FOOD SAFETY/Pesticides Sun Z-T et al (1999) Increased risk of hepatocellular carcinoma in male hepatitis B surface antigen carriers with chronic hepatitis who have detectable urinary aflatoxin metabolite M1 Hepatology 30: 379–383 Wang L-Y, Hatch M, Chen CJ et al (1996) Aflatoxin exposure and the risk of hepatocellular carcinoma in Taiwan International Journal of Cancer 67: 620–625 Wild CP and Turner PC (2002) The toxicology of aflatoxins as a basis for public health decisions Mutagenesis 17: 471–481 Yu MW, Lien JP, Chiu YH et al (1997) Effect of aflatoxin metabolism and DNA adduct formation on hepatocellular carcinoma among chronic hepatitis B carriers in Taiwan Journal of Hepatology 27: 320–330 Pesticides M Saltmarsh, Alton, UK ª 1998 Elsevier Ltd All rights reserved This article is reproduced from the previous edition, pp 869–874, ª 1998, Elsevier Ltd What are Pesticides? Pesticide is a generic term that covers a wide range of natural and synthetic chemicals (over 700 in total) that are used to protect crops from attack from pests, both before and after harvest There are many different sorts of pests The term includes insects, slugs and snails, nematode worms, mites, rodents, weeds, molds, bacteria and viruses The chemicals can be applied before and during growth of the plant or on to the stored crop as, for example, fumigants, which are used to kill pests that have infested stored cocoa or grain Chemicals used to treat pests on animals are not included; they are considered as veterinary medicines The pesticide formulation used by the farmer will include the pesticide chemical itself and a number of other chemicals that enable it to be applied and to work as effectively as possible These will include solvents, adhesives, and surface-active agents such as emulsifiers In some cases other chemicals, known as ‘safeners,’ are applied to minimize the damage done to the crop while maintaining the effectiveness of the spray on the target It is estimated that worldwide usage of pesticides is around 2.5 million tons with a cost in 1997 of US$21 billion Why Do We Need Pesticides? Food crops are subject to attack by a multitude of pests and diseases and pesticides are applied to minimize the damage to the crop It has been estimated 323 that without protection world cereal crop yields would fall by between 46 and 83% History is littered with records of crop failures and famine caused primarily by rodent, insect or fungus Some of these events have had a wide-ranging and longlasting effect, like the 1845–1846 Irish potato famine and the 1917–1918 German ‘turnip winter,’ the latter so called because the potatoes rotted and turnips were the only stored root crop that was available to feed the population through the winter Both these events, in which 1.5 million and 700 000 people died, respectively, were caused by potato blight, infection by the fungus Phytophthora infestans Famine caused by massive swarms of locust is still all too common in Northern Africa and Arabia Less spectacular but as disastrous is the loss of an estimated 30% of harvested crops in India to rodents In addition to the loss of the crop, pesticides are used to control agents which make the crop toxic rather than healthy Two examples are the toxins caused by fungi When an insect bores into a peanut it allows spores of the fungus Aspergillus flavus to enter and grow, producing the aflatoxins, a series of carcinogens When rye (Secale cereale) grows in damp conditions a fungus, Claviceps purpurea, can grow on the seed If this seed is subsequently ground into flour and made into bread it can cause consumers to suffer hallucinations, gangrene, and death Outbreaks amounting to epidemics were common in the Middle Ages in Europe and one occurred as recently as 1951 in France A second reason relates not so much to quantity as to quality Supermarkets in the developed nations offer a wide range of fresh produce at competitive prices Consumers not like holes made by slugs and snails in their fresh lettuce They not expect scab marks on their apples, or holes made by small maggots in their carrots Flour millers not expect to have to clean the grain from weed seeds before milling Even small defects can dramatically reduce the value of the crop, or indeed make it unsaleable, and the need for a competitive price requires minimal labor input so that application of pesticide is essential Types of Pesticides There are currently around 600 pesticides, both natural and synthetic Natural pesticides include both chemicals derived from plant sources and biological agents such as parasitic wasps, mites, bacteria, and chemicals contained within or exuded by plants or bacteria While there is no inherent reason why natural products should be any safer than synthetic ones (after all, insect 324 FOOD SAFETY/Pesticides venoms and toxins and poisonous plants are natural), it appears that the risks lie in their potential impact on the environment rather than on their effect in food There are also increasing numbers of cases where plants have been given a gene which expresses a natural pesticide (see Bacillus thuringiensis, below) At the time of writing, naturally derived pesticides make up less than 5% of the world pesticide market, but a great deal of work is being devoted to the screening of natural sources and this proportion will certainly increase The most successful natural product development so far has been that of the pyrethrin insecticides, of which 33 are currently available The largest classes of pesticides are pyrethrins, organochlorines, organophosphates, and carbamates, although there are many smaller classes with only one or two members The chemical structures of the key members of the major groups are given in Table Important Pesticide Groups This list covers the important pesticide groups and some individual pesticides but does not attempt to be comprehensive Pyrethrins Pyrethrins are chemically related to pyrethrin, which is a secondary metabolite found in the flowers of the pyrethrum plant (Chrysanthemum cinerariaefolium) Dried pyrethrum flowers were used as an insecticide in ancient China and in the middle ages in Persia The dried flowers are still used Current production is around 20 000 tons per annum centered in Kenya and Tanzania The pyrethrins are effective insecticides, having very low dose rates and rapid knockdown of insects but being harmless to mammals under all normal conditions Natural pyrethrins break down rapidly under the influence of oxygen and UV light This limits their use in agriculture, but recently synthetic analogs have been developed to overcome these problems Starting from the structure of the natural product a large number of synthetic compounds have been made It is worth noting how they differ in effectiveness: deltamethrin is a broad range insecticide; allethrin is particularly toxic to house flies (Musca domestica) but much less effective with other insects; flumethrin is active against cattle ticks; while others are acaricides or miticides with little or no insecticidal activity Bacillus thuringiensis Bacillus thuringiensis is a widely distributed bacterium that during sporulation produces a crystal inclusion which is insecticidal when ingested by the larvae of a number of insect orders Susceptible orders include Lepidoptera, Diptera, and Colcoptera The action of B thuringiensis was first observed in 1901 as the cause of a disease of silkworms Several strains of the bacterium have been identified with activity against a range of insects including cabbage looper, tobacco budworm, mosquito, black fly, and more recently nematodes, ants and fruit flies While the bacterium appears an ideal insecticide (having a toxicity 300 times greater than synthetic pyrethroids), it requires careful use It is most effective against neonates and early larval instars so that spraying must be timed for egg hatch It also has no contact activity and must be ingested so the plant must be well covered to ensure the insect receives a lethal dose Furthermore it has a half-life in the field as short as h, so careful timing is essential for it to be effective Despite these limitations, it has been shown to be an important component of crop management programes One way of overcoming the problems of application of B thuringiensis is to incorporate the gene responsible for expression of the protein into the crop plant This has been achieved with maize (Zea mays) to protect against the European corn borer, with cotton (Gossypium hirsutum) to protect against a range of budworms and bollworms, and with potato (Solanum tuberosum) against Colorado beetle (Cotton may seem irrelevant in a text on food but cottonseed oil is used extensively in cooking oils, margarines, and industrial fats.) This genetic modification has great benefits but care has to be taken that the food product has not changed in some unpredicted way All genetically modified foods have to be extensively tested and cleared by regulatory agencies before release Neem oil This is an oil obtained from the neem tree, Azadirachta indica A Juss It has been used as an insecticide in India and Africa but is increasingly being developed as a significant commercial product It contains a number of compounds, one of the most active being azardirachtin, which is an insect antifeedant but also shows growth inhibitory and endocrine disrupting effects This product and its individual components is at the beginning of its commercial development, which is likely to result in a series of products as significant as those from pyrethrum FOOD SAFETY/Pesticides 325 Table Chemical structure and acceptable daily intake (ADI) of some pesticides Compound Class Structure Deltamethrin Pyrethrin Br C ADI (mg per kg body weight) CH CH3 Br 0.01 CO2 CN C O H H H CH3 DDT 0.02 Organochlorine CH Cl Lindane (HCH) Organochlorine Cl CCl3 Cl Cl 0.008 Cl Cl Cl Chlorfenvinphos Organophosphate (mixture of two isomers) Cl O (CH3CH2O)2P O Cl C (CH3CH2O)2P C H C Cl Cl , Cl Organophosphate O C H Cl Malathion 0.002 O Cl 0.02 S (CH3O)2PSCHCH2CO2CH2CH3 CO2CH2CH3 Propoxur Carbamate 0.02 OCONHCH3 OCH(CH3)2 Simazine Triazine Cl N N NHCH2CH3 0.005 N NHCH2CH3 Glufosinate 0.02 O CH3PCH2CH2CHCO2H OH Glyphosate NH2 O 0.3 HO2CCH2NHCH2P(OH)2 Microbial phytotoxins These are herbicides and include the highly commercially successful glufosinate, a synthetic form of phosphinothricin, first isolated from Streptomyces hygroscopicus, a soil-borne microbe This compound is a potent, irreversible inhibitor of glutamine synthetase which is used in plants for photorespiration Many attempts have been made to make synthetic variants of phosphinothricin 326 FOOD SAFETY/Pesticides without success Other members of this group include anisomycin and herboxidiene, derived from other Streptomyces strains The veterinary insecticide, avermectin is derived from Streptomyces avermitilis Organochlorines The organochlorines were the first group of synthetic insecticides and without them the dramatic decrease in malaria observed in the 1950s would have been impossible The best known of this class is DDT (dichlorodiphenyltrichloroethane) but others include 2,4 DD, hexachlorbenzene, and lindane Of these only lindane (
-hexachlorocyclohexane, see Table 1) is still in use in the developed world These compounds are very slow to break down in the environment and one result of this persistance was the decline in bird numbers graphically described by Rachel Carson in the book Silent Spring The problem was that DDT was concentrated through the food chain and predator birds in particular were failing to raise chicks Since the organochlorine pesticides and other sources of organochlorines in the environment have been largely phased out, numbers of many species of birds are rising again It is recognized that pesticides are still having an adverse influence on numbers of some birds that inhabit farmland However, this is not a straightforward effect In the case of the grey partridge, for example, it is because herbicides have reduced the number of weeds, which in turn has reduced the number of insects that feed on the weeds, resulting in fewer insects for the chicks to eat The mechanism of action of the organochlorines is not known in detail although they appear to act on the central nervous system In humans the organochlorine compounds tend to accumulate in the body fat and in mothers’ milk While there is no direct evidence that they cause mutations or cancers, there is concern that lindane may be a carcinogen and its role in breast cancer is still under review However, in contrast, DDT and -HCH have both been shown to inhibit tumors in mice initiated by aflatoxin B1 Although organochlorine pesticides have largely been phased out in Europe, analysis for them continues and low levels of lindane are still being detected in milk in the UK (typically at 0.005 mg kgÀ1 compared with the maximum residue limit (see below) of 0.008 mg kgÀ1 and an acceptable daily intake (see below) of 0.05 mg per kg body weight) Organophosphorus compounds Organophosphorus compounds generally contain both sulfur and phosphorus linked to carbon atoms Their discovery was a by-product of the development of nerve gases The group includes parathion, malathion, dimethoate, diazinon, and chlorfenvinphos They are used as herbicides, insecticides, and fungicides They break down quickly in the environment and not concentrate in body fats, although they may be stored for some time However, their mode of action – inhibition of acetylcholine esterase – means that they affect both insects and mammals and their use depends on the effective dose in the target species being below the sensitivity of other species Acute effects of sublethal doses of organophosphates in man include sweating, salivation, abdominal cramps, vomiting, muscular weakness, and breathing difficulties Concern has also been expressed about long-term effects following acute exposure Research suggests that some victims may show reductions in some neurobehavioral tests when tested some months after exposure There are also concerns that people who not appear to have suffered acute poisoning have subsequently developed debilitating illnesses Symptoms include extreme exhaustion, mood changes, memory loss, depression, and severe muscle weakness Carbamates Carbamates are derived from carbamic acid and are used against both insects and weeds They are also acetylcholine esterase inhibitors They are very reactive and are used up rapidly after application Methyl bromide Methyl bromide was for many years the fumigant of choice for destroying insects in stored crops, but it is now being withdrawn as part of the general restriction on volatile organohalogen compounds because of their damaging effect on the ozone layer It is being replaced by a number of less environmentally damaging compounds, including phosphine, although none currently available is as effective or as cheap as methyl bromide Phosphine Phosphine has been used as a fumigant for many years It is highly reactive and leaves no residues but great care has to be taken in its application because it is very toxic to humans FOOD SAFETY/Pesticides Control of Pesticides Control over pesticides is exercised in two ways: stringent testing on new pesticides before they are permitted and measurement of the residue in the crop Testing pesticides There are a number of national and international bodies that approve new pesticides within their areas of responsibility These include Codex Alimentarius, the European Union, and the US Food and Drug Administration (USFDA) Currently, within the European Union, registration of pesticides is being harmonized under Directive 91/414 EEC Annexe of this directive will identify all active ingredients permitted in pesticides As yet this annexe is incomplete and member states are still acting under their national laws Within the UK pesticide registration is carried out under the Control of Pesticide Regulations 1986 and is the responsibility of the Ministry of Agriculture, Fisheries, and Food who are advised by the Advisory Committee on Pesticides In the USA a new Food Quality Protection Act of 1996 replaced both the Food, Drug, and Cosmetic Act and the Insecticide, Fungicide, and Rodenticide Act to provide a comprehensive regulatory scheme for pesticides In order to gain approval for use, pesticides are subjected to an extensive testing program including toxicity tests on mammals, plants, insects, fungi, birds, bees, fish, earthworms, and other soil organisms The toxicity studies include effects of pesticides on fetuses and infant animals There are also environmental tests which include laboratory tests on the breakdown and movement of the chemical in plants, soil, water, air, mammals, birds, and fish These latter tests determine the rate of decay in the various species Laboratory tests are followed by prolonged field trials to determine the fate of the chemical and its breakdown products in the environment and to estimate how the pesticide is concentrated up the food chain On average it takes about 10 years to develop a new pesticide at a cost of about £50 million The complete dossier of results has to be submitted to the approval body who determine whether the tests have been sufficiently rigorous to allow an acceptable daily intake (ADI) of the pesticide to be set The ADI is defined as the amount of a pesticide that can be taken in each day throughout a person’s life with the practical certainty, on the basis of all known facts, that no harm will result This is determined on the basis of the highest level at which the pesticide has no observable effect in 327 animal tests This is then reduced by a factor of 10 in case humans are more sensitive than the animals used in the tests, and by a further factor of 10 to allow for cases where some humans may be more sensitive than others In some cases, where the data show unusual effects, the safety factor can be increased from 100 to 500 or 1000 In practice the amount of pesticides to which the population is exposed is far below this level Table includes the ADI for a number of the more common pesticides There is no evidence that there are any cases where the combined effects of two pesticides are greater than the sum of their individual effects, in other words there is no evidence of synergy in toxicology between the different pesticides Once maximum residue limits (MRL see below) for foodstuffs have been set on the basis of good agricultural practice, a total dietary intake is determined by considering all commodities in which the pesticide is likely to be used, and assuming the upper range of consumption, all foodstuffs at the MRL and no losses during transport, storage or food preparation This figure is then compared with the ADI For all permitted pesticides in the UK the figure is below the ADI Maximum residue limits Maximum residue limits (MRLs) are statutory limits set on individual active ingredient and foodstuff combinations They are based on residue levels which result when the pesticide is used according to the instructions on the label and in accordance with good agricultural practice (GAP) MRLs may be used to ensure that the pesticides are only being used in accordance with GAP Many countries have codes of good operating practice with training for farmers and operators to ensure that pesticides are used at optimal levels Some countries rely on the Codex Alimentarius Committee on Pesticide Residues to establish MRLs, while others set their own (Codex Alimentarius is an international body which has over 120 countries as members and their standards are increasingly being accepted as the basis of world trade in foodstuffs.) In the USA the FDA used to set tolerances for pesticide/foodstuff combinations but under the 1996 Act it sets a level for each pesticide in all foods based on the principle of a reasonable certainty of no harm This is defined as a lifetime cancer risk of less than in a million There is also a requirement that residue tolerances must be specifically determined as being safe for children Within the EU, individual member states have historically set their own MRLs which differ from 328 FOOD SAFETY/Pesticides state to state Directive 76/895 established a common MRL setting regime and a series of subsequent directives has fixed the levels for a series of pesticides in fruit, vegetables, cereal products, and products of animal origin There is an ongoing program to harmonize the levels throughout the Union Most industrialized countries have pesticide surveillance programs which cover both homeproduced and imported commodities and these report annually The EU has an annual specific coordinated program to check compliance in nominated combinations of pesticide and foodstuff MRLs require sophisticated equipment for their determination because the levels are so low and the minimum detectable limit depends on the foodstuff For example the tolerance for aldrin and dieldrin (two organochlorines) in the USA is between 0.05 and 0.1 mg kgÀ1 (parts per million), depending on the foodstuff There are over 600 different active ingredients available commercially Because there are so many, laboratories around the world have developed sophisticated rapid analytical techniques to allow them to screen pesticides by class so that retailers, food manufacturers, and governments can carry out analyses as a matter of routine The MAFF 7th Report of the UK Working Party on Pesticide Residues in 1996 showed 68% of samples had no detectable residue, 31% had residues below the MRL, and ... the UK the figure is below the ADI Maximum residue limits Maximum residue limits (MRLs) are statutory limits set on individual active ingredient and foodstuff combinations They are based on residue... debilitating illnesses Symptoms include extreme exhaustion, mood changes, memory loss, depression, and severe muscle weakness Carbamates Carbamates are derived from carbamic acid and are used against... program to check compliance in nominated combinations of pesticide and foodstuff MRLs require sophisticated equipment for their determination because the levels are so low and the minimum detectable