Organophosphate Toxicity

Updated: May 04, 2016
  • Author: Kenneth D Katz, MD, FAAEM, ABMT; Chief Editor: Asim Tarabar, MD  more...
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Overview

Background

Organophosphate (OP) compounds are a diverse group of chemicals used in both domestic and industrial settings. Examples of organophosphates include insecticides (malathion, parathion, diazinon, fenthion, dichlorvos, chlorpyrifos, ethion), nerve gases (soman, sarin, tabun, VX), ophthalmic agents (echothiophate, isoflurophate), and antihelmintics (trichlorfon). Herbicides (tribufos [DEF], merphos) are tricresyl phosphate–containing industrial chemicals.

Organophosphate compounds were first synthesized in the early 1800s when Lassaigne reacted alcohol with phosphoric acid. Shortly thereafter in 1854, Philip de Clermount described the synthesis of tetraethyl pyrophosphate at a meeting of the French Academy of Sciences. Eighty years later, Lange, in Berlin, and, Schrader, a chemist at Bayer AG, Germany, investigated the use of organophosphates as insecticides. However, the German military prevented the use of organophosphates as insecticides and instead developed an arsenal of chemical warfare agents (ie, tabun, sarin, soman). A fourth agent, VX, was synthesized in England a decade later. During World War II, in 1941, organophosphates were reintroduced worldwide for pesticide use, as originally intended.

Massive organophosphate intoxication from suicidal and accidental events, such as the Jamaican ginger palsy incident in 1930, led to the discovery of the mechanism of action of organophosphates. In 1995, a religious sect, Aum Shinrikyo, used sarin to poison people on a Tokyo subway. Mass poisonings still occur today; in 2005, 15 victims were poisoned after accidentally ingesting ethion-contaminated food in a social ceremony in Magrawa, India.

Nerve agents have also been used in battle, notably in Iraq in the 1980s. Sarin, delivered by rockets, was used in the chemical warfare attack in Damascus, Syria in 2013. [1] Additionally, chemical weapons still pose a very real concern in this age of terrorist activity.

In farm workers, chronic occupational exposure to organophosphate insecticides has been linked to neuropsychological effects in some studies. These have included difficulties in executive functions, psychomotor speed, verbal, memory, attention, processing speed, visual-spatial functioning, and coordination. [2]

Exposure to organophosphates is also possible via intentional or unintentional contamination of food sources. Although no clinical effects of chronic, low-level organophosphate exposure from a food source have been shown, advancements in risk assessment and preparedness are ongoing. [3, 4]

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Pathophysiology

The primary mechanism of action of organophosphate pesticides is inhibition of carboxyl ester hydrolases, particularly acetylcholinesterase (AChE). AChE is an enzyme that degrades the neurotransmitter acetylcholine (ACh) into choline and acetic acid. ACh is found in the central and peripheral nervous system, neuromuscular junctions, and red blood cells (RBCs).

Organophosphates inactivate AChE by phosphorylating the serine hydroxyl group located at the active site of AChE. The phosphorylation occurs by loss of an organophosphate leaving group and establishment of a covalent bond with AChE.

Once AChE has been inactivated, ACh accumulates throughout the nervous system, resulting in overstimulation of muscarinic and nicotinic receptors. Clinical effects are manifested via activation of the autonomic and central nervous systems and at nicotinic receptors on skeletal muscle.

Once an organophosphate binds to AChE, the enzyme can undergo one of the following:

  • Endogenous hydrolysis of the phosphorylated enzyme by esterases or paraoxonases
  • Reactivation by a strong nucleophile such as pralidoxime (2-PAM)
  • Irreversible binding and permanent enzyme inactivation (aging)

Organophosphates can be absorbed cutaneously, ingested, inhaled, or injected. Although most patients rapidly become symptomatic, the onset and severity of symptoms depend on the specific compound, amount, route of exposure, and rate of metabolic degradation. [5]

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Epidemiology

Frequency

United States

In 2014, the American Association of Poison Control Centers reported 2180 single exposures to organophosphate insecticides, with 20 major outcomes and three deaths. In addition, 5138 single exposures to organophosphate insecticides in combination with carbamate or non-carbarbamate insecticides were reported, with three major outcomes and no deaths. [6]

International

Pesticide poisonings are among the most common modes of poisoning fatalities. In countries such as India and Nicaragua, organophosphates are easily accessible and, therefore, a source of both intentional and unintentional poisonings. The incidence of international organophosphate-related human exposures appears to be underestimated. [7]

Mortality/Morbidity

Worldwide mortality studies report mortality rates from 3-25%. [8] The compounds most frequently involved include malathion, dichlorvos, trichlorfon, and fenitrothion/malathion.

Mortality rates depend on the type of compound used, amount ingested, general health of the patient, delay in discovery and transport, insufficient respiratory management, delay in intubation, and failure in weaning off ventilatory support.

Complications include severe bronchorrhea, seizures, weakness, and neuropathy. Respiratory failure is the most common cause of death.

Age

Organophosphates (OPs) may affect children or other at-risk populations differently. The increased susceptibility has not been elucidated but may involve delayed or persistent effects. More work in this area is in progress and should help identify the true risk potential. [9]

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