Organophosphate pesticides: A general review

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Agricultural Science Research Journals Vol. 2(9), pp. 512- 522, September 2012
Available online at http://www.resjournals.com/ARJ
ISSN-L:2026-6073 ©2012 International Research Journals
Review
Organophosphate pesticides: A general review
*M. Kazemi1, A. M. Tahmasbi1, R. Valizadeh1, A. A. Naserian1 and A. Soni2
1
Department of Animal Science, Excellence Center for Animal Science, Faculty of Agriculture, Ferdowsi University of
Mashhad, P. O. Box 91775-1163, Mashhad, Islamic Republic of Iran.
2
Plant Protection Company of Pars Taravat.
*Corresponding Author’s Email: [email protected]
Abstract
Pesticides are chemicals to control a variety of pests that can damage crops and livestock and reduce
farm productivity. Organophosphate (OP) compounds are a group of pesticides that includes some of
the most toxic chemicals used in agriculture. OP toxicity is due to the ability of these compounds to
inhibit an enzyme, acetyl cholinesterase at cholinergic junctions of the nervous system. This review will
deal with the history and composition, its uses and role in pollution, metabolism of OPs, health
impacts, and clinical manifestations of its toxicity, diagnostic methods and treatment. We suggest that
in future, the ministry of agriculture of developing countries especially Iran, should concentrate on the
optimization and monitoring of usage of OP compounds as pesticides and furthermore, encouraging
the farmers to use natural pesticides and organic agriculture rather than chemical pesticides. Also
animal feeds and Milk may serve as a vector for the transmission of substances of extrinsic origin
which can be potentially toxic to the consumer. These toxins may originate in cow's milk from the
ingestion of plants known to contain toxic substances or feeds contaminated with OP pesticides. In this
article, OP pesticide pollution in livestock feed, its excretion from animal and general data about OP
pesticides are reviewed.
Key words: pesticide, organophosphate, acetyl cholinesterase, agriculture.
INTRODUCTION
Many Iranian farmers have become reliant on toxic
pesticides and their excessive use leads to dangerously
high exposure levels for farm workers and consumers in
fruits, vegetables, nuts and field crops. OP compounds
are used as pesticides, herbicides, and chemical gases
used World War (Bowls et al., 2003). OPs constitute a
heterogeneous category of chemicals specifically
designed for the control of pests, weeds or plant
diseases. Their application is still the most effective and
accepted means for the protection of plants, and has
contributed significantly to enhanced agricultural
productivity and crop yields (Bolognesi, 2003). The
consumption of OP pesticides in developing country
especially in Iran for pest control in agriculture is
increasing, for example Iran is the largest producer of
pistachios in the world and farmers apply chemicals in
pistachio orchards to pest control. About 13 different
pests and diseases have been found to attack pistachios.
Farmers currently apply 20 different chemicals in
pistachio orchards, including OP pesticides (such as
phosalone, diazinon and malathion) (Aghasi et al., 2010).
Also animal feeds are routinely subjected to
contamination
from
diverse
sources,
including
environmental pollution and activities of insects and
microbes. Animal feeds may also contain endogenous
toxins arising principally from spraying pesticides against
pests. Although residual of OP pesticides are often
considered separately, because of their different origins.
Thus, particular compounds such as OP pesticides may
exert anti-nutritional effects or reduce reproductive
performance in farm animals. Furthermore, the combined
effects may be the result of additive or synergistic
interactions between the environment and animals. The
extent and impact of these interactions in practical
Kazemi et al.
513
Figure 1. General chemical composition of organophosphate pesticides
livestock feeding remain to be quantified. Feed
contaminant with pesticides especially OPs occur on a
global scale but there are distinct geographical
differences in the relative impact of individual
compounds. The term "feed" is generally used in its
widest context to include compound blends of straight
ingredients as well as forages. With such a broad
perspective, it is necessary and more instructive to
introduce some focus. So the common use of pesticides
in public health and agricultural schedules has caused
severe environmental pollution and potential health
hazards including severe acute and chronic cases of
human and animal poisonings (Moghadamnia and
Abdollahi, 2002). So we developed a search strategy to
find publications about OP poisoning and its
management using the key phrases causes of OP
compounds, diagnosis, management of OP poisoning
and drugs under clinical trials and also this article discuss
about the contaminant with OP pesticides that pose
significant risks to farm livestock.
History and composition
OP compounds were first developed by Schrader shortly
before and during the Second World War. They were first
used as an agricultural insecticide and later as potential
chemical warfare agents (Taylor, 1996). In the late 1990
and 2000 years, with the advent of increased awareness
of terrorism, nerve agents have gained prominence as
weapons of mass destruction. In these compounds, the
OPs are a group of both synthetic and biogenic OP
compounds, characterized by the presence of the binding
covalent, carbon to phosphorus (C-P) bond. In OPs, this
carbon to phosphorus bond replaces one of the four
carbon-to-oxygen-to-phosphorus bonds of the more
common phosphate ester (Wanner and Metcalf, 1992).
While the vast majority of phosphorus-containing organic
compounds contain the phosphate ester bond, both
synthetic and naturally occurring phosphonates are still of
importance (Blackburn, 1981). The direct C-P linkage is
chemically and thermally inert, with the result, most of
organophosphonate compounds are resistant to chemical
hydrolysis, thermal decomposition and photolysis
compared with analogous compounds containing the
more reactive N-P, S-P or O-P linkages (Figure 1). The
letter R represents either ethyl or methyl (Figure 2). The
insecticides with a double bonded sulfur are OP in the
liver. Phosphonate contains an alkyl(R-) in place of one
alkoxy group (RO-). X is called leaving group and is the
principal metabolite for a specific identification.
The use of organophosphate pesticides: pollution
and toxicity
The projected household demands for food until 2020
show that, between 1995 and 2020 years, the demand
for food grains is likely to be doubled, for vegetables
more than 2.5 times and for fruits 5 times. Thus, increase
in the consumption of pesticides is likely to be at least
two to three times more in years to come (Kanekar et al.,
2004). The most important effects of the synthetic
pesticides, especially OP pesticides are water and soil
pollutions, as well as the contamination of vegetables,
fruits, milk, food products and other living organisms
(Denistrop, 2000; Ahmad, 2001; Erin et al., 2001).
Pollution of the water in the river and depleting its
resources can put the lives of many people in danger
(Chimwanza et al., 2006). A wide range of organic
compounds may occur in feedstuffs, including OP
pesticides. Pesticides that may contaminate feeds
originate from most of the major groups, including organ
chlorine, OP and pyrethroid compounds (Van Barneveld,
1999). OP pesticides are examples of agriculture
pollutants that may contaminate feed of livestock,
particularly herbage. Cows grazing pastures that are
sprayed with OP produce milk with higher pesticide
content than cows grazing in unsprayed pastures.
Moreover, it has also been reported that the ground
water, surface water and drinking water are contaminated
with pesticide (Raju et al., 1982; Hallberg, 1989).
Diazinon levels in the Babol Rud River on the Caspian
coast of 4.1 parts per billion (ppb) were recorded, with
diazinon, ethion and methyl parathion pesticides the
commonest pesticide contaminants in this river (Nasehi,
1999). Another study determined the level of diazinon as
9.5 ppb in the Shirud River (Vaez, 2000). To put these
figures in perspective, the UK Environment Agency set
their environmental quality standard for annual average
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Agric. Sci. Res. J.
Figure 2: Chemical structures of some OP compounds.
Figure 2. Chemical structures of some OP compounds.
exposure for diazinon in freshwater at 0.01 ppb, to
protect aquatic life (EA, 1999). In Karaj, 40 km west of
Tehran, more than 200 people were hospitalized
following
consumption
of
pesticide-contaminated
cucumber in May 2002, while total poisoning cases were
suspected to be even higher. Malathion was earlier
reported as the commonest residue in cucumber
obtained from wholesale markets in Tehran (Farshad et
al., 2001). Number of OP pesticides is countless, so we
discuss about some OP pesticides herein. Metabolism in
animals is characterized by rapid elimination of
phosalone and its metabolites in the urine and feces.
Although in most tissues most of the residue has not
been identified, the available data indicate oxidation of
phosalone to its oxygen analogue, cleavage of phosalone
to yield O, Odiethyl dithiophosphoric acid and of the oxon
to the corresponding thiolic acid, and of both phosalone
and
its
oxon
to
2-oxo-3-mercaptomethyl-6hlorobenzoxazole (Demoras and Fournel, 1968). The
transformation of phosalone in rats has been described
(Demoras and Fournel, 1968; Demoras and Laurent,
1980; Smith et al., 1988). Phosalone was reported to be
oxidized in the rat to its oxon and both of these converted
to the putative intermediate thiol, with the phosphorus
portion of phosalone forming diethyldithiophosphoric acid
and that of the oxon forming diethylthiophosphoric acid.
The thiol is presumably subsequently transformed
sequentially into the sulphide, sulphoxide and sulphone.
In one disposition study 926 mg of radioactive phosalone
uniformly labelled in the aromatic ring was administered
via a fistula directly into the rumen of a 530 kg lactating
Holstein cow whose daily dietary ration was
approximately 8.2 kg. Urine and feces samples were
collected over a 100-hour period, and the cow was milked
in the morning and evening each day after feeding
(Craine, 1974). The total recovery of radioactive carbon
from the experiment was 100.1%, with urine containing
93.7%, feces 6.1% and milk 0.3%. The lack of radioactive
C02 in the urine suggested that no degradation of the
benzene ring had occurred. The solubility and stability of
phosalone in water at pH 5, 7 and 9 have been
investigated, So the solubility of phosalone was
determined by suspending 50 g phosalone in 500 mL
water, stirring, heating to 50°C and cooling to 20°C or
agitating 1 g/100 mL water with ultra-sound. Phosalone in
dichloromethane extracts of the filtered solutions by GLC
Kazemi et al.
was determined. The solubility was estimated to be about
1.7 mg/kg at 20°C (Laurent and Buys, 1975). Little
degradation (<10%) was observed during 28 d at pH 5.0
or 7 at 20°C. At pH 9.0 degradation was more
pronounced, giving a half-life of about 9 d. Phosalone is a
compound of moderate toxicity (Hayes and Laws, 1990;
Hayes, 1982). The acute oral lethal dose (LD50) values
ranged between 82-205 mg/kg for male rats; and
between 90-170 mg/kg for female rats (EPA, 1987). The
acute dermal LD50 values for rats ranged between 350
mg/kg and 390 mg/kg (EPA, 1987). The acute
percutaneous LD50 for rats is 1,500 mg/kg (Worthing,
1987). Phosalone rates of 700 g/ha were not found to be
hazardous to honeybees, provided they were not actively
foraging at the time of spraying (Worthing, 1987).
Pesticides, one of which was phosalone, applied to host
eggs at field rates in the laboratory were highly toxic to
Trichogramma brasiliensis released on the eggs, causing
84-100% mortality in 24 h. However, percentage
parasitism after 4 d was higher with phosalone (36-73%)
than with other pesticides studied, and emergence from
treated host eggs did not appear to be affected.
Phosalone had little or no effect on adults or cocoons of
Apanteles plutellae (Elzen, 1989). Soil acts as filter,
buffer and degradation potentials with respect to storage
of pollutant with the help of soil organic carbon (Burauel
and Bassmann, 2005). It is recognized that the soil is
also a potential pathway of pesticide transport to
contaminate water, air, plants, food and ultimately in the
human via, runoff and subsurface drainage; interflow and
leaching; and the transfer of mineral nutrients and
pesticides from soils into the plants and animals that
constitute the human food chain (Abrahams, 2002).
Pesticides which are very persistent in soil slowly break
down and result in source of contamination (Stephenson
and Solomon, 1993). The sources of contamination are
closely related to anthropogenic pollution, such as
domestic and industrial discharges, agricultural chemical
applications and soil erosion due to deforestation
(Bhattacharya et al., 2003). Effects of pesticides have
been reported in milk, feed, cottonseed, different fruits,
vegetables and fish meal at different intervals (Hussain et
al., 2002; Munshi et al., 2004; Saqib et al., 2005). Lloyd
and Matthysse (1971) were unable to detect diazinon in
milk of dairy cows after feeding diazinon with a protein
supplement. Szerletics et al. (2000) reported low
amounts (<0.025–1 mg/kg) in milk the first day after
treatment of cattle and sheep with diazinon. Diazinon
thus can occur in animal products after exposures, but
excretion occurs quite rapidly within a few days.
Health impacts by organophosphate pesticides
Concerns have been raised about the increasing levels of
cancer incidence and possible links with high levels of
pesticide exposure. Each year 500 people are recorded
515
as dying from cancer in Golestan province (Iran), 350
from stomach and 150 from throat cancer. The main crop
in this province is cotton. The investigation also revealed
that the incidence of cancer among people in the belt
between Ramsar and Behshahr in Mazandaran province
was 70% higher than in other parts of the Caspian coast
and this area suffers 30% more cancer cases than
elsewhere in the Iran (IDNH, 2002). Among pesticides,
OP is responsible for more than 50% of total poisoning
cases (Abdollahi et al., 1997). In addition, OP has been
used as harmful nerve gases (Morgan et al., 1980).
Oxidative stress is another mechanism for toxicity of
pesticides resulting in cell death (necrosis and apoptosis)
and changes in metabolic and vital functions of the cells
that leads to cancer types (Abdollahi et al., 2004). Many
studies reviewed by the Ontario College show positive
associations between solid tumors and pesticide
exposure, including kidney cancer. Children are
constantly exposed to low levels of pesticides in their
food and environment, an elevated risk of kidney cancer
was associated with paternal pesticide exposure through
agriculture. It has also been reported that the chronic
exposure to pesticides leads to kidney failure (Abdollahi
et al., 2004).
Pesticide metabolism
Presently, there are more of 900 pesticides and more of
600 active pesticide ingredients on the market (Hall et al.,
2001). Millions of ton of pesticides are applied annually;
however, less than 5% of these products are estimated to
reach the target organism, with the remainder being
deposited on the soil and nontarget organisms, as well as
moving into the atmosphere and water (Pimental and
Levitan, 1986). The metabolic fate of pesticides is
dependent on abiotic factors (temperature, moisture, soil
pH, etc.), microbial community or plant species (or both),
pesticide characteristics (hydrophilicity, pKa/b and etc.),
and biological and chemical reactions (Figure 3). Abiotic
degradation is due to chemical and physical
transformations of the pesticide by processes such as
photolysis, hydrolysis, oxidation, reduction, and
rearrangements. Further, pesticides may be biologically
unavailable because of compartmentalization, which
occurs as result of pesticide adsorption to soil and soil
colloids without altering the chemical structure of the
original molecule. However, enzymatic transformation,
which is mainly the result of biotic processes mediated by
plants and microorganisms, is by far the major route of
detoxification. Metabolism of pesticides may involve a
three-phase process (Shimabukuro, 1985; Hatzios,
1991). In phase I metabolism, the initial properties of a
parent compound are transformed through oxidation,
reduction, or hydrolysis to produce a more water-soluble
and less toxic product than the parent. The second phase
involves conjugation of a pesticide or pesticide metabolite
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Agric. Sci. Res. J.
Figure 3. Schematic metabolism pesticides in body(Barr and Needham, 2002).
to a sugar, amino acid, or glutathione, which increases
further, the water solubility and reduces toxicity compared
with the parent pesticide. Phase II metabolites have little
or no phytotoxicity and may be stored in cellular
organelles. The third phase involves conversion of Phase
II metabolites into secondary conjugates, which are also
nontoxic (Hatzios, 1991). In leafy spurge (Euphorbia
esula L.), examples of Phase III metabolism are the
conjugation of the N-glycoside metabolite of picloram with
malonate and the formation of a gentibioside from the
picloram glucose ester metabolite (Frear et al., 1989).
McKellar et al. (1976) fed chlorpyrifos to dairy cattle for 2
weeks. The parent compound and two (oxidized and
hydroxylated) metabolites were found at low levels in milk
and cream (fat) and the concentrations of all three
compounds decreased rapidly after cessation of
administration. Johnson et al. (1974) obtained similar
results. Hsu et al. (1995) recovered a maximum of 0.14%
of intake via the eggs and depletion of residues from the
body was rapid. Researcher had shown that pesticides
have series deleterious effects on the rumen fluid (Cook,
1969). Cook (1957) was perhaps the first to suggest that
rumen liquor played an active role in hydrolyzing OPs,
particularly parathion. Additional evidence by Cook
(1957) indicated that metabolism of parathion by rumen
microorganisms accounted for its apparent lack of toxicity
to cattle. Certain OP pesticides were shown by Williams
et al. (1963) to stimulate gas production In vitro by rumen
holotrich protozoa, whereas these compounds had no
appreciable effect when rumen bacteria served as the
inoculums source. Kutches et al. (1970) reported
toxaphene were ineffectual in causing a depression of In
vitro dry matter disappearance at the 100, 250, and 500
µcg per ml of culture environment(consist of rumen fluid).
Mechanism of action
OP pesticides avidly bind to acetyl cholinesterase (AChE)
molecules and share a similar chemical structure (Figure
4). This leads to accumulation of acetylcholine and
subsequent over-activation of cholinergic receptors at the
neuromuscular junctions and in the autonomic and
central nervous systems (Paudyal, 2008). The rate and
degree of AChE inhibition differs according to the
structure of the OP compounds and the nature of their
metabolite. After the initial formation of AChE-OP
complex, two reactions are happened: at the first
reaction, Spontaneous reactivation of the enzyme may
occur at a slow pace, much slower than the enzyme
Kazemi et al.
517
Figure 4. Schematic inhibitory of AChE by organophosphate pesticides(Paudyal, 2008)
inhibition and requiring hours to days to occur. The rate
of this regenerative process solely depends on the type
of OP compound: spontaneous reactivation half life that it
last 0.7 h for dimethyl and 31 h for diethyl compounds. In
general, AChE-dimethyl OP complex spontaneously
reactivate in less than one day whereas AChE-diethyl OP
complex may take several days and reinhibition of the
newly activated enzyme can occur significantly in such
situation. The spontaneous reactivation can be hastened
by adding nucleophilic reagents like oximes, liberating
more active enzymes. These agents thereby act as an
antidote in OP poisoning (Eddleston et al, 2002). At the
second step, with time, the AChE enzyme-OP complex
loses one alkyl group making it no longer responsive to
reactivating agents. This progressive time dependent
process known as ageing. The rate of ageing depends on
various factors like pH, temperature, and type of OP
compound; dimethyls OPs have ageing half life of 3.7 h
whereas it is 33 h for diethyl OP (Worek et al., 1997;
Worek et al., 1999). The slower the spontaneous
reactivation, the greater the quantity of inactive AChE
available for ageing. Oximes, by catalyzing the
regeneration of active AChE from enzyme-OP complex,
reduce the quantity of inactive AChE available for ageing.
Since ageing occurs more rapidly with dimethyl OPs,
oximes are hypothetically useful before 12 h in such
poisoning. However, in diethyl OP intoxication they may
be useful for many days (Worek et al., 1997; Worek et al.,
1999).
Alternatives
pesticides
to
reduce
the
organophosphate
Several methods either independent or in conjunction
have been used for the removal of these pesticides
including chemical oxidation with ozone, photo
degradation (Zertal et al., 2005), combined ozone and UV
irradiation (Malato et al., 1999), fenton degradation
(Watts, 1996), biological degradation (Chen et al., 2009),
ozonation (Hua et al., 2006), membrane filtration
(Hofman et al., 1997) and adsorption (Daneshvar et al.,
518
Agric. Sci. Res. J.
2007). Among various cleanup technologies, the
adsorption on activated carbon (AC) is one of the wellestablished and effective techniques (Hind et al., 1999).
AC adsorbents are now widely employed for product
purification and wastewater treatment, because of their
exceptionally large surface areas, well-developed internal
pore structure as well as their surface reactivity attributed
to the existence of a wide spectrum of oxygen containing
surface groups (Yin et al., 2007; Adhoum and Monser,
2002). Being mechanically robust and highly
hydrophobic, molecules with large hydrophobic groups
can be strongly adsorbed onto carbon surface. The main
drawback of adsorption methods is their environmentally
incomplete character because they only transfer but not
degrade pollutants. In this case, the used adsorbent
becomes a hazardous material demanding further
treatment subsequently. Nevertheless, some interesting
options based on the application of advanced oxidation
methods, are under development and may constitute in
the near future an efficient solution to degrade the
adsorbed contaminants and regenerate the activated
carbon on site and in situ (Okawa et al., 2007). Several
works (Moreno Castilla, 2004; Boehm, 1994) emphasized
the key role of surface chemistry in adsorption of organic
solutes from aqueous phase and concluded that
adsorptive properties of AC are mainly determined by its
chemical composition. It is well known that adsorption
behavior is influenced by surface oxygen complex
content, which determines the charge of the surface, its
hydrophobicity and the electronic density of the graphene
layers. To increase the concentration of surface oxygen
groups a wide range of oxidizing or/and acid agents such
as HNO3, H2O2, HClO4, (NH4)2S2O8, O3 have been
successfully applied (Santiago et al., 2005; Canizares et
al., 2006). The introduction of acid surface groups was
always accompanied by an important destruction of the
basic sites excepting for (NH4)2SO4 and H2O2. Possibly,
H2O2 was capable to introduce some recognized basic
groups as quinones, chromenes or pyrones due to its
rather soft oxidation strength. In contrast, surface oxides
can be reduced by treatment with alkaline or reductants
solutions, e.g., NH3, NaOH, NaHSO3, etc. (Przepiorski,
2006; Alcanaz-Monge and Illan-Gomez, 2008). In
general, several research groups have recorded
significant decrease in the adsorption of organic
compounds such as dodecanoic acid, methyl isoborneol
and phenol upon increasing the oxygen content of the
carbon
adsorbent
(mainly
by
surface
oxidation)(Pendleton et al., 1997; Terzyk, 2003). It is well
known that ozone was effectively applied to drinking
water treatment and wastewater treatment for its powerful
oxidization potential (Wu et al., 2005). In animal
especially ruminant, there are three general types of
antidotes for poisons. First, a mechanical antidote is one
that binds a poison in the gut and prevents absorption of
the poison. Second, a chemical antidote stimulates the
body such that the poison is metabolized and detoxified
at a faster rate. Third, a physiologic antidote counteracts
the toxic effects of the poison. An example of a
mechanical antidote is the so-called "Universal Antidote"
which consists of 2 parts charcoal, 1 part magnesium
oxide and 1 part tannic acid. Charcoal is an adsorbent,
tannic acid is a precipitant for alkaloids, and magnesium
oxide is both an adsorbent and an antacid. The
barbiturate phenobarbital is an example of a chemical
antidote. Phenobarbital very markedly increases the rate
of metabolism of drugs and poisons such as
zoxazolamine and Warfarin by the liver (Burns, 1969).
Examples of physiologic antidotes are atropine and
phenobarbital. Atropine is an antidote for poisons such as
parathion that interfere with AChE. Phenobarbital can act
both as a chemical antidote and a physiologic antidote.
Physiologic antidotes are routinely used in veterinary and
human medicine but mechanical and chemical antidotes
are not used as extensively. A comprehensive review of
the literature on the use of activated carbon as an
emergency antidote for treatment of ingested poisons
was presented in 1963 by Holt and Peter. In this study,
authors demonstrated that activated carbon is an
excellent adsorbent for many poisons.
Toxicological procedure
The degree of absorption depends on the contact time
with the skin, the lipophilicity of the agent involved and
the presence of solvents, for example xylene, and
emulsifiers in the formulation which can facilitate
absorption. For powders, the finer the powder the more
rapid and complete is skin absorption. Other important
factors include volatility of the pesticide (e.g. dichlorvos is
much more volatile than malathion), the permeability of
clothing, the extent of coverage of the body surface and
personal hygiene. The rate of absorption also varies with
the skin region affected. For example, parathion is
absorbed more readily through scrotal skin, maxillae and
skin of the head and neck than it is through the skin of
the hands and arms. It is probable that traumatized skin
or the presence of dermatitis allows greater absorption of
OP compounds. In one study, the mean amount of liquid
parathion absorbed dermally was only 1.23% of the
measured potential dermal exposure (Durham et al.,
1972). Following absorption, OP compounds accumulate
rapidly in fat, liver, kidneys and salivary glands. The
phosphorothioates (P=S), for example diazonin,
parathion, and bromophos, are more lipophilic than
phosphates (P=O), for example dichlorvos, and are
therefore stored extensively in fat which may account for
the prolonged intoxication and clinical relapse after
apparent recovery which has been observed in poisoning
from these OP insecticides. OP compounds generally are
lipophilic and therefore cross the blood / brain barrier in
most cases (Vale, 1998). Phosphates (P=O) are
biologically active as AChE inhibitors, whereas
Kazemi et al.
phosphorothioates (P=S) need bioactivation to their
phosphate analogues (oxon) to become biologically
active. As a consequence, the features of intoxication
after exposure to phosphorothioates (P=S) are delayed
unless aerial oxidation has occurred already to generate
traces of oxon. OP compounds other than phosphates
(P=O) are metabolically activated to their corresponding
oxon by oxidation desulfuration mediated by cytochrome
P450 isoforms, by flavincontaining mono-oxygenase
enzymes, by N-oxidation and by S-oxidation. The oxons
which inhibit AChE can be deactivated by hydrolases,
such as the carboxylases and by A-esterases, for
example paraoxonase (Vale, 1998). Elimination of
metabolites occurs mostly in urine with lesser amounts in
feces and expired air. Some OPs, for example dichlorvos
which is not stored in fat to any great extent, may be
eliminated in hours whereas the inhibitory oxon of
chlorpyrifos or dementon-S-methyl may persist for days
because of their extensive storage in fat (Vale, 1998).
Sign and symptom of poising with OP pesticides
Symptoms of acute OP poisoning develop during or after
exposure, within minutes to hours, depending on the
method of application. Exposure due to inhalation results
in the fastest appearance of toxic symptoms, followed by
the gastrointestinal route and finally the dermal route.
Some of the most commonly reported early symptoms
include, in possible order of occurrence: headache,
nausea, dizziness, hyper secretion (swearing and
salivation), muscle twitching, weakness, and tremors, in
coordination, paralysis and starvation (Rickett et al.,
1986; Petras, 1981).
Diagnostic Strategies
The diagnosis of poisoning by AChE inhibitors is
confirmed by demonstrating reduced levels of AChE
activity in
plasma
(serum)
and
erythrocytes.
Unfortunately, many hospital laboratories do not have the
in-house capability to determine cholinesterase levels.
Most patients with a significant, acute exposure
demonstrate sharply reduced to absent plasma
cholinesterase levels within a few hours of exposure. Any
patient who has a full-blown cholinergic syndrome should
be treated empirically without waiting for laboratory
confirmation of decreased cholinesterase activity. Known
or suspected exposure to AChE inhibitors should be
confirmed by ordering both plasma and erythrocyte
(RBC) AChE levels. In acute exposures, the plasma
AChE levels fall first, with decreases in RBC AChE levels
lagging behind. Patients with chronic exposures may
demonstrate only reduced RBC AChE activity, and their
normal plasma AChE levels may impart a false sense of
security. The true reflection of depressed AChE activity is
519
found in the RBC activity, and even a very mild acute
exposure may result in severe clinical poisoning in these
individuals. Red blood cell AChE levels recover at a rate
of 1% per day in untreated patients and take about 6 to
12 weeks to normalize, whereas plasma cholinesterase
levels may recover in 4 to 6 weeks. Other ancillary
studies should be geared toward the evaluation of
pulmonary, cardiovascular, and renal function, and fluid
and electrolyte balance (Tafuri and Roerts, 1987;
Gagandeep and Khurana, 2009; Worek et al., 2005).
Treatment
The initial objective should be the establishment of an
airway and adequate ventilation because the patient with
acute OP poisoning commonly presents with respiratory
distress secondary to excessive oropharyngeal
secretions, bronchospasm, respiratory muscle paralysis
and, rarely, acute respiratory distress syndrome and
pulmonary edema. Airway management in these cases
consists of suctioning the copious oropharyngeal
secretions and vomitus, if present. Endotracheal
intubation and mechanical ventilation often are required.
It is essential to improve tissue oxygenation as much as
possible prior to administration of atropine in order to
minimize the risk of ventricular fibrillation. After
stabilization of the airway, IV atropine sulfate should be
administered. Atropine acts as a physiologic antidote in
anticholinesterase intoxication by competitively blocking
the action of acetylcholine at muscarinic receptors, thus
ameliorating the excessive parasympathetic stimulation
caused by AChE inactivation. Repeated doses of
atropine should be administered until signs of
atropinization
(mydriasis,
tachycardia,
flushing,
xerostomia, anhydrosis, etc) appear (Haddad and
Winchester, 1983; Namba et al., 1971). When cows
become contaminated with organ phosphorus pesticides
such as phosalone it is recommended that the following
steps be taken: firstly check feed, water and insecticide
sprays to determine the source of contamination. Then
discontinue use of contaminating materials. Secondly in
cases of acute pesticide poisoning cows should be
drenched with 2 to 2.5 kg of activated carbon. The drench
can be slurry made of 2 to 3 parts water and 1 part
activated carbon. Thirdly if cows are in convulsions, the
veterinarian may also treat them with intravenous
injections of phenobarbital or some other barbiturate.
When intravenous injections cease, phenobarbital should
be fed at a rate of 10 mg per kg of body weight per day.
This amounts to about 5 g (one tablespoon) daily.
Treatment should be continued for 6 weeks or until milk
tests show that the pesticides are below tolerance levels.
Phenobarbital can be added to the grain ration. Milk
should not be marketed until 7 d after phenobarbital
feeding has stopped. Fourthly activated carbon should be
fed at the rate of 1 kg per head daily for a 550 kg cow.
520
Agric. Sci. Res. J.
The carbon can be fed successfully by mixing either with
silage or grain. Also some dairy cooperatives add
activated carbon to the concentrate at a level of 10% and
then pellet the mixture. When both activated carbon and
phenobarbital are fed, some phenobarbital may be
trapped in the gut by activated carbon. It is probably best
to feed phenobarbital about 2 h before feeding activated
carbon. The level of Phenobarbital administered no doubt
could be reduced if the drug was given by intramuscular
injections (Cook, 1969).
Conclusion
OP pesticides have become increasingly popular for
agricultural, industrial, livestock husbandry and home use
and represent a significant potential health risk for human
and livestock. At now and especially in the future, the
ministry of agriculture of developing countries especially
Iran, should concentrate on the optimization and
monitoring of usage of OP pesticides and furthermore
encouraging the farmers to use natural pesticides rather
than chemical pesticides. Also the farmers must be
aware to methods of dealing with OP pesticides in Iran
for preventing from undesirables problems, for example it
is possible to accelerate removal of OP pesticides from
livestock feed and environment according to various
methods. No doubt, more effective and less harmful
pesticides in agriculture and animal husbandry sections
or organic agriculture must be developed in Iran. There is
considerable scope for promoting organic farming,
although no certification system exists yet for organic
products for local or export markets in Iran. On the basis
of the author’s studies, 70-80% of current pesticide use in
most crops is unnecessary, demonstrating the huge
potential for pesticide reduction in Iran. With further
support from the government, society, institutions and
public education, farmers can make huge advances to
protect human health and the environment and to
improve their income.
Acknowledgement
The authors would like to thank Dr. Abdolmansour
Tahmasbi and prof. Reza Valizadeh for their helps during
the preparation of this manuscript.
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