sectional data were collected from agriculture workers in Trang Province, Thailand in 2013–2014. Agricultural workers in Trang Province filled out questionnaires provided by the Ministry of Public Health and gave blood samples. Risk scores were calculated based on questionnaire responses and a reactive-paper finger-blood test was used to evaluate AChE levels.

Results: A total of 6,222 agricultural workers filled out questionnaires and 3,431 workers gave blood samples. The prevalence of abnormal AChE results was 12.5%. Females had a higher prevalence of abnormal serum AChE levels (15.2%) than males (10.2%). The most common self-reported symptoms were coughing (5.7%), being tired (5.4%), dizziness (5.1%), and dry skin and irritation (4.8%). However, those with abnormal serum AChE levels were most likely to report dizziness (12.4%) as their main symptom. Male agricultural workers had significantly higher risk than female farmers after adjusting for sex and risk level (OR = 1.56, P < 0.05).

Conclusions: Abnormal serum AChE levels associated with pesticide exposure are associated with AChE levels and symptoms such as coughing, being tired, dizziness, and dry skin and irritation. Educating farmers about the effects of pesticide use and encouraging better personal protective equipment practices could also assist in reducing pesticide exposure.     

Effect of temperature and pirimiphos methyl on biochemical biomarkers in Chironomus riparius Meigen

Fourth-instar Chironomus riparius Meigen larvae were exposed to the organophosphate (OP) insecticide pirimiphos methyl (0, 0.1, 1.0, and 10 microg/L) for 48, 72, or 96 h at three temperatures (3, 12, or 22 degrees C). Two biochemical biomarkers, acetylcholinesterase (AChE) and glutathione S-transferase (GST), were measured in individual larvae from each treatment. AChE activity was inhibited by the OP in a dose-responsive fashion. This response remained similar at all three temperatures, demonstrating that AChE is a robust and specific biomarker. Exposure duration had little effect on AChE activity. In contrast, GST activity was induced at the highest OP insecticide concentration, but induction was also evident at 3 degrees C. There was a significant effect of exposure duration, with an overall decline in GST activity over time. This result agrees with previous work suggesting that GSTs are not particularly suitable for use as a biomarker of pesticide exposure or effect in Chironomus.     

Effects of repeated inhalation-exposure to fenitrothion powder on blood cholinesterase activity in rats

Effects of repeated exposure to Fenitrothion (Sumithion) powder on erythrocyte membrane acetylcholinesterase (AChE) and plasma cholinesterase (PChE) were studied. Rats (Wistar, male) were exposed to fenitrothion powder for 2 hrs/day, 5 days/week for up to 3 months. The suppression of AChE and PChE increased as the exposure period and exposure concentration increased, although no suppression of AChE was found after a single 4-hr exposure. The persistence of AChE inhibition after the termination of exposure increased with the increase of the exposure period and was longer than that of PChE. Therefore, the AChE activity seems to be useful in examining the response to the repeated exposure to fenitrothion powder.     

Inhibition of acetylcholinesterase activity in the terrestrial isopod Porcellio scaber as a biomarker of organophosphorus compounds in food

This paper describes the toxicity of organophosphorus pesticide diazinon in juvenile and adult terrestrial isopods Porcellio scaber (Isopoda, Crustacea). The woodlice were exposed to different concentrations of diazinon added to food (5, 10, 50, and 100 or 150 micrograms/g dry food). Weight change and food assimilation efficiency were determined two and four weeks after the exposure. The activity of acetylcholinesterase (AChE) in surviving animals was measured at the end of the experiment. The results show that woodlice exposed to diazinon do not significantly differ from controls in growth and feeding rate. The reduction of AChE activity was observed at the lowest diazinon exposure (5 and 10 micrograms/g dry food). These results suggest that AChE activity might prove a useful biomarker, indicating low levels of organophosphates in food.     

Seasonal exposure of fish to neurotoxic pesticides in an intensive agricultural catchment, Uma-oya, Sri Lanka: linking contamination and acetylcholinesterase inhibition

The annual cultivation pattern in the Uma-oya catchment in Sri Lanka is characterized by Yala and Maha rainfall periods and associated cropping. Two cultivation seasons were compared for pesticide residues: base flow, field drainage, and the runoff and supplementary sediment data for three sites in the catchment. Organophosphate and N-methyl carbamate pesticide analysis confirmed a higher concentration in the Yala season with low-flow conditions. Acetylcholinesterase (AChE) activity was measured by standard spectrometry in the brain, muscle, and eye tissues of three freshwater cyprinid fishes, Garra ceylonensis, Devario malabaricus, and Rasbora daniconius from three study sites during months overlapping two seasons in 2010 (December) and 2011 (July). Baseline AChE data were measured from fish samples from a forested reserve in the Knuckles. A 73% inhibition in muscle AChE activity in G. ceylonensis was associated with intense pesticide exposure months in the Yala season. The AChE inhibition more than 70% in G. ceylonensis eyes in both Yala (76%) and Maha (72.5%) seasons indicates particular sensitivity of eye tissue to inhibitors. The less dramatic AChE inhibition in the eye tissues in D. malabaricus and R. daniconius in both seasons indicates exemplary protective capacity of muscle AChE in fish. The highest inhibition of AChE (up to 60% in brain and up to 56% in muscle AChE activity in R. daniconius and up to 47.8% in brain and up to 64.6% in muscle AChE activity in D. malabaricus) occurred during the Yala season. Tissue AChE activity and physiological activity in fish were correlated. The results collectively indicate that AChE is a consistent biomarker for diffused contaminant exposure in agricultural catchments.     

The monitoring of cholinesterases in farm workers and tradesmen exposed to phosphoric esters and carbamates]

Cholinesterase (ChE) levels (Ellman method) were monitored in 90 subjects (69 males and 21 females) exposed to carbamate and organophosphate pesticides (78 agricultural workers and 12 pesticide vendors). Pre-exposure baseline values of plasma and red blood cell cholinesterase activities were defined for each subject with two blood samples (23 workers) or three blood samples (59 workers) taken almost thirty days after the last exposure. After control of intra-individual variation, 8 subjects with only one pre-exposure value and 13 with a coefficient of variation above 30% were excluded. For the other 59 subjects, the intra-individual variation of erythrocyte ChE (16%) was similar to the inter-individual one (15%), whereas the inter-individual variation of plasma ChE (21%) was higher than the intra-individual one (14%). Laboratory variation for plasma ChE measurements was 8%. Baseline values were analyzed (ANOVA) for sex, age, task and hour and season of sampling. Both erythrocyte and plasma enzymes, corrected for hematocrit, were lower in females. Plasma cholinesterase activity was lower in "re-entry" agricultural workers and in pesticide vendors. Post-exposure cholinesterase activity was measured in 54 workers within a few (1-21) days after last handling. Average relative reduction was 15.2% (95% C.I. = 4.9%-25.5%) in erythrocyte cholinesterase activity and 29.1% (95% C.I. = 18.2%-40.1%) in plasma cholinesterase activity. The one-way variance analysis showed marked plasma ChE reduction in mixers, loaders and appliers (36%, 95% C.I. = 24%-48%) and in parathion handlers (35%, 95% C.I. = 21%-49%. No significant reduction in blood cell cholinesterase activity in relation to task and to pesticide handled was observed. We conclude that the intra-individual variations of the baseline values were higher for three repetitions (88% and 84% of the population were within a variability of less than 30%, for AChE and for ChE respectively) than for two repetitions (91% and 88% of the population were within 30% of variability for AChE and for ChE respectively). The figures show a greater sensitivity of plasma ChE activity in acute exposure, probably due to a poor reliability in detection of erythrocyte ChE by local laboratories. The maximum reduction (38%, 95% C.I. = 22%-53%) in plasma ChE activity was observed within six days of the last exposure in loaders and appliers.     

The 2,4-D herbicide effects on acetylcholinesterase activity and metabolic parameters of piava freshwater fish (Leporinus obtusidens)

The effects of 2,4-D (1 or 10 mg/L) on acetylcholinesterase (AChE) and metabolic parameters were evaluated in piava (Leporinus obtusidens) after 96 h. AChE activity was significantly reduced in the brain at a concentration of 10 mg/L and in the muscle at both concentrations tested. Muscle glycogen and lactate were significantly reduced for both 2,4-D concentrations but no significant change was observed in liver glycogen. Muscle protein levels were enhanced after exposure at 10 mg/L, but no significant changes were observed in muscle and liver glucose. Liver lactate and protein were significantly reduced after exposure to this herbicide. 2,4-D exposure produced a decrease in blood glucose at both concentrations and enhanced lactate levels at 10mg/L. Plasma protein increased at both concentrations tested. In conclusion, the results obtained indicate that 2,4-D affects brain and muscle AChE activity and some blood and tissue metabolic parameters of L. obtusidens. The stress generated by 2,4-D is the probable cause of alterations observed and measured parameters can be used to monitor 2,4-D fish toxicity.     

Influence of physical exercise and gender on acetylcholinesterase and butyrylcholinesterase activity in human blood samples

The determination of cholinesterase activity in plasma and erythrocytes serves as a useful and sensitive biomarker of exposure to organophosphorus and carbamate pesticides. However, acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) activity may be influenced by factors such as age, gender, drugs, and physical exercise. The aim of this study was to evaluate the effects of gender and physical exercise on the activity of AChE and BuChE in healthy individuals. The values for both enzymes were lower for women. Physical exercise increased the levels of BuChE, and had no significant effect on AChE. To our knowledge, this is the first clinical study evaluating the influence of physical exercise in levels of these enzymes. Considering that cholinesterase activity is a useful parameter in assessing the exposure of individuals to pesticides, it is important to understand factors that influence the determination of the enzymes in order to avoid the erroneous interpretation of results.     

Body size-related differences in the inhibition of brain acetylcholinesterase activity in juvenile Nile tilapia (Oreochromis niloticus) by chlorpyrifos and carbosulfan

Influence of body size on inhibition of brain acetylcholinesterase (AChE) activity of juvenile Nile tilapia, Oreochromis niloticus by chlorpyrifos and carbosulfan was investigated concerning its potential use in the biomonitoring of anticholinesterase pesticides in tropical water bodies. Three size groups of fish (fry: 3-4 cm, fingerlings: 6-8 cm, sub-adults: 10-12 cm in total length) were exposed to a series of concentrations of chlorpyrifos (0.5-12 microg L(-1)) or carbosulfan (1-10 microg L(-1)), and concentration-response for inhibition and recovery of the AChE enzyme was evaluated in comparison to the controls at different time points, 2, 6, 10, and 14 d. The AChE activities of the control fish followed the order of decreasing activity, fry>fingerlings>sub-adults. AChE activities of the fry were nearly 2-fold higher than that of the sub-adults. Following 48 h of pesticide exposure, the AChE activity of the three size groups of fish decreased significantly in comparison to the respective controls in a concentration-dependent manner. The activity was greatly inhibited in the fry (39-85%) compared to sub-adults (18-47%) exposed to the most of the similar concentrations of the pesticides. Median effective in vivo inhibition concentrations (48 h IC50) of chlorpyrifos for fry, fingerlings, and sub-adult stages were 0.53, 0.75, and 3.86 microg L(-1), respectively, whereas the corresponding values for carbosulfan were 3.37, 7.02, and 8.72 microg L(-1). When fish were maintained in the initial pesticide medium for 14 days, AChE activity restored gradually depending on the initial pesticide exposure concentration and the size group of the fish. Results indicate that brain AChE of Nile tilapia is a promising biomarker for assessment of anticholinesterase pesticide contaminations in water. However, body size of Nile tilapia should be taken into account when using this biomarker in biomonitoring programmes.     

Acute health effects of organophosphorus pesticides on Tanzanian small-scale coffee growers

Acute health effects of organophosphorus (OP) pesticides on coffee farmworkers in 1991-1992 in Tanzania are reported to provide a basis for concern over farmworkers being overexposed during application. Workers exposed to OP pesticides (N=133) were drawn from a population of about 240,000 coffee farmers. They were interviewed on symptoms and personal protection, and their erythrocyte acetylcholinesterase (AChE) activity was determined during both spraying and nonspraying period. AChE activities during spraying and nonspraying period were comparable (mean 32.0, SD 7.8 vs. 33.0, SD 8.7 U/g HgB, P=0.26). The prevalence of cough, headache, abdominal pain, excessive sweating, nausea, excessive salivation, diarrhea, and vomiting did not differ significantly between spraying and nonspraying periods. There was no suggestion of decreased AChE in exposed subjects who complained of OP-related symptoms compared to symptomless exposed subjects. Use of gloves, long boots, head cover, face cover, and coverall was not significantly associated with AChE activity. No marked AChE depression was found during spraying season, which may explain the lack of association between symptoms and AChE. The fact that only moderately toxic OP pesticides were used may indicate that toxicity was not sufficiently high to cause depression. Experience, however, suggests that occupational poisoning remains a potential serious danger in coffee cultivation in Tanzania.     

Behavioral effects of acute sublethal exposure to dimethoate on wood mice, Apodemus sylvaticus: II—Field studies on radio-tagged mice in a cereal ecosystem

The effects of direct exposure to an organophosphate (OP) pesticide, dimethoate, were examined in free-living wood mice, Apodemus sylvaticus, in a wheat field. Male mice were radio-tagged at night and followed during 2–3-d periods, before and after an intraperitoneal injection of 50 mg/kg dimethoate which previous laboratory studies had demonstrated causes a maximum depression in brain acetylcholinesterase (AChE) activity of 75% relative to non-exposed mice. In subsequent weeks, survival was estimated by tracking and trapping data. Exposure to dimethoate significantly decreased locomotor activity in the first 6 h after administration resulting in a significant decrease in the area over which animals moved. These effects were limited to the night of treatment and disappeared 24 h later. The transient behavioral impairment of the dimethoate-treated animals appeared to have no effect on medium-term survival. Direct exposure to OPs sufficient to cause 75% depression of brain AChE is unlikely to be hazardous to wood mice if exposure is transient, as in the present study; it would be expected that sustained exposure sufficient to cause such behavioral effects would, however, be detrimental.     

Symptoms and cholinesterase activity among rural residents living near cotton fields in Nicaragua.

OBJECTIVES: To explore whether symptoms resulted from pesticide spray drift on residentially exposed populations in rural Nicaragua. METHODS: 100 residents, each 10 years of age or older, were randomly selected from a Nicaraguan community surrounded by actively sprayed cotton fields (the exposed community) and from a socioeconomically similar community far from agricultural spraying (the control community). Subjects working with pesticides were excluded, and the study was conducted at the end of the 1990 cotton spraying season (August-December). Demographic information, exposure questions, and prevalence of 11 acute symptoms and 17 chronic symptoms were gathered from a structured interview. Finger stick erythrocyte cholinesterase (AChE) was measured with a portable colorimeter. Acute symptoms were grouped according to their previously known associations with cholinesterase (ChE) inhibitors into four ordinal categories (asymptomatic, non-specific, possible, probable). RESULTS: Residents from the exposed community were significantly more likely to report recently sighting a spray plane near their community, exposure to pesticide from drift, crossing recently sprayed fields, eating home grown food, and feeling ill after drift exposure. The mean AChE value was significantly lower for residents of the exposed community (4.9 v 5.3 IU/dl). The proportion of subjects complaining of one or more chronic or acute symptoms was significantly higher for the exposed community (87%) than for the controls (53%). Odds ratios for residents in the exposed community, by symptom categories, were non-specific 1.6 (95% confidence interval (95% CI) 0-8 to 3.2), possible 4.1 (95% CI 1.7 to 10.2), and probable 9.93 (95% CI 2-9 to 34.4). CONCLUSION: These findings indicate a strong association between exposure to aerial pesticides and symptoms. This study should be replicated with more quantitative exposure measures, for if confirmed, the results have relevance for millions in rural communities worldwide.     

Response and Recovery of Brain Acetylcholinesterase Activity in the European Eel,Anguilla anguilla,Exposed to Fenitrothion

European eel (Anguilla anguilla) were exposed to sublethal fenitrothion concentrations in a continuous flow-through system for 4 days. Brain acetylcholinesterase (AChE) activity was evaluated after 2, 8, 12, 24, 32, 48, 56, 72, and 96 h pesticide exposure. Results indicated that AChE activity in eel brains decreased as the concentration of fenitrothion increased. The pesticide induced significant inhibitory effects on the AChE activity ofA. anguilla,ranging from >40% inhibition at a sublethal concentration of 0.02 ppm to >60% inhibition at a sublethal concentration of 0.04 ppm. Eel were exposed to both fenitrothion concentrations for 96 h and then allowed a period of recovery in pesticide-free water. Samples were removed at 8, 12, 24, 48, 72, 96, 144, and 192 h and eel brain AChE activity was evaluated. Following 1 week of recovery, the AChE activity of those animals previously exposed to 0.02 and 0.04 ppm fenitrothion was still different from that of the controls. So, the AChE activity of eel brains at the end of the recovery phase remained significantly depressed.