Organophosphate poisoning

Organophosphate poisoning results from exposure to organophosphates (OPs), which cause the inhibition of acetylcholinesterase (AChE), leading to the accumulation of acetylcholine (ACh) in the body. Organophosphate poisoning most commonly results from exposure to insecticides or nerve agents. OPs are one of the most common causes of poisoning worldwide, and are frequently intentionally used in suicides in agrarian areas. There are around 1 million OP poisonings per year with several hundred thousand resulting in fatalities annually.

Organophosphates inhibit AChE, causing OP poisoning by phosphorylating the serine hydroxyl residue on AChE, which inactivates AChE. AChE is critical for nerve function, so the irreversible blockage of this enzyme, which causes acetylcholine accumulation, results in muscle overstimulation. This causes disturbances across the cholinergic synapses and can only be reactivated very slowly, if at all. Paraoxonase (PON1) is a key enzyme involved in OP pesticides and has been found to be critical in determining an organism’s sensitivity to OP exposure.

Examples

 * Insecticides including malathion, parathion, diazinon, fenthion, dichlorvos, chlorpyrifos, ethion
 * Nerve Gases including soman, sarin, tabun, VX
 * Ophthalmic agents: echothiophate, isoflurophate
 * Antihelmintics such as trichlorfon
 * Herbicides including tribufos [DEF], merphos are tricresyl phosphate–containing industrial chemicals.

Exposure to any one of the above listed organophosphates occurs on a daily basis through inhalation, absorption, and ingestion, most commonly of food that has been treated with an organophosphate herbicide or insecticide. Exposure to these chemicals can occur at public buildings, schools, residential areas, and in agricultural areas. The chemicals chlorpyrifos and malathion have been linked to reproductive effects, neurotoxicity, kidney/liver damage, and birth defects. Dichlorvos has also been linked to reproductive effects, neurotoxicity, and kidney/liver damage, as well as being a possible carcinogen.

Diagnosis
A number of measurements exist to assess exposure and early biological effects for organophosphate poisoning. Measurements of OP metabolites in both the blood and urine can be used to determine if a person has been exposed to organophosphates. Specifically in the blood, metabolites of cholinesterases, such as butyrylcholinesterase (BuChE) activity in plasma, neuropathy target esterase (NTE) in lymphocytes, and of acetylcholinesterase (AChE) activity in red blood cells. Due to both AChE and BuChE being the main targets of organophosphates, their measurement is widely used as an indication of an exposure to an OP. The main restriction on this type of diagnosis is that depending on the OP the degree to which either AChE or BuChE are inhibited differs; therefore, measure of metabolites in blood and urine do not specify for a certain OP. However, for fast initial screening, determining AChE and BuChE activity in the blood are the most widely used procedures for confirming a diagnosis of OP poisoning.

Other procedures used in the diagnosis of OP exposure are the identification and qualitative analysis of nerve agents in the plasma after exposure. This method is highly sophisticated, but this analysis of unbound nerve agents is the most specific method for diagnosis. However, the drawback to this method of diagnosis is the short life-time of nerve agents in the body, which will limit the time frame in which exposure can be detected. Other methods of diagnosis include, analysis of protein adducts and the quantitative analysis of decomposition products in by the plasma and urine, but again due to the rapid elimination of the nerve agents in the body. One method that works to expand the time interval for diagnosis involves incubating blood or plasma levels with high fluoride concentration. The drawback to this method is that it can be used for retrospective detection of OP exposure, but cannot be used as a method for verification.

Symptoms
The health effects associated with organophosphate poisoning are a result of excess acetylcholine (ACh) present at different nerves and receptors in the body because acetyocholinesterase is blocked. Accumulation of ACh at motor nerves causes overstimulation of nicotinic expression at the neuromuscular junction. When this occurs symptoms such as muscle weakness, fatigue, muscle cramps, fasciculation, and paralysis can be seen. When there is an accumulation of ACh at autonomic ganglia this causes overstimulation of nicotinic expression in the sympathetic system. Symptoms associated with this are tachycardia, hypertension, and hypoglycemia. Overstimulation of nicotinic acetylcholine receptors in the central nervous system, due to accumulation of ACh, results in anxiety, headache, convulsions, ataxia, depression of respiration and circulation, tremor, general weakness, and potentially coma. When there is expression of muscarinic overstimulation due to excess acetylcholine at muscarinic acetylcholine receptors symptoms of visual disturbances, tightness in chest, wheezing due to bronchoconstriction, increased bronchial secretions, increased salivation, lacrimation, sweating, peristalsis, and urination can occur.

The effects of organophosphate poisoning are recalled using the mnemonic SLUDGEM (Salivation, Lacrimation, Urination, Defecation, Gastrointestinal motility, Emesis, miosis) An additional mnemonic is MUDDLES: miosis, urination, diarrhea, diaphoresis, lacrimation, excitation, and salivation.

The onset and severity of symptoms, whether acute or chronic, depends upon the specific chemical, the route of exposure, the dose, and the individuals ability to degrade the compound, which the PON1 enzyme level will affect.

Reproductive effects
Certain reproductive effects in fertility, growth, and development for males and females have been linked specifically to OP pesticide exposure. Most of the research on reproductive effects has been conducted on farmers working with pesticides and insecticdes in rural areas. For those males exposed to OP pesticides, poor semen and sperm quality have been seen, including reduced seminal volume and percentage motility, as well as a decrease in sperm count per ejacuate. In females menstrual cycle disturbances, longer pregnancies, spontaneous abortions, stillbirths, and some developmental effects in offspring have been linked to OP pesticide exposure. Prenatal exposure has been linked to impaired fetal growth and development. The effects of OP exposure on infants and children are at this time currently being researched to come to a conclusive finding.

Neurotoxic effects
Neurotoxic effects have also been linked to poisoning with OP pesticides and cause four neurotoxic effects in humans: cholinergic syndrome, intermediate syndrome, organophosphate-induced delayed polyneuropathy (OPIDP), and chronic organophosphate-induced neuropsychiatric disorder (COPIND). These syndromes result after acute and chronic exposure to OP pesticides.

Cholinergic syndrome occurs in acute poisonings with OP pesticides and is directly related to levels of AChE activity. Symptoms include miosis, sweating, lacrimation, gastrointestinal symtoms, respiratory difficultires, dyspnea, bradycardia, cyanosis, vomiting, diarrhea, as well as other symptoms. Along with these central effects can be seen and finally seizures, convulsions, coma, respiratory failure. If the person survives the first day of poisoning personality changes can occur, aggressive events, psychotic episodes, disturbances and deficits in memory and attention, as well as other delayed effects. When death occurs, it is most commonly due to respiratory failure from the combination of central and peripheral effects, paralysis of respiratory muscles and depression of the brain respiratory center. For people afflicted with cholinergic syndrome, atropine sulfate combined with an oxime is used to combat the effects of the acute OP poisoning. Diazepam is sometimes also administered in combination with the atropine and oximes.

The intermediate syndrome (IMS) appears in the interval between the end of the cholinergic crisis and the onset of OPIDP. Symptoms associated with IMS manifest within 24–96 hours after exposure. The exact etiology, incidence, and risk factors associated with IMS are not clearly understood, but IMS is recognized as a disorder of neuromuscular junctions. IMS occurs when a person has a prolonged and severe inhibition of AChE and has been linked to specific OP pesticides such as methylparathion, dichlorvos, and parathion. Patients present with weakness of neck flexion and proximal limb muscle weakness, as well as respiratory deficiency. Recovery from IMS can occur within 5–18 days after the onset of the symptoms, as long as respiratory failure is promptly recognized and treated. Respiratory support, as well as atropine and oximes should be administered as soon as identified, since IMS is considered a life threatening syndrome.

OPIDP occurs in a small percentage of cases, roughly two weeks after exposure, where temporary paralysis occurs. This loss of function and ataxia of peripheral nerves and spinal cord is the phenomenon of OPIDP. Once the symptoms begin with shooting pains in both legs, the symptoms continue to worse for 3–6 months. In the most severe cases quadriplegia has been observed. Treatment only affects sensory nerves, not motor neurons which may permanently lose function. The aging and phosphorylation of more than 70% of functional NTE in peripheral nerves is one of the processes involved in OPIDP. Standard treatments for OP poisoning are ineffective for OPIDP.

COPIND occurs without cholinergic symptoms and is not dependent on AChE inhibition. COPIND appears with a delay and is long lasting. Symptoms associated with COPIND include cognitive deficit, mood change, autonomic dysfunction, peripheral neuropathy, and extrapyramidal symptoms. The underlying mechanisms of COPIND have not been determined, but it is hypothesized that withdrawal of OP pesticides after chronic exposure or acute exposure could be a factor.

Effects on developing animals
Evidence of exposure to OP pesticides during gestation and early postnatal period have been linked to neurodevelopmental effects in animals, specifically rats. Animals exposed in utero to chlorpyrifos exhibited decreased balance, poorer cliff avoidance, decreased locomotion, delays in maize performance, and increased gait abnormalities. Early gestation is believed to be a critical time period for the neurodevelopmental affects of pesticides. OP’s affect the cholinergic system of fetuses, so exposure to chlorpyrifos during critical periods of brain development potentially could cause cellular, synaptic, and neurobehavioral abnormalities in animals. In rats exposed to methyl parathion, studies found reduced AChE activity in all brain regions and subtle alterations in behaviors such as locomotor activity and impaired cage emergence. Organophosphates as whole have been linked to decreases in the length of limbs, head circumference, and slower rates of postnatal weight gain in mice.

PON1 Influence
Paraoxonase (PON1) is a key enzyme in the metabolism of organophosphates. PON1 can inactivate some OPs through hydrolysis. PON1 hydrolyzes the active metabolites in several OP insecticides such as chlorpyrifos oxon, and diazoxon, as well as, nerve agents such as soman, sarin, and VX. PON1 hydrolyzes the metabolites, not the parent compounds of insectides. The presence of PON1 polymorphisms causes there to be different enzyme levels and catalytic efficiency of this esterase, which in turn suggests that different individuals may be more susceptible to the toxic effect of OP exposure. The level of PON1 plasma hydrolytic activity provides more protection against OP pesticides. Rats injected with purified PON1 from rabbit serum were more resistant to acute cholinergic activity than the control rats. PON1 knockouts in mice are found to be more sensitive to the toxicity of pesticides, like chlorpyrifos. Animal experiments indicate that while PON1 plays a significant role in regulating the toxicity of OPs its degree of protection given depends on the compound (i.e. Chlorpyrifos oxon or diazoxon). The catalytic efficiency with which PON1 can degrade toxic OPs determines the degree of protection that PON1 can provide for organism. The higher the concentration of PON1 the better the protection provided. PON1 activity is much lower in neonates, so neonates are more sensitive to OP exposure. In 2006, reports up to a 13-fold variation was seen in PON1 levels in adults, as well as, specifically regarding sensitivity to diazoxon, a variation up to 26 and 14-fold was reported in a group of newborns and Latino mothers. This wide range in variability of enzyme levels determining a humans sensitivity to various OPs is being researched further.

Treatment and Prevention
Current antidotes for OP poisoning consist of a pretreatment with carbamates to protect AChE from inhibition by OP compounds and post-exposure treatments with anti-cholinergic drugs. Anti-cholinergic drugs work to counteract the effects of excess acetylcholine and reactivate AChE. Atropine can be used as an antidote in conjunction with pralidoxime or other pyridinium oximes (such as trimedoxime or obidoxime), though the use of "-oximes" has been found to be of no benefit, or possibly harmful, in at least two meta-analyses. Atropine is a muscarinic antagonist, and thus blocks the action of acetylcholine peripherally. These antidotes are effective at preventing lethality from OP poisoning, but current treatment lack the ability to prevent post-exposure incapacitation, performance deficits, or permanent brain damage.

Enzyme bioscavengers are being developed as a pretreatment to sequester highly toxic OPs before they can reach their physiological targets and prevent the toxic effects from occurring. Significant advances with cholinesterases (ChEs), specifically human serum BChE (HuBChE) have been made. HuBChe can offer a broad range of protection for nerve agents including soman, sarin, tabun, and VX. HuBChE also possess a very long retention time in the human circulation system and because it is from a human source it will not produce any antagonistic immunological responses. HuBChE is currently being assessed for inclusion into the protective regimen against OP nerve agent poisoning. Currently there is potential for PON1 to be used to treat sarin exposure, but recombinant PON1 variants would need to first be generated to increase its catalytic efficiency.

One other agent that is being researched is the Class III anti-arrhythmic agents. Hyperkalemia of the tissue is one of the symptoms associated with OP poisoning. While the cellular processes leading to cardiac toxicity are not well understood, the potassium current channels are believed to be involved. Class II anti-arrhythmic agents block the potassium membrane currents in cardiac cells, which makes them a candidate for become a therapeutic of OP poisoning.

Five days of an organic food diet has been shown to dramatically reduce the organophosphorus pesticides in the urine of children.

Ginger Jake
A striking example of OPIDN occurred during the 1930s Prohibition Era when thousands of men in the American South and Midwest developed arm and leg weakness and pain after drinking a "medicinal" alcohol substitute. The drink, called "Ginger Jake," contained an adulterated Jamaican ginger extract containing tri-ortho-cresyl phosphate (TOCP) which resulted in partially reversible neurologic damage. The damage resulted in the limping "Jake Leg" or "Jake Walk" which were terms frequently used in the blues music of the period. Europe and Morocco both experienced outbreaks of TOCP poisoning from contaminated abortifacients and cooking oil, respectively.

Gulf War Syndrome
Research has linked the neurological abnormalities found in Persian Gulf War veterans, who suffer from Gulf War syndrome, to exposure to wartime combinations of organophosphate chemical nerve agents. Before, it was believed that veterans were suffering from a psychologically based disorder or depression, most likely post-traumatic stress disorder (PTSD). Many veterans were given pyridostigmine bromide (PB) pills to protect against nerve gas agents such as sarin and soman. During the war veterans were exposed to combinations of organophosphate nerve agents, which produced symptoms associated with chronic organophosphate-induced delayed polyneuropathy (OPIDP)syndrome. Similar symptoms found in the veterans were the same symptoms reported for individuals in occupational settings who were acutely poisoned by organohosphates, such as chlorpyrifos. Studies found veterans experienced deficits in intellectual and academic abilities, simple motor skills, memory impairment, and impaired emotional function. These symptoms indicate brain damage, not a psychologically based disorder.

Governmental review
The U.S. Food Quality Protection Act (FQPA), passed in 1996, designated the United States Environmental Protection Agency (EPA) to conduct a 10 year review process of the health and environmental effects of all pesticides, beginning with the Organophosphates. The process has taken longer than expected, but was recently concluded and eliminated or modified thousands of uses.

Many non-governmental and research groups, as well as the EPA's Office of Inspector General, have published concerns that the review did not take into account possible neurotoxic effects on developing fetuses and children, an area of developing research. OIG report. A group of leading EPA scientists sent a letter to the chief administrator, Stephen Johnson, decrying the lack of developmental neurotoxicity data in the review process. EPA Letter EHP article  New studies have shown toxicity to developing organisms during certain "critical periods" at doses much lower than those previously suspected to cause harm.

Organophosphate Pesticides Reports

 * Epidemiological study of the relationships between exposure to organophosphate pesticides and indices of chronic peripheral neuropathy, and neuropsychological abnormalities in sheep farmers and dippers.Phase 1. Development and validation of an organophosphate uptake model for sheep dippers by C Sewell and others. Institute of Occupational Medicine Research Report TM/99/02a
 * Epidemiological study of the relationships between exposure to organophosphate pesticides and indices of chronic peripheral neuropathy, and neuropsychological abnormalities in sheep farmers and dippers. Phase 2. Cross-sectional exposure response study of sheep-dippers by A Pilkington and others. Institute of Occupational Medicine Research Report TM/99/02b
 * Epidemiological study of the relationships between exposure to organophosphate pesticides and indices of chronic peripheral neuropathy, and neuropsychological abnormalities in sheep farmers and dippers. Phase 3. Clinical neurological, neurophysiological and neurophysiological study by A Pilkington and others. Institute of Occupational Medicine Research Report TM/99/02c

Future Research
Currently, more research is being done on animals and human fetuses to determine the effects of OP’s during critical periods of development. Due to children's decreased size, faster rate of respiration, and continuing organ development, this area is important to research. Research to determine the variability on the PON1 enzyme in humans is also currently being researched.

The use of the organophosphates in aviation lubricating oils and hydraulic fluids and its impact on health and flight safety is currently being researched. Aerotoxic Syndrome is a medical condition allegedly caused by exposure to contaminated bleed air.

Purdey (1998) suggested that organophosphates, in particular Phosmet, induced the transmissible spongiform encephalopathy epidemic of BSE. A European Union food safety Scientific Steering Committee examined the evidence and did not find a link.