1. Definition of a Pesticide
Any substance intended for preventing, destroying, repelling, or mitigating any pest
Insecticides
Herbicides
Fungicides

2. Pesticides are Not New Fumigants Insecticides Rodenticides
Sulfur (1000 BC)
Bordeaux mixture (CuSO4, lime (Ca(OH)2, H2O)- 1882)
Insecticides
Arsenic (16th Century)
Tobacco leaves (Nicotiana tabacum )
Rotenone (Derris eliptica – 1800s)
Pyrethrum (Chrysanthemum cinerriafolum- 1800s)
Paris Green (copper arsenite 1800s)
Rodenticides
Nux vomica (Strychnos nux-vomica)

3. There is No Such Thing as a Completely SAFE Pesticide
But Pesticides can be USED safely.
All Pesticides have some type of biological activity
The trick is to balance efficacy and safety.

4. Pesticide Regulations
Wiley or Sherman Act (1906)
Federal Food, Drug, and Cosmetic Act (1938)
Pesticide tolerances set in 1954 and 1958
1958: Delaney Act
No additive is deemed safe if found to cause cancer
Pesticides were one type of additive
Federal Insecticide, Fungicide, and Rodenticide Act
1947 (initially administered by USDA)
1972 (switch to EPA)
Defines registration and labeling requirements
Can cancel registration
Set tolerances
FDA monitors residue levels
Food Quality Protection Act
1996
Special considerations for children
Higher susceptibility
Consider child consumption patterns

5. Development of a New Pesticide is a Lengthy and Costly Process
FIFRA
Product and residue chemistry
Environmental fate
Toxicology
Acute toxicity
Oral, dermal, inhalation, irritation, ocular, delayed neurotox, dermal sensitization
Subchronic
Rat, mouse, dog (dermal, inhalation, neurotox)
Chronic
Rodent, dog, carcinogenicity
Reproductive
Fertility, teratology, in vitro mutagenicity
Biotransformation
Occupational exposure
Spray drift
Environmental impact on non-target species
Efficacy
Costs: $30 to 80M (full development and testing of a new pesticide may take 10 years)

6. The Majority of Insecticides are Neurotoxic

7. Toxicological Syndromes Associated with Insecticides
Acute syndromes
Cholinesterase inhibitors
Arsenic
Pyrethrins and pyrethroids
Chronic toxicosis
Delayed neuropathy (OPIDN)
Residues
Organochlorine insecticides
DDT
Wild-life problem

8. Anticholinesterase Insecticides
Organophosphorus Insecticides
Carbamate Insecticides

9. Acetylcholinesterase Inhibitors
Organophosphates (OP)
Phosphoric acid derivatives
30 in use
First synthesized in 1937 (Bayer)
Tetraethyl pyrophosphate (TEPP)
Chemical warfare agents
Tabun (GA)
Sarin (GB) – Japanese subway attacks
Soman (GD) VX
CMPF (GF)
Parathion (O,O-diethyl O p-nitrophenyl phosphate)
Replaced DDT in 1950s
Number of acute poisoning cases resulted
Chlorpyrifos
Parathion

10. Acetylcholinesterase Inhibitors
Carbamates
Carbamic acid derivatives
~25 in use
Originally developed as fungicides (1930s)
Carbaryl
Physostigmine (alkaloid from Calabar bean)

11. Interference With Neurotransmitter (Acetylcholine) Function
Synthesized in neuron cell body
Release triggered by an action potential
Sudden influx Ca2+ ---> ACh release
Broken down by acetylcholinesterase
Primary neurotransmitter in PNS
Smooth and skeletal muscle
CNS (distributed throughout)
In the developing brain, every neuron expresses cholinesterase activity even if it isn’t cholinergic in adulthood. 4

12. Effector organs: Smooth muscle
Mode of Action
Cholinergic
neuron
Acetylcholine
Effector organs: Smooth muscle
Skeletal muscle CNS

13. Mode of Action Acetylcholinesterase inhibitors Carbamates
Carbamylation of enzyme occurs
Reversible
Organophosphates
Enzyme phosphorylation occurs
“Aging”
Irreversible
Aging occurs over a hr time period

14. Mode of Action C Carbamate Organophosphate Op Choline + Acetylcholine
Treatment:
Atropine
2-PAM
Choline +
Acetylcholine
Acetate
Acetycholinesterase inhibitors C Op
Acetylcholinesterase
Synaptic cleft
Carbamate
Organophosphate
Muscarinic
Nicotinic

15. Clinical Signs
Clinical signs related to excessive stimulation of nicotinic and muscarinic receptors
Muscle tremors
CNS effects
Respiratory paralysis (this is what causes death)
Salivation, Lacrimation, Urination, and Defecation
CNS signs
Bronchospasm
Bronchial secretions
Miosis
Bradycardia

16. Organophosphate Insecticides
Toxicology
Delayed effects (OPIDN)
Ginger Jake paralysis
Tri-o-tolyl phosphate (TOTP)
Leptophos, mipafox, chlorpyrifos, DFP
All organophosphates are required to be tested for their ability to produce OPIDN before they are marketed (Hen test) 5

17. OPIDN Organophosphate Induced Delayed Neuropathy Clinical signs
Ataxia and paralysis
Develop 10 to 14 days after exposure
Neuropathology
Wallerian-type degeneration
Mode of Action
Inhibition of neurotoxic esterase (NTE) is generally predictive 10

18. As Animals Mature, They Become Less Sensitive to Chlorpyrifos Toxicity
(Moser et al, Toxicol. Sci, 1998)

19. Generic OP Pathway Oxon Parent Pesticide Bind to CaEs Inhibit AChE
Hepatic Activation
Bind to
CaEs
Hydroylzed by
A-Esterases
Inhibit
AChE

20. Acetylcholinesterase in the Young Brain is NOT more Sensitive to Organophosphorus Pesticide Inhibition
Mortensen et al., Toxicol. 1998

21. Developmental Profiles of Carboxylesterase and A-Esterase in Rats
(Mortensen et al., J. Biochem. Toxicol. 1996; Moser et al, Toxicol. Sci, 1998)

22. Generic OP Pathway Adult vs Young Rat
Oxon
Hepatic Activation and
probably Deactivation
Parent Pesticide
Bind to
CaEs
Hydroylzed by
A-Esterases
Inhibit
AChE

23. Developmental Profiles of
Arylesterase (U/mL)
Arylestease (U/mL)
Age (months) A B C
Developmental Profiles of
A-Esterase in Humans
Cole et al, 2003

24. Pyrethrin and Pyrethroid Insecticides

25. Pyrethrin and Pyrethroid Insecticides
Pyrethrins
Naturally derived insecticide
Chrysathemum
Natural pyrethrins include pyrethrin I, pyrethrin II, jasmolin I, jasmolin II, cinerin I and cinerin II.3
Pyrethroids
Synthetic insecticides
Slightly more persistent
Fenvalerate
Deltamethrin

26. Pyrethroids
Natural pyrethrins are light sensitive and undergo rapid photodegradation
Pyrethroids that contain a cyano substituent at the alpha-carbon of the phenoxy-benzyl moiety have been classified as type II; pyrethroids which lack this alpha cyano moiety as type I
Type I (T syndrome): Tremors, tachypnea, "running fits," hyperthermia, and salivation within 1-2 hours of injection
Allethrin
Pyrethrin I
Resmethrin
Tetramethrin
Type II (CS syndrome): Whole body tremors, hypersensitivity, occasional running fits, choreoathetosis (sinuous writhing), hypothermia, and generalized seizures
Cypermethrin
Deltamethrin
Fenvalerate

27. Pyrethrin and Pyrethroid Insecticides
Toxicity
Low oral toxicity
LD50 quite variable
25 to mg/kg
Rapid hydrolysis in the gastrointestinal tract
Liver metabolism
Synergists
Microsomal enzyme inhibitors
Piperonyl butoxide

28. Mode of Action (Overstimulation of the Nervous System)
Interference with voltage gated sodium channels.
Type I keep channels open for shorter period vs. Type II
Enhanced sodium ion conductance

30. Clinical Signs Muscle tremors Mixed CNS effects Increased salivation
CNS depression
CNS excitation
Seizures
Increased salivation
Vomiting
Ataxia
Hypo and hyperthermia
Paresthesia

31. Generic Pyrethroid Pathway
Keep Na Channels
Open
Hydrolyzed
by P450s
Hydroylzed by
CAEs

32. Time After Dosing (minutes) Choreoathetosis Salivation50
100
150
200
250
300
350
400
Choreoathetosis Score
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Weanling, 4 mg/kg DLT
Adult, 30 mg/kg DLT
Choreoathetosis
Salivation Score
Weanling, 4 mg/kg DLT
Adult, 30 mg/kg DLT
Salivation

33. Newer Pesticides:
“Mectins” (Nicotinic receptor agonists, specific for non-mammalian receptors)
Avermectin
Ivermectin
Anthelmentic, insecticide
Management of river blindness (Onchocerca)
Low dermal absorption
Minimal biotransformation

34. Final Thoughts
Most of what has been presented relates to acute toxicology.
What about long term effects of low level exposure to pesticides?
What about effects on the developing nervous system?