Toxicity of Herbicides

Chapter 44

Toxicity of Herbicides Pawan K. Gupta

INTRODUCTION

BACKGROUND

Herbicides, also commonly known as weed killers, are chemical substances used to control unwanted plants. They are phytotoxic chemicals used for destroying various weeds or inhibiting their growth. They have variable degrees of specificity. The worldwide consumption of herbicides is almost 48% of the total pesticides usage. The consumption of herbicides in developing countries is low because weed control is mainly done by hand weeding (Gupta, 2004). Early chemicals used as herbicides include sulfuric acid, sodium chlorate, arsenic trioxide, sodium arsenate, and petroleum oils. Iron and copper sulfate or sodium borate were generally difficult to handle and/or toxic, relatively nonspecific, or phytotoxic to the crop as well as the unwanted plant life if not applied at exactly the proper time (Gupta, 2016a,b). During the last few decades, the herbicides have represented the most rapidly growing section of the pesticide industry due in part to (1) movement into monoculture practices and (2) mechanization of agricultural practices because of increased labor costs. The result has been a plethora of chemically diverse structures rivaling the innovative chemistry so as to develop synthetic organic herbicides and biopesticides that are quite selective for specific plants and have low mammalian toxicity. The aim is to protect desirable crops and obtain high yields by selectively eliminating unwanted plant species, thereby reducing the competition for nutrients (Gupta, 2006). Most of the animal/human health problems that result from exposure to herbicides are due to their improper use or careless disposal of containers (Gupta, 2010a). Very few problems occur when these chemicals are used properly. However, there is increased concern about the effects of herbicides on animal health because of runoff from agricultural applications and entrance into drinking water supply (Gupta, 1986, 1988).

The first discovery in the field of selective weed control was the introduction of 2,4-dinitro-o-cresol (DNOC) in France in 1933. This is very toxic to mammals and can cause bilateral cataract in humans. In 1934, phenoxy herbicides were developed and 2,4-dichlorophenoxyacetic acid (2,4-D) was introduced (Gupta, 2010b). During World War II, considerable effort was directed toward the development of effective, broad-spectrum herbicides with a view to both increasing food production and finding potential chemical warfare agents (Gupta, 1989). One chemical class of phenoxy derivatives including the acids, salts, amines, and esters represents the first commercially available products evolving from this research in 1946. Some other herbicides used from this class include 4-(2,4-dichlorophenoxy) butyric acid (2,4-DB), 2-(2,4-dichlorophenoxy propionic acid) (dichlorprop), 2-(2-methyl-4-chlorophenoxy) propionic acid (MCPP or mecoprop), and 2-methyl-4-chlorophenoxyacetic acid (MCPA) (Kennepohl et al., 2010). This class of herbicides has been in continuous, extensive, and uninterrupted use since 1947 and is the most widely used family of herbicides. Another chemical class of herbicides deserving particular attention is the bipyridyl group, especially paraquat and diquat. Weidel and Russo first described the structure of paraquat in 1882. In 1933, Michaelis and Hill discovered its redox properties and called the compound methyl viologen. Its herbicidal properties were discovered by ICI in 1955, and it became commercially available in 1962 (Smith, 1997; Lock and Wilks, 2010). The first urea herbicide, N,N-dimethyl-N0 -(4-chlorophenyl)-urea, was introduced in 1952 by DuPont under the common name of monuron. In subsequent years, many more derivatives of this class of compounds have been marketed (Liu, 2010). Protopyrinogen oxidase (Protox)-inhibiting herbicides have been used since the 1960s and currently represent a relatively large and growing segment of the herbicide

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554 SECTION | VII Herbicides and Fungicides

market. Nitrofen was the first Protox-inhibiting herbicide to be introduced for commercial use in 1964. This diphenyl ether (DPE) herbicide was eventually recognized as a relatively weak inhibitor of Protox, but it was a lead compound of an entire class of structurally related herbicides that were much more active. Subsequently, several DPE herbicides have been successfully commercialized (Nandihalli et al., 1992; Anderson et al., 1994). Substituted aniline, an alachlor herbicide, was registered and introduced in 1967 for the preplant or preemergent control of a broad spectrum of grass, sedge and broadleaf weeds (Heydens et al., 2010). Subsequently, inhibitors of aromatic acid biosynthesis herbicides (organic phosphorus) such as glyphosate, broad-spectrum, nonselective, postemergent, systemic herbicide with activity on essentially all annual and perennial plants have been developed. Monsanto discovered the herbicidal properties of glyphosate in 1970, and the first commercial formulation was introduced in 1974 under the Roundup brand name. Other triazine and triazole herbicides have been extensively used in agriculture in the United States and other areas of the world for more than 50 years. The triazines inhibit photosynthesis by blocking photosynthetic electron transport (Gysin and Knuesli, 1960; Steven and Summer, 1991; Breckenridge et al., 2010). Dicamba, which was first registered in the United States in 1967, is another organic (benzoic) acid herbicide that acts by mimicking the effects of auxins (i.e., natural plant growth hormones), causing enhanced but uncontrolled growth rates, alterations in plant function homeostasis and death (Harp, 2010). Another class of synthetic chemical compounds called the imidazolinone herbicides was discovered in the 1970s, with the first US patent awarded in 1980 for imazamethabenzmethyl. New families of herbicides introduced since the 1970s account for increasing shares of use and include bipyridyl (paraquat), bentazon, fenaxalactogen, oxyfluorfen, clomazone, clorpyralid, fluazifop, and norfluorazon. Today, the use of newer compounds that have low toxicity is quite common (Osteen and Padgitt, 2002).

TOXICOKINETICS Toxicokinetics studies provide important data on the amount of toxicant delivered to a target as well as species-, age- and gender-specific metabolism. Animals are exposed to herbicides of different chemical classes. In general, liver is the primary site for biotransformation and may include activation as well as detoxification reactions through the cytochrome P450-dependent monooxygenase system, the flavin-containing monooxygenase, esterases and a variety of transferases, most notably the glutathione (GSH) S-transferases (Hodgson and Meyer, 1997).

2,4-D is the most extensively studied phenoxy acid derivative herbicide. Absorption of 2,4-D occurs rapidly from the gastrointestinal (GI) tract, and peak levels are reached in 10 min to 24 h depending on species, dose and chemical form. Following oral exposure to 2,4-D, plasma half-life ranges from 3.5 to 18 h. Dermal absorption was reported to occur rapidly but was usually less than 6%. The compound is protein bound in vivo and is rapidly distributed to the liver, kidneys, lung and brain. 2,4-D has also been reported to cross the placental barrier in laboratory animals and pigs. 2,4-D is not metabolized to reactive intermediates, does not accumulate in tissues and is excreted predominantly as the parent compound in urine. However, the rate of excretion via urine is inversely proportional to dose. 2,4-D has been detected in the milk of lactating rats dosed with 2,4-D. The salts and esters of 2,4-D undergo acid and/or enzymatic hydrolysis to form 2,4-D acid, and small amounts may be conjugated with glycine or taurine. Excretion can be markedly enhanced by ion trapping using alkaline agents because most of these herbicides are organic acids (Erne, 1966a,b; Pelletier et al., 1989; Kennepohl et al., 2010). Another organic acid herbicide, dicamba, is rapidly and nonselectively distributed to most of the organs; however, dermal absorption is minimal. Ninety percent of excretion is through urine, and a small amount is excreted in feces. Dicamba is mostly unmetabolized but may be conjugated with glucuronic acid or glycine. Elimination occurs rapidly, and there is no evidence of bioaccumulation in the mammalian system (Harp, 2010). Bipyridyl derivative paraquat is rapidly but incompletely absorbed from the GI tract of laboratory animals and humans, with plasma concentration of 3090 min, and it is poorly absorbed through contact with skin. It has been reported that dogs absorb more paraquat than do rats, resulting in greater susceptibility of dogs toward paraquat toxicity (Lock and Wilks, 2010). Paraquat is very poorly metabolized, and bulk is excreted unchanged in the urine and feces. The transport mechanism for organic cations in renal proximal tubular cells is not fully understood; however, two membrane proteins, organic cation transporter 1 (OCT1) and organic cation transporter 2 (OCT2), have been isolated from rat kidney. OCT1, located at the basolateral membrane, transports tetraethylammonium, and this can be inhibited by other organic cations such as quinine. OCT2 stimulates the uptake of tetraethylammonium, and this can be markedly inhibited by cimetidine. The transport of paraquat can be blocked by the addition of the divalent cation quinine, cimetidine and, to a lesser extent, tetraethylammonium, suggesting that paraquat may be transported by both transport systems, an electro neutral organic cation/H1 exchange and P-glycoprotein (Chan et al., 1998). It was found that the hMATE1-mediated transport of agmatine was inhibited

Toxicity of Herbicides Chapter | 44

H+ PQ+2

PQ+2

Na+

0

0 2H+ PQ+2 P-glycoprotein

PQ+2

OCT2 pH 7.4 Basolateral membrane

pH 7.2

pH 6.7 Apical membrane

MATE1

Urine (Bile)

H+

H+

2+

2+

AGM

AGM AGM

2+

AGM

OCT2 (OCT1)

(hepatocytes)

Renal proximal tubules

FIGURE 44.1 Schematic representation of the proposed transport systems for paraquat across renal tubular cells. The transporters are OCT1 and OCT2 at the basolateral membrane and P-glycoprotein and the cation/H1 exchange system at the brush border membrane. Reproduced with permission from Chan, B.S.H., Lazzaro, V.A., Seale, J.P., Duggin, G.G., 1998. The renal excretory mechanisms and the role of organic cations in modulating the renal handling of paraquat. Pharmacol. Ther. 79, 193203.

AGM

2+

2+

OCT2 (OCT1)

Cimetidine quinine OCT1 PQ+2

pigs. In rats, diquat monopyridone has been identified in the feces, at approximately 5% of an oral dose, whereas diquatdipyridone has been detected in urine. These results indicate that diquat is probably metabolized by GI bacteria (JMPR, 1993). Ureas and thioureas such as diuron are readily absorbed through the GI tract in rats and dogs and are mainly metabolized by dealkalization of the urea methyl groups. The predominant metabolite of diuron in urine is N-(3,4-dichlorophenyl)-urea. Diuron is partially excreted unchanged in feces and urine. The storage of diuron does not occur in tissues (Boehme and Ernst, 1965; Hodge et al., 1967; Liu, 2010). Organophosphorus herbicides such as glyphosate and glufosinate are poorly absorbed both orally and via the dermal route. There is rapid elimination, and these are not biotransformed and do not accumulate in tissues. More than 70% of an orally administered dose of glyphosate is rapidly eliminated through feces and 20% through urine. The main metabolite of glyphosate is aminomethylphosphonic acid (AMPA); AMPA is of no greater toxicological concern than its parent compound (JMPR, 2004). The proton class of oxidase inhibitor herbicides is either not readily absorbed or is rapidly degraded by metabolism and/or excreted. In mammals, there are remarkable species differences in the levels of porphyrin accumulation resulting from exposure to Protox inhibitors. There is no bioaccumulation risk to animals. The carboxyester group of the triazolinone herbicide carfentrazone ethyl is initially metabolized to a carboxylic acid group. Other metabolites identified in rats and lactating goats include hydroxymethylpropionic acid and cinnamic acid derivatives, which are further metabolized to yield a benzoic acid derivative (Aizawa and Brown, 1999).

MATE1

by paraquat, which indicates the involvement of MATE-1 (multidrug and toxin extrusion) in paraquat renal transport (Winter et al., 2011). It is clear that paraquat can enter a renal cell via OCT2 and, to a lesser extent, OCT1 and then be transported out of the cell by MATE-1. However, whether MATE-2k can transport paraquat is not known (Chan et al., 1998; Lock and Wilks, 2010; Winter et al., 2011). A schematic representation of the proposed transport systems for paraquat across renal tubular cells is shown in Figs. 44.1 and 44.2. Unlike paraquat, diquat does not accumulate in the lungs; however, it is observed in liver, kidney, plasma and adrenal gland. Diquat does not enter the brain (Rose et al., 1976). Following oral administration, 90%98% of the dose is eliminated via the urine (Daniel and Gage, 1966). Metabolism studies indicate some unidentified metabolites of diquat in the urine of rabbits and guinea

555

Blood

AGM

2+

FIGURE 44.2 Proposed mechanism of agmatine transport in tissues (i.e., kidney and liver) widely recognized to express OCT1, OCT2 and MATE-1. Organic cation transporter (OCT) 1 and 2 mediate the facilitated influx transport of organic cations at the basolateral membrane of hepatocytes and renal proximal tubule cells, respectively. The multidrug and toxic compound extrusion (MATE) transporter 1, an H1/cation antiporter, is critical in the efflux elimination of various organic cations from the brush border and canalicular membrane of the kidney and liver, respectively. (Descriptions in parentheses refer to equivalent structures in the liver.) Reproduced with permission from Winter, T.N., Elmquist, W.F., Fairbanks, C.A., 2011. OCT2 and MATE1 provide bidirectional agmatine transport. Mol. Pharm. 8, 133142.

556 SECTION | VII Herbicides and Fungicides

Substituted anilines are well absorbed in rats orally. The dermal penetration in monkeys is relatively slow. The metabolism of alachlor in rats is complex due to extensive biliary excretion, intestinal microbial metabolism and enterohepatic circulation of metabolites. The main routes of excretion are urine and feces, and nearly 90% of the dose is eliminated in 10 days. Dimethenamid, an amide derivative, is slowly but well absorbed after oral administration (90% in rats) and is extensively metabolized in rats. The maximum concentration in blood is not achieved until approximately 72 h. Excretion is primarily via bile. By 168 h after treatment, an average of 90% of the administered dose is eliminated. In rats, the triazolopyrimidine compounds are rapidly absorbed and urinary elimination is rapid, with half-lives ranging from 6 to 12 h. Excretion is mainly through urine, and small amounts are excreted in feces.

MECHANISM OF ACTION There are a number of biochemical changes or free radical-mediated processes; some may also be produced by other mechanisms that have been used to assess tissue injury. For example, the loss of tissue GSH may reflect alkylation reactions, not oxidation. Furthermore, some free radical-mediated changes that may cause injury are also the result of injury. In most situations, it is difficult to pinpoint the exact mechanism of action. The mechanism of action of phenoxy derivatives, triazines, triazolopyrimidines, imidazolinones, dinitroaniline, and many other classes of herbicides is not precisely known. However, phenoxy compounds are known to depress ribonuclease synthesis, uncouple oxidative phosphorylation and increase the number of hepatic peroxisomes. The relationship of these biochemical changes to clinical effect is not clear. In dogs, these herbicides may directly affect muscle membranes (Sandhu and Brar, 2000). Herbicides such as 2,4-D, 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and dicamba act as peroxisome proliferators. Oxadiazinon causes hepatic porphyria in both mice and rats. The phenyl urea herbicides linuron and monuron are rodent liver carcinogens. Chloroacetanilide and metolachlor have shown weak hepatocarcinogenicity in female rats and are nongenotoxic, suggesting a tumor-promoting action. The dinitro compounds markedly stimulate respiration while simultaneously impairing adenosine triphosphate synthesis. The main toxic action is uncoupling of oxidative phosphorylation, converting all cellular energy in the form of heat and causing extreme hyperthermia. In addition, the gut flora in ruminants is able to further reduce the dinitro compounds to diamine metabolites, which are capable of inducing methemoglobinemia. The available information on substituted anilines indicates that there is a nongenotoxic mechanism of action

and lack of relevance to humans for the nasal turbinate, stomach and/or thyroid oncogenic effects produced in rats. The data support grouping of alachlor, acetochlor, and butachlor with respect to a common mechanism of toxicity for nasal turbinate and thyroid tumors, and grouping of alachlor and butachlor for stomach tumors (Heydens et al., 2010). The mechanism of action of paraquat and diquat is very similar at the molecular level and involves cyclic reductionoxidation reactions, which produce reactive oxygen species and depletion of nicotinamide adenine dinucleotide phosphate hydrogen (NADPH). However, the critical target organ differs for the two compounds, so the mammalian toxicity is quite different. Although both herbicides affect kidneys, paraquat is selectively taken up in the lungs. Paraquat causes pulmonary lesions as a result of type I and type II pneumocytosis. The primary event in the mechanism of toxicity within cells is paraquat’s ability to undergo a single electron reduction from the cation to form a free radical that is stable in the absence of oxygen. If oxygen is present, a concomitant reduction of oxygen takes place to form superoxide anion (O2 2 ). Superoxide radical, in turn, is nonenzymatically converted to singlet oxygen, which attacks polyunsaturated lipids associated with cell membranes to form lipid hydroperoxides. Lipid hydroperoxides are normally converted to nontoxic lipid alcohols by the selenium-containing GSHdependent enzyme, GSH peroxidase. Selenium deficiency, deficiency of GSH, or excess lipid hydroperoxides allows the lipid hydroperoxides to form lipid-free radicals. Lipid hydroperoxides are unstable in the presence of trace amounts of transition metal ions and decompose to free radicals, which in turn cause further peroxidation of polyunsaturated lipid in a process that is slowed by vitamin E. Peroxidation of the membranes could in turn cause cellular dysfunction and hence lead to cell damage or death (Smith, 1997). Genes associated with oxidative stress, redox cycling and apoptosis have been shown to play a key role in the development of lung fibrosis (Tomita et al., 2007; Lock and Wilks, 2010). The neurotoxicity of paraquat is under debate (Lock and Wilks, 2010), but its involvement in neurodegenerative diseases like Parkinson’s is well established (Jones et al., 2014). A schematic diagram incorporating these elements of the mechanism of paraquat-induced lung toxicity is shown in Fig. 44.3. The mechanism of action of diquat differs somewhat from that of paraquat because it undergoes alternate reduction followed by reoxidation—a process known as redox recycling. Like paraquat, diquat can redox cycle, with the major difference being that diquat can more readily accept an electron than can paraquat (Gage, 1968). The major target organs are the GI tract, the liver and the kidneys. Unlike paraquat, diquat shows no special

Toxicity of Herbicides Chapter | 44

Hexose monophosphate shunt IN

OUT

557

3

Paraquat + NCH3

>0.5 nm

ce p Re

1

PQ+2

PQ2+ 2 O+2

5

Putrescine

+ NH3

+ NH3

(CH2)4

H2O2

0.622 nm

GSSH

Then O+2 + O2+ Fe3+ + O2+

Glutathione peroxidase

NADPH

2H+

O2 H2O2 Fe2+ + O2

.

OH + OH− + Fe3+

Fe2+ + H2O2

4

GSH

Glutathione reductase Alveolar epithelial cell wall

NADP+

NADPH

0.702 nm

tor

+ CH3 N

Lipid peroxidation

NADP+

Cell death

Hexose monophosphate shunt FIGURE 44.3 Schematic representation of mechanism of toxicity of paraquat. (1) Structure of paraquat and putrescine; (2) putative accumulation receptor; (3) redox cycling of paraquat utilizing NADPH; (4) formation of hydroxyl radical (OHG) leading to lipid peroxidation; and (5) detoxication of H2O2 via GSH reductase peroxidase couple, utilizing NADPH. Reproduced with permission from Smith, L.L., 1997. Paraquat. In: Sipes, I.G., McQueen, C.A., Gandolfi, A.J. (Eds.), Comprehensive Toxicology: Toxicology of the Respiratory System, vol. 8. Pergamon, New York, NY, pp. 581589.

affinity for the lung and does not appear to involve the same mechanism that selectively concentrates paraquat in the lung (Rose and Smith, 1977). Glyphosate, a member of the phosphonomethyl amino acid group, selectively inhibits the enzyme 5-enolpyruvoylshikimate 3-phosphate synthetase. The enzyme plays a key role in the biosynthesis of the intermediate, chorismate, which is necessary for the synthesis of the essential amino acids phenylalanine, tyrosine and tryptophan. This aromatic amino acid biosynthesis pathway is found in plants as well as in fungi and bacteria but not in insects, birds, fish, mammals and humans, thus providing a specific selective toxicity to plant species (Franz et al., 1997).

TOXICITY More than 200 active ingredients are used as herbicides; however, some are believed to be obsolete or have been discontinued. Of these, several have been evaluated for their toxic potential, and acceptable daily intake has been recommended by the Joint Meeting on Pesticide Residues (IPCS, 2002). In general, with a few exceptions, most of the newly developed chemicals have a low order of toxicity to mammals. However, there is increasing experimental and anecdotal evidence that exposure to herbicides

also affects at least some form of development and/or reproduction in one or more species of animals. Some herbicides have been associated with birth defects in humans. For example, 2,4-D in combination with 2,4,5-T or dioxin, oryzalin, butiphos, picloram, Silvex (2-(2,4,5-trichlorophenoxy) propionic acid) and TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) are known to cause reproductive problems/malformations in humans. A nonexhaustive list of herbicides that cause developmental toxicity in experimental animals is presented in Table 44.1.

Inorganic Herbicides and Organic Arsenicals Substances such as sodium arsenite, arsenic trioxide, sodium chlorate, ammonium sulfamate and borax were formerly used on a large scale. The disadvantage of such herbicides from an agricultural standpoint is that they are nonselective; thus, their use has declined due to the availability of better and selective organic preparations (Marrs, 2004; Gupta, 2016a,b).

Phenoxy Acid Derivatives This class of herbicides includes 2,4-D, 2,4,5-T, 2,4-DB, dalapon, dichlorprop or 2,4-DP, mecoprop or MCPP, MCPA and Silvex. Some of the phenoxy derivatives are

558 SECTION | VII Herbicides and Fungicides

TABLE 44.1 Nonexhaustive List of Herbicides That Are Known to Cause Developmental Toxicity in Experimental Animals (Gupta, 2017) Chemical

Malformations

Atrazine

Disruption of ovarian cycle and induced repetitive pseudopregnancy (rats, at high doses)

Buturon

Cleft palate, increased fetal mortality (mice)

Butiphos

Teratogenic (rabbit)

Chloridazon

Malformations

Chlorpropham

Malformations or other developmental toxicity (mice)

Cynazine a

Malformations such as cyclopia and diaphragmatic hernia (rabbits). Skeletal variations in rats a

2,4-D , 2,4,5-T alone or in combination

Malformations such as cleft palate, hydronephrosis, teratogenic (mice, rats)

Dichlorprop

Teratogenic (mice), affects postnatal behavior (rats)

Dinoseb

b

Multiple defects (mice, rabbits)

Dinoterb

Skeletal malformations (rats), skeletal, jaw, head and visceral (rabbits)

Linuron

Malformations (rats)

Mecoprop

Malformations (mice)

Monolinuron

Cleft palate (mice)

MCPA

Teratogenic and embryotoxic (rats), teratogenic (mice)

Prometryn

Head, limbs and tail defects (rat)

Propachlor

Slightly teratogenic (rats)

Nitrofenb

Malformations (mice, rats, hamsters)

Silvex

Teratogenic (mice) a

TCDD

Malformations/teratogenic (fetotoxicity in chicken, rats, mice, rabbits, guinea pigs, hamsters and monkeys)

Tridiphane

Malformations such as cleft palate (mice), skeletal variations (rats)

a

TCDD is a common contaminant of 2,4-D and 2,4,5-T. Obsolete.

b

no longer agents of choice because of the formation of chlorinated dibenzofurans and dibenzodioxins, particularly TCDD, as a consequence of poorly monitored manufacturing practices. Some formulations of 2,4,5-T contain dioxin contaminants that increase the toxicity of technical-grade herbicides and therefore the safe use of phenoxy herbicides has been questioned. Reports indicate the occurrence of three rare forms of cancer (Hodgkin’s disease, soft tissue carcinoma, and non-Hodgkin’s lymphoma) in workers exposed to these herbicides contaminated with dioxins (Kennepohl et al., 2010). However, 2,4-D contains less than the quantitation limits of dioxins set by regulatory agencies (e.g., USEPA). 2,4-D is permitted for use in many countries throughout the world, including the United States and Canada. As a group, these are essentially nontoxic, and acute oral/dermal exposure to phenoxy herbicides is slightly to

moderately hazardous in normal use. Dermal irritation in rabbits is considered slight for the acid form of 2,4-D and minimal for the salt and ester forms. Eye irritation in rabbits, on the other hand, is severe for the acid and salt forms, but it is minimal for the ester. The oral LD50 for phenoxy acid derivatives in dogs is 100800 mg/kg body weight (BW). The dog is more sensitive and may develop myotonia, ataxia, posterior weakness, vomiting, bloody diarrhea and metabolic acidosis because of difficulty in the renal elimination of such organic acids (Gehring et al., 1976). Kidney effects consisting of reduced cytoplasmic eosinophilia of the epithelial cells lining and some convoluted tubules have been reported in dog. 2,4-D does not produce any testicular/ovarian damage or induce any abnormal reproductive disorders. However, some of the molecules of this class have been reported to cause teratogenic effects in animals at maternally toxic

Toxicity of Herbicides Chapter | 44

doses (Table 44.1) and reproductive problems in humans (Gupta, 2017). The group of compounds neither induces adverse effects in the nervous and immune systems nor has any potential to induce cancer or mutagenicity in laboratory animals. 2,4-D was found to be noncarcinogenic to rats, mice and dogs. Dogs are the most sensitive animals, whereas sheep, cattle and poultry are less sensitive (Yano et al., 1991a,b; Munro et al., 1992; Kennepohl et al., 2010). In dogs and pigs, GI signs of toxicity include anorexia, rumen atony, diarrhea, ulceration of oral mucosa, bloat and rumen stasis in cattle and vomiting, diarrhea, salivation, etc. Neuromuscular signs include depression and muscular weakness in cattle and ataxia, posterior weakness (particularly the pelvic limbs) and periodic clonic spasms (at high doses) in dogs. Silvex is unusual for this group because it is very toxic and small doses (26 mg/kg BW) may cause ill effects in dogs (Sandhu and Brar, 2000).

Bipyridyl Derivatives This chemical class of herbicides includes paraquat (1,10 -dimethyl-4,40 -bipyridylium dichloride) and diquat (1,10 -ethylene-2,20 -bipyridylium dibromide). Paraquat is usually formulated as dichloride salt (also known as methyl viologen). The bis(methyl sulfate) salt is no longer commercialized. Paraquat is nonselective and is a fastacting contact herbicide. This compound is one of the most toxic of the commonly used herbicides, and the toxicity varies in different animals depending on the formulation and species used. The toxic doses (oral LD50) of paraquat and diquat in rats are 150 and 231 mg/kg BW, respectively, and this class of herbicides is classified as moderately hazardous. Paraquat is a skin and eye irritant but not a skin sensitizer in animals. Mice are less sensitive than rats to orally administered paraquat, whereas guinea pigs, cats, monkeys and rabbits are more sensitive (Murray and Gibson, 1972; Bus et al., 1976a,b; Nagata et al., 1992; Lorgue et al., 1996; JMPR, 2003; Lock and Wilks, 2010). Cattle and sheep are more sensitive than other species. As indicated previously, paraquat and diquat have somewhat different mechanisms of action. Diquat exerts most of its harmful effects in the GI tract. The major cause of death after exposure to paraquat is lung damage. However, rabbits do not show signs of respiratory distress. Immediate toxic effects include convulsions or depression and incoordination, gastroenteritis and, finally, difficult respiration due to pulmonary edema and alveolar fibrosis (27 days). Animals that survive the first few days develop dehydration, pallor or cyanosis, tachycardia, tachypnea, harsh respiratory sounds and emphysema or pneumomediastinum.

559

Upon long-term exposure, there is progressive pulmonary fibrosis and increased respiratory distress. The morphological changes seen in animals include degeneration and vacuolization of pneumocytes, damage to type I and type II alveolar epithelial cells, destruction of the epithelial membranes and proliferation of fibrotic cells. The animals die as a consequence of reduced gas exchange and the development of severe hypoxia. Gross lesions include pulmonary congestion, edema and hemorrhages. Lingual ulcers may be seen. Other findings include failure of lungs to collapse when chest is opened and areas of hemorrhages, fibrosis and atelectasis. Microscopic lesions include necrosis of type I alveolar epithelial cells followed by progressive alveolar and intestinal fibrosis and alveolar emphysema. Renal proximal tubular degeneration and moderate centrilobular hepatic degeneration may also be seen (Smith, 1997). In mice, paraquat did not readily cross the placenta, whereas in rats it readily crossed the placenta, being detected in fetuses within 30 min of an intravenous injection to pregnant rats (Lock and Wilks, 2010). It has neither carcinogenic nor mutagenic potential; however, high doses injected into pregnant rats and mice on various days of gestation may cause significant maternal toxicity but do not produce teratogenic effects (Bus and Gibson, 1975). Diquat is formulated as dibromide salt and is slightly less toxic to dogs than is paraquat. After chronic exposure, the major target organs are the GI tract, the liver and the kidneys; however, lungs are not affected (Hayes, 1982). The presence of cataracts in both dogs and rats has been observed. Similar signs of toxicity have been seen in mice, guinea pigs, rabbits, dogs, and monkeys. Diquat has no effect on fertility, is not teratogenic and produces fetotoxicity only at doses that are maternally toxic. In a multigeneration study, at high doses, cataracts were observed in rats (FAO/WHO, 1993).

Ureas and Thioureas The ureas and thioureas (polyureas) are available under different names, such as diuron, fluometuron, isoproturon, linuron, buturon, chlorbromuron, chlortoluron, chloroxuron, difenoxuron, fenuron, methiuron, metobromuron, metoxuron, monuron, neburon, parafluron, siduron, tebuthiuron, tetrafluron and thidiazuron. In general, polyureas have low acute toxicity and are unlikely to present any hazard in normal use, with the exception of tebuthiuron, which may be slightly hazardous. Diuron and monuron are potent inducers of hepatic metabolizing enzymes compared to those polyurea herbicides with one or no halogen substitutions (chlortoluron and isoproturon). Male rats are more sensitive than females to the enzyme-inducing activity of diuron, and

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this can lead to detoxication of EPN and O-demethylation of p-nitro anisole. N-demethylation of aminopyrine increases for 13 weeks and then returns to normal (Hodgson and Meyer, 1997). Recovery from diuron intoxication is quick (within 72 h), and no signs of skin irritation or dermal sensitization have been reported in guinea pigs. Linuron in sheep causes erythrocytosis and leukocytosis with hypohemoglobinemia and hypoproteinemia, hematuria and ataxia, enteritis, degeneration of the liver and muscular dystrophy (Liu, 2010). In chickens, it leads to loss of weight, dyspnea, cyanosis and diarrhea. It is nontoxic to fish (Lorgue et al., 1996). Fluometuron is less toxic than diuron. In sheep, depression, salivation, grinding of teeth, chewing movements of the jaws, mydriasis, dyspnea, incoordination of movements and drowsiness are commonly seen. On histopathology, severe congestion of red pulp with corresponding atrophy of the white pulp of the spleen and depletion of the lymphocyte elements have been reported (Mehmood et al., 1995). The acute LD50 of isoproturon in rats is similar to that of diuron and does not produce any overt signs of toxicity, except at very high doses. A single oral dose of isoproturon in mice may produce some neurotoxic effects at very high doses and may reduce spontaneous and forced locomotor activity (Sarkar and Gupta, 1993a,b). Polyurea herbicides have been suspected to have some mutagenic effects but do not have carcinogenic potential (Liu, 2010). In general, the compounds do not cause developmental toxicity; however, buturon, linuron and monolinuron are known to cause some teratogenic abnormalities in experimental animals (Table 44.1). Isoproturon has been reported to cause maturational malformation of sperm and decreased spermatogenesis in rats (Liu, 2010).

Phosphonomethyl Amino Acids or Inhibitors of Aromatic Acid Biosynthesis Two organophosphorus compounds, glyphosate (Roundup and Vision; N-(phosphonomethyl) glycine) and glufosinate (Basta; N-(phosphonomethyl)homoalanine), are broad-spectrum, nonselective systemic herbicides. Although they exist as free acids, due to their low solubility, they are marketed as the isopropyl amine or trimethylsulfonium salts of glyphosate and the ammonium salt of glufosinate. Glyphosate has low acute oral toxicity in mice and rats and is unlikely to pose acute hazard in normal use. The LD50 of trimethylsulfonium salt is 750 mg/kg BW. The animals most affected are cattle, sheep and dogs. Dogs and cats show eye, skin and upper respiratory tract signs when exposed during or subsequent to an application to weeds or grass. Nausea, vomiting, staggering and hind leg weakness have been reported in dogs and cats

that were exposed to fresh chemical on treated foliage (Susan, 2003). Glyphosate is not a dermal irritant and does not induce photosensitization; and formulations can cause severe occupational contact dermatitis. Glyphosate is an ocular irritant in the rabbit and human, with minor to moderate conjunctival irritation and slight iritis that usually disappears within 48 h after exposure (Acquavella et al., 1999; JMPR, 2004). Formulations of glyphosate can cause intoxication in humans, which may be due to the presence of surfactants such as polyoxyethyleneamine. This class of surfactants has been associated with hemolysis and with GI tract and central nervous system (CNS) effects (Talbot et al., 1991). There is no evidence of mutagenic or carcinogenic potential of glyphosate. Several investigations do indicate the teratogenicity and reproductive toxicity of glyphosate. However, there is further need to conduct such studies and other parameters involving risk assessment of these compounds (Antoniou et al., 2012). It does not have adverse effects on reproductive performance in animals, except at very high doses maternal toxicity has been reported (JMPR, 2004). The testicular seminiferous tubules of rats treated with glyphosate indicated decreased epithelium lengths. The commercial formulation of glyphosate (Roundup, Monsanto Co.) is a potent endocrine disruptor in vivo and causes disturbances in the reproductive development of rats (Romano et al., 2010) and may lead to human breast cancer cells growth via estrogen receptors (Thongprakaisang et al., 2013). The acute oral toxicity of glufosinate is low, and glufosinate is slightly more hazardous than glyphosate. Common signs of toxicity include CNS excitation and hypothermia in animals (Ebert et al., 1990; Hack et al., 1994). Glufosinate ammonium formulation has been involved in a number of poisoning cases (cardiovascular and CNS adverse effects) possibly due to surfactantinduced penetration into the CNS (Watanabe and Sano, 1998). The compound is not considered to be mutagenic, teratogenic, or carcinogenic, except in whole-embryo culture. Teratogenic effects in mice have been observed, resulting in apoptosis in the neuroepithelium of the developing embryo (Watanabe, 1997).

Protoporphyrinogen Oxidase Inhibitors In the past, the Protox-inhibiting herbicides were often termed “DPE-type herbicides,” and almost all of the Protox inhibitors were DPEs. This nomenclature led to confusion concerning the classification of these herbicides because other DPE herbicides have an entirely different molecular site of action (i.e., inhibition of acetyl-CoA carboxylase). Since then, many other structurally related Protox inhibitors have been commercialized. In general, the newer products are more potent Protox inhibitors,

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resulting in lower application rates than those of the older herbicides of this class. Some of them appear to be analogs of the substrate or a substrate/product transition state of the enzyme (Reddy et al., 1998; Dayan and Duke, 2010). After the first generation of Protox inhibitors (with the exception of oxadiazon), which were based on the DPE, numerous other nonoxygen-bridged compounds (non-DPE Protox inhibitors) with the same site of action (carfentrazone, JV 485 and oxadiargyl) were commercialized (Dayan and Duke, 2010). Protox inhibitors have little acute toxicity and are unlikely to pose any acute hazard in normal use. These compounds increase the porphyrin levels in animals when administered orally, and the porphyrin levels return to normal within a few days. Rats and mice are sensitive and variegate porphyria-like symptoms can be generated in mice with high doses of Protox inhibitors. The majority of these compounds are neither mutagenic nor carcinogenic in nature, and the developmental toxicity correlates with Protox accumulation. Most Protox inhibitors, except bifenox and oxyfluorfen, are non- to moderately toxic to aquatic wildlife (Dayan and Duke, 2010). It has been reported in rats that prenatal exposure to sulfentrazone leads to neurodevelopmental effects (de Castro et al., 2007).

Triazines and Triazoles These herbicides are inhibitors of photosynthesis and include both the asymmetrical and the symmetrical triazines. Examples of symmetrical triazines are chloro-Striazines (atrazine, simazine, propazine, terbuthylazine and cyanazine); the thiomethyl-S-triazines (ametryn, prometryn and terbutryn), and the methoxy-S-triazine (prometon) (Breckenridge et al., 2010). The commonly used asymmetrical triazine is metribuzin. These herbicides have low oral toxicity and are unlikely to pose acute hazards in normal use, except for ametryn and metribuzin, which may be slightly to moderately hazardous. They are generally neither irritants to the skin or eye nor skin sensitizers. The exceptions are atrazine, which is a skin sensitizer in guinea pigs, and cyanazine, which is toxic by the oral route. However, sensitivity of sheep and cattle to these herbicides is appreciably high. The main symptoms are anorexia, hemotoxia, hypothermia, locomotor disturbances, irritability, tachypnea and hypersensitivity (Sandhu and Brar, 2000). Doses of 500 mg/kg of simazine or 30 mg/kg atrazine for 3060 days are lethal to sheep. Deaths have been reported in sheep and horses grazing triazine-treated pasture 17 days after spraying. Cumulative effects are not seen. Metribuzin is slightly more toxic than simazine, but it does not produce any harmful effects in dogs fed at 100 ppm in the diet. Simazine is excreted in milk, so it is

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a public health concern (Susan, 2003). Atrazine is more toxic to rats but comparatively less toxic to sheep and cattle than is simazine. These herbicides are classified as liver microsomal enzyme inducers and are converted to N-dealkylated derivatives. In contrast to simazine, it is not excreted in milk. Triazines seem to have no potential to be mutagenic or to produce carcinogenicity in animals. However, feeding of very high levels of some triazines resulted in mammary tumors in rats. Terbutryn also caused thyroid and liver tumors in female rats (Breckenridge et al., 2010). The exception is cyanazine, which is more acutely toxic, weak mutagenic, and results in developmental toxicity, presumably because of the presence of cyano moiety (Hodgson and Meyer, 1997).

Substituted Anilines Substituted anilines are used as systemic herbicides. The commonly used herbicides are alachlor, acetochlor, butachlor, metolachlor and propachlor. This class of herbicides is slightly hazardous, except butachlor, which is not likely to pose any hazard. The compounds are nonirritant to eyes, slight to moderate skin irritant, and produce skin sensitization in guinea pigs. Lower doses in rats and dogs do not produce any adverse effects; however, long-term exposure in dogs causes hepatotoxicity and splenic effects. The ocular lesions (progressive uveal degenerative syndrome) produced by alachlor are considered to be unique to the LongEvans rat because the response has not been observed in other strains of rats, mice, or dogs. At high oral doses, it may lead to maternal and fetal toxicity but may not cause any adverse effect on reproduction. It is neither teratogenic nor produces any microbial genotoxicity. Alachlor has the potential to produce thyroid tumors and adenocarcinomas of the stomach and nasal turbinates of LongEvans rats and in the lungs (bronchoalveolar) of CD-1 mice at high doses. It is considered to be a human carcinogen (Ahrens, 1994; Monsanto, 1997a,b; Heydens et al., 2010). Long-term exposure of acetochlor to rats has no adverse effects on reproductive performance. Acetochlor is converted into a rat-specific metabolite that may be related to the nasal and thyroid tumors, thus posing no genetic or carcinogenic hazard to humans (Ashby et al., 1996). Butachlor does not adversely affect reproductive performance or pup survival. It is nongenotoxic. Butachlor induced multiple tumors in SD rats but not in F344 rats or CD-1 mice (Heydens et al., 2010). Metolachlor can increase the incidence of liver tumors in rats and has been classified as a possible human carcinogen (Monsanto, 1991; Wilson and Takei, 1999; Heydens et al., 2010). Compared to other substituted anilines, propachlor is severely irritating to the eye and slightly irritating to the skin. Propachlor produces skin sensitization in guinea

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pigs. In rats, high doses of propachlor produce erosion, ulceration, and hyperplasia of the gastric mucosa; herniated mucosal glands in the pyloric region of the stomach; hypertrophy; and necrosis of the liver. In dogs, there is poor diet palatability, which results in weight loss and poor consumption of food. Propachlor may produce slight developmental or adverse reproductive effects (Table 44.1). It is not genotoxic or clastogenic in mammals. However, there is evidence that it produces benign hepatic tumors in male mice (Heydens et al., 2010). The previously discussed data support grouping alachlor, acetochlor and butachlor based on a common mechanism of toxicity for evaluation of risk assessment to humans and animals (Heydens et al., 2010).

Amides and Acetamides The commonly used amides and acetamides include bensulide, dimethenamid-P and propanil and are slightly to moderately hazardous in normal use. Dimethenamid is a racemic mixture of the M and P stereoisomers, whereas P isomer has useful herbicidal activity. Both substances produce only mild reversible skin and eye irritation and skin sensitization in guinea pigs. Comparison of racemic dimethenamid with dimethenamid-P indicates that there is little difference in their toxicological profiles. The signs of toxicity in mice, rats and dogs are similar, with reduced BW gain and liver enlargement with induction of liver xenobiotics metabolizing enzyme. There is strong binding to hemoglobin in rats, but this has no relevance to humans. Dimethenamid can reduce fetal BW but is not teratogenic. There is no compound-related mutagenic or carcinogenic potential (JMPR, 2005).

Dinitrophenol Compounds Several substituted dinitrophenols alone or as salts, such as DNP (2,4-dinitrophenol), DNOC (dinitro-o-cresol) and dinoseb (2-(1-methylpropyl)-4,6-dinitro), are used as herbicides. The main source of poisoning in animals is human negligence in removing the preparation if it spills, in disposing of the containers and in preventing animals access to treated fields. In general, the dinitro compounds are not very watersoluble and are highly hazardous to animals. The oral acute LD50 of DNOC in mice, guinea pigs, rabbits, hens, dogs, pigs and goats ranges from 25 to 100 mg/kg BW. In sheep, a dosage of 25 mg/kg/day causes toxicosis in 25 days. Clinical signs include fever, dyspnea, acidosis, oliguria, muscular weakness, tachycardia and convulsions followed by coma and death with a rapid onset of rigor mortis. Abortions have been reported in sows. In cattle and ruminants, methemoglobinemia, intravascular hemolysis and hemoproteinemia have been observed. Cataract

can occur with chronic dinitrophenol intoxication. Exposure to these compounds may cause yellow staining of skin, conjunctiva, or hair (Lorgue et al., 1996).

Triazolopyrimidine Herbicides Triazolopyrimidine herbicides include cloransulammethyl, diclosulam, florasulam, penoxsulam, flumetsulam, metosulam, and pyroxsulam. The generic structure of the triazolopyrimidine herbicides connected to a substituted phenyl ring through a sulfonamide bridge is shown in the second edition of this book. The acute oral toxicity of triazolopyrimidine herbicides is very low. On repeated exposure, the primary organs are the kidney (rat and mouse), liver (rat, mouse, and dog) and thyroid (rat) (Billington et al., 2010). In dogs, the target organ is eye as compared to other species (Timchalk et al., 1996). No adverse effects on neurotoxicity, reproductive performance and mutagenic abnormalities have been observed. The compound has no carcinogenic potential in humans (EPA, 1997a,b).

Imidazolinones Imidazolinone herbicides include imazapyr, imazamethabenzmethyl, imazapic, imazethapyr, imazamox, and imazaquin. These are selective broad-spectrum herbicides discovered in the 1970s. These herbicides are relatively nontoxic. Results from primary eye irritation studies range from no irritation (imazaquin) to slightly irritating (imazamethabenzmethyl) and moderately irritating (imazapic and imazethapyr), showing complete recovery within 7 days postdosing. The rabbit primary irritation study with imazapyr showed irreversible irritation. Toxicological effects of imidazolinone herbicides are slight to moderate skeletal myopathy and/ or slight anemia in dogs occurring in the 1-year dietary toxicity studies with three structurally similar imidazolinones (imazapic, imazaquin and imazethapyr). There is no evidence of any adverse effect on reproductive performance and on fetal abnormalities in the rat and the rabbit. Neither mutagenicity nor any carcinogenicity has been reported in either of these species (Hess et al., 2010).

Benzoic Acids The herbicides in this group include chloramben, dicamba, and naptalam. These have a low order of toxicity. In practice, dicamba is often combined with other herbicides and is used to control a wide spectrum of weeds. The signs and lesions are similar to those described for the chlorophenoxy acids. Poisoning after

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normal use has not been reported in domestic animals. Dicamba either alone or combined with another herbicide induced significant levels of apoptosis in mouse preimplantation embryo assay (Greenlee et al., 2004). It is a skin and eye irritant, and high doses may cause neurobehavioral symptoms in rats and rabbits. The compound did not show any adverse effects in a three-generation study in rats (Harp, 2010). Dicamba induces peroxisomal enzymes in rat liver and causes transcription up-regulation of the peroxisome proliferator-activated receptor. Long-term exposure to dicamba may induce tumors in rats due to its action as a peroxisome proliferator; however, the implications of these findings are not clear and may require further study (Espandiari et al., 1998). Dicamba-induced oxidative stress-mediated cytogenotoxicity has been demonstrated in an in vitro cell model (Gonzalez et al., 2009; Harp, 2010).

Carbamates, Thiocarbamates and Dithiocarbamate Compounds The compounds in this category include derivatives of carbamic acid (asulam, barban, chlorpropham, chlorbufam, karbutilate and phenmedipham), derivatives of thiocarbamic acid (butylate, cycloate, diallate, EPTC, molinate and triallate) and derivatives of dithiocarbamic acid (metham sodium). These herbicides have low to moderate toxicity in rats and do not pose acute hazards. They are used at low concentrations, and poisoning problems have not been reported. In general, these herbicides do not produce skin or eye irritation. With repeated exposure, there is a possibility of alopecia for some time after ingestion (Lorgue et al., 1996; Hurt et al., 2010). In ruminants, diallate results in anorexia, ataxia, muscular contractions, exhaustion, prostration, and alopecia in sheep, which is an indication of chronic poisoning. Thiobencarb has induced toxic neuropathies in neonatal and adult laboratory rats. It appears to increase permeability of the bloodbrain barrier. The nonspecific lesions include hepatic, renal and pulmonary congestion, enteritis, ascites, and hydrothorax (Susan, 2003).

Others Bromacil and terbacil are commonly used methyluracil compounds. These compounds can cause mild toxic signs at levels of 50 mg/kg BW in sheep, 250 mg/kg BW in cattle and 500 mg/kg BW in poultry when given daily for 810 days. Signs of toxicity include bloat, incoordination, depression and anorexia. Toxic doses of bromacil can be hazardous, especially for sheep, but no field cases of toxicity have been reported.

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The nitrile herbicides, ioxynil and bromoxynil, may uncouple and/or inhibit oxidative phosphorylation. Ioxynil, presumably due to its iodine content, causes enlargement of the thyroid gland in the rat (Marrs, 2004). Members of polycyclic alkanoic acids (diclofop, fenoxaprop, fenthiaprop, fluazifop and haloxyfop) have moderately low toxicity, whereas haloxyfop-methyl is an exception, and has high toxicity. They tend to be more toxic if exposure is dermal. The dermal LD50 of diclofop in rabbits is only 180 mg/kg (Susan, 2003). Some members of the amide group, such as bensulide and propanil, are used as plant growth regulators, and some of them are more toxic than others. A lethal dose of bensulide for dogs is 200 mg/kg. The prominent clinical sign is anorexia; other signs and lesions are not definitive and are similar to those of chlorophenoxy acid poisoning. Hemolysis, methemoglobinemia and immunotoxicity have occurred after experimental exposure to propanil (Lorgue et al., 1996). The toxicity of sulfonylureas (chlorsulfuron, sulfometuron, metsulfuron, chloremuron, and kensulfuron) appears to be quite low (Susan, 2003). A number of substances are used as defoliants in agriculture, including sulfuric acid to destroy potato haulms and two closely related trialkylphosphorothioates (DEF and merphos) to defoliate cotton. A notable feature of the latter is that they produce organophosphate-induced delayed neuropathy in hens (Baron and Johnson, 1964). Chlomequat is used as a growth regulator on fruit trees. The signs of toxicity in experimental animals indicate that it is a partial cholinergic agonist (JMPR, 2000).

ENDOCRINE DISRUPTION In both males and females, some herbicides affect reproduction through different mechanisms of action of endocrine disruption; exogenous agents interfere with reproduction and the development process. In males, normal reproductive function involves interaction of the hypothalamic-pituitary-testis axis and the thyroid gland. In females, increased concentrations of xenoestrogens may affect ovarian function through the disruption of feedback mechanisms in the hypothalamus-pituitarygonadal axis (Flaws and Hirshfield, 1997; Bretveld et al., 2006). Herbicides, like other chemicals, may disrupt all stages of hormonal function of the reproductive system. In females, during pregnancy and, to a greater extent, during lactation, a portion of the maternal body burden of these chemicals is transferred to the offspring. For example, herbicides such as linuron produce hypothyroidism (Gupta, in press). The herbicide glyphosate in low nontoxic concentrations caused disruption of the aromatase enzyme in human placental cells in vitro. It reduced the aromatase enzyme activity responsible for the synthesis of estrogens (Richard et al., 2005). A study indicated that

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male reproductive toxicity of glyphosate is due to the inhibition of a StAR protein and an aromatase enzyme, which caused an in vitro reduction in testosterone and estradiol synthesis. The study further suggested that commercial formulation of glyphosate (glyphosateRoundup Transorb, Monsanto) is a potent endocrine disruptor in vivo because it caused disturbances in the reproductive development of rats when the exposure was performed during the puberty period (Romano et al., 2010). From various experimental studies, it has been concluded that herbicides can disturb reproduction and developmental processes of both males and females through endocrine signals in organisms indirectly exposed during prenatal or early postnatal life. Such effects during fetal development may be permanent and irreversible. According to one estimate, eight herbicides (2,4-D, 2,4,5-T, alachlor, amitrole, atrazine, metribuzin, trifluralin and nitrofen) were identified as endocrine disruptors. Most of these were identified accidentally rather than as a result of an exhaustive screening process (Pocar et al., 2003).

TREATMENT The successful management of herbicide poisoning depends on (1) the clinicians’ understanding of the mechanism of herbicide toxicity and applying that understanding to the treatment options, (2) accurate diagnosis and assessment of the severity of intoxication, (3) maintenance of vital body functions and adequate clinical monitoring, (4) minimization of further absorption of the compound, and (5) appropriate use of specific treatment. Treatment is usually symptomatic and supportive. Intravenous fluid should be given to promote diuresis. Toxicity of paraquat is enhanced by selenium/vitamin E deficiency, oxygen, and low tissue GSH peroxidase activity. Therefore, vitamin E and selenium with supportive therapy may be useful in the early stages of paraquat intoxication. Excretion of bipyridyl compounds may be accelerated by forced diuresis induced by mannitol infusion and furosemide administration. Oxygen therapy and fluid therapy are contraindicated (Clark, 1971; Smith et al., 1974). Dinis-Oliveira et al. (2006, 2007, 2008, 2009) experimentally found dexamethasone, sodium salicylate and lysine acetylsalicylate to be an effective treatment for paraquat-induced toxicity. These authors concluded that the antioxidant properties of these agents might be responsible for their effectiveness. An effective antidote for dinitrophenol compounds is not known. Affected animals should be cooled and sedated to help control hyperthermia. Phenothiazine tranquilizers are contraindicated; however, diazepam can be used to calm the animal. Atropine sulfate, aspirin, and antipyretics should not be used; rather, physical cooling

measures such as cool baths or sponging and keeping the animal in a shaded area are advocated. Intravenous administration of large doses of sodium bicarbonate (in carnivores) solutions, parenteral vitamin A, and intense oxygen therapy, where possible, may be useful. If the herbicide is ingested and the animal is alert, an emetic should be administered; if the animal is depressed, gastric lavage should be performed. Treatment with activated charcoal should follow. Dextrosesaline infusions in combination with diuretics and tranquilizers (not barbiturates) are very useful. In ruminants, for methemoglobinemia, methylene blue solution and administration of ascorbic acid are useful (Lorgue et al., 1996).

CONCLUDING REMARKS AND FUTURE DIRECTIONS Herbicides are routinely used to control noxious plants. Most of these chemicals, particularly the synthetic organic herbicides, are quite selective for specific plants and have low toxicity for mammals; other less selective compounds (e.g., arsenicals, chlorates and dinitrophenols) are more toxic to animals. Most animal health problems including reproduction, which is affected by endocrine disruption, result from exposure to excessive amounts of herbicides because of improper or careless use or disposal of containers. The residue potential for most of these chemicals is low.

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