Module 10 Insecticides and Public Health

[BULB 1] What is Stockholm Convention?

Main objective is to protect human health and the environment from persistent organic pollutants (POPs).

[BULB 1] What are some characteristics of Stockholm Convention?

• An example of POP is DDT

• Aims to reduce and eliminate DDT

• DDT is recommended by WHO for malaria control

[BULB 1] What is the definition of a pest?

Any organism that is harmful to humans or human concerns

EX: plant, animal, or micro-organism

[BULB 1] What are pesticides?

Substances or mixtures of substances used in agriculture or public health programs to protect against pests.

[BULB 1] This is a list of some pesticides and their target pests. The lecture outlined some in red, what are these?

Acaricides - for mites and ticks

Algicides - algae

Herbicides - plants

[BULB 1] What is the purpose of Insecticides and what are some characteristics of it?

Pesticides that are formulated to kill, repel, or
mitigate the effects of insects.
• Control of insect pests of crops and vector-
borne diseases.
• Saved millions of human and animal lives.
• Remain the most important element of
integrated approaches to vector control.

[BULB 1] Who registers several pesticide products that may be used to control vectors that spread diseases?

EPA

[BULB 2] What are the classification of Insecticides?

• Chemical composition

• Mode of entry

• Mode of action

• Toxicity

• Stage specificity

[BULB 2] Chemical Composition:

What are organic and inorganic insecticides and what are examples of each?

• Organic insecticides have carbon as the basis of their molecular structure.

  • Example: pyrethrum, glyphosate

• Inorganic insecticides are crystalline,
  environmentally stable, do not contain carbon, and are usually derived from mineral ores extracted from the Earth.

  • Example: copper, sulphur, mercury, and boron

[BULB 2] Mode of Entry:

Contact poisons enter the insects body in what way?

Capable of entering the insect body through the cuticle

EX: DDT

[BULB 2] Mode of Entry:

Stomach poisons enter the insects body in what way?

Enter through the mouth or digestive system of the insect when ingested by the insect

EX: lead arsenate

[BULB 2] Mode of Entry:

Fumigants enter the insects body in what way?

Volatile chemical compounds that enter the bodies of insects in a gaseous phase through the spiracles

EX: methyl bromide, naphthalene

[BULB 2] What does Mode of action mean?

Means by which a toxin affects an insect’s anatomy, physiology, or biochemistry

[BULB 2] The mode of action is determined by the insecticide’s _ _ (the chemical compound responsible for the toxic effect).

Active ingredient

[BULB 2] Mode of Action:

DDT and pyrethroids are broad-spectrum nerve poisons that work by _ .

• They work by opening the sodium ion channel in insects’ neurons or nerve cells, causing them to fire spontaneously

• The insect goes into spasms and eventually dies

Contact

[BULB 2] Mode of Action:

What are characteristics of pyrethroid and pyrethrin?

• Pyrethroids are synthetic compounds produced to mimic the effects of Pyrethrin.

• Pyrethrum refers to the product extracted from Chrysanthemum flowers, while Pyrethrins are the active ingredients in Pyrethrum.

• Synthetics tend to have a longer residual efficacy than naturally occurring compounds.

[BULB 2] Mode of Action:

Carbamate and organophosphate are broad-spectrum insecticides that work by contact.

What are some characteristics about this?

• They interfere with pathways in the
  nervous systems of animals, humans, and insects, posing a risk to applicators and field workers.

• They function by inhibiting
  acetylcholinesterase, which catalyzes the breakdown of acetylcholine, a neurotransmitter at nerve junctions.

[BULB 2] Mode of Action:

Bacillus thuringiensis (Bt) is a natural, soil-borne bacteria that produce proteins known as cry toxins.
What are some characteristics about this?

• When Bt is consumed by certain insects, the toxic crystal is released, penetrating the insect’s stomach, and killing the insect.

• Several strains kill specific species of insects.

  • Bacillus thuringiensis var israelensis kills mosquitoes and black flies.

[BULB 2] Mode of Action:

Neonicotinoids (new nicotine-like
insecticides) are chemically related to nicotine.
What are some characteristics about this?

• They mimic acetylcholine and make
  insects jumpy, with leg tremors, rapid wing motion, disoriented movement, paralysis, and may cause death.

• Target piercing-sucking insects through ingestion exposure.

• Concerns of pollinator health.

[BULB 2] Mode of Action:

Insect growth regulators (IGRs) are new
chemicals that mimic hormones in young
insects and interfere with the normal growth or development of the pest.
IGRs can control many insects, including fleas, cockroaches, and mosquitoes.
What are some characteristics of this?

• Chitin synthesis inhibitors

• Juvenoids or juvenile hormone analogs

• Ecdysone interference

[BULB 2] Toxicity:

The toxicity of an insecticide is its capacity or ability to cause injury or illness.

What does LD 50 mean and its characteristics?

• LD 50 (lethal dose, 50%) is the amount of a
  substance given all at once, which causes
  the death of 50% of the animals in the
  dose group. The LD 50 is one way to
  measure a material's short-term poisoning
  potential (acute toxicity). Expressed as
  mg/kg.

• LC 50 is the chemical concentration in the
  air that kills 50% of the test animals during
  observation. Expressed as mL/kg.

[BULB 2] Stage Specificity:

What are the insect stages?

• Ovicides - eggs

• Larvicides - larval stage

• Pupicides - pupal stage

• Adulticides - adult stage

[BULB 2] What are some benefits of insecticides on public health?

• Control of vector-borne diseases
  • Successful Chagas disease control program
  in the Americas
  • Onchocerciasis in eight countries of West
  Africa by weekly application of safe larvicides

• Reduction of pest nuisance and annoyances

• Protection and enhancement of lawns, gardens,
  parks, and ponds for public enjoyment

• Protection of pets

• Control of invasive pests that may vector diseases

[BULB 2] Insecticides kill organisms that share many _ _ and physiological processes with non-target species.

• Difficult to develop insecticides that have
  ample margins of safety

• Human exposure can be through contact,
  ingestion, or inhalation through
  occupational effects and risks, food
  residues, and soil, air, and water exposures.

Biochemical pathways

[BULB 2] Insecticides can have _ (immediate or short-term) or _ (long term) effects.

Acute, chronic

[BULB 2] What are the effects of acute and chronic pesticide poisoning symptoms?

• Acute: Effects are immediately after
  pesticide exposure.

• Chronic: May not appear for weeks,
  months, or even years after pesticide
  exposure.

• Children, farm workers, and insecticide
  applicators are more vulnerable.

[BULB 2] What are some ecological impacts of insecticide usage?

• Most insecticides are non-specific
  and will kill pest species as well as
  non-target species.

• May kill beneficial insects such as
  honeybees and natural insect
  enemies (predators, and
  parasitoids) that normally control
  pest insects.

[BULB 2] What is insecticide resistance?

Selection of a heritable characteristic in an insect population that results in the repeated failure of an insecticide product to provide
the intended level of control when used as recommended.

Added note: Genetics and intensive application of
insecticides are responsible for the rapid
development of resistance in many
arthropods

[BULB 2] What are some facts about insecticide resistance?

• Almost all public health insecticides are also
  used in agriculture.

• Many vector species of public health
  importance have already developed
  resistance to one or more insecticides.

[BULB 2] What is Behavioral resistance to insecticides?

Resistant insects may detect or recognize a danger and avoid the toxin.

[BULB 2] What is Reduced penetration resistance to insecticides?

Resistant insects may absorb the toxin more slowly than susceptible insects.

[BULB 2] What is Metabolic resistance to insecticides?

Resistant insects may detoxify or destroy the toxin faster than susceptible insects, or quickly rid their bodies of the toxic molecules.
• Is the most common mechanism and often presents the greatest challenge.

[BULB 2] What is Altered target-site resistance to insecticides?

The site where the toxin usually binds in the insect becomes modified to reduce the insecticide’s effects

• Second most common mechanism of resistance

[BULB 2] What is Cross-resistance?

Occurs when insects that exhibit resistance to one insecticide are also resistant to other insecticides with the same mode of action.

• DDT and pyrethroids have the same mode of action. Both are chemically unrelated but act on the same target site.

[BULB 2] What is Multiple resistance?

Occurs when several different resistance mechanisms are present simultaneously in resistant insects. The different resistance mechanisms may combine to provide resistance to multiple classes of products.

[BULB 2] How can insecticide resistance be managed?

• Rotation

• Mosaics

• Mixtures

[BULB 2] What is the “Rotation” strategy when managing insecticide resistance?

The best strategy for controlling disease vectors is the rotational use of insecticides of different modes of action altogether.

[BULB 2] What is the “Fine - scale mosaic” strategy when managing insecticide resistance?

Spatially separated applications of different compounds against the same insect. For example, using two insecticides in different houses within the same village or space so there is the potential for insects within a single generation to encounter both insecticides, and therefore reduce the rate of resistance selection.

[BULB 2] What is the “Mixtures” strategy when managing insecticide resistance?

Theory is that if resistance to each of the two insecticide compounds within a mixture is rare then multiple resistance to both will be extremely rare.

[BULB 2] What is DDT and what are some facts about it?

(Dicholoro-diphenyl-trichloroethane)

• Developed as the first of the modern synthetic insecticides in the 1940s.

• Initially used with great effect to combat malaria, typhus, and the other insect-borne human diseases among both military and civilian populations.

• Also effective for insect control in crop and livestock production, institutions, homes, and gardens.

[BULB 2] What are some facts about “Silent Spring”?

• Published in 1962 by Rachel Caron

• It revolutionized how people understand their relationship with the natural environment.

• Explained how the application of agricultural chemicals, pesticides, and other chemicals polluted our streams, damaged bird and animal populations, and caused severe medical problems for humans.

• DDT was largely used in the U.S. after WWII to
  prevent various insect-borne diseases.

• Presented the use of DDT in ecological terms rather than portraying the effectiveness.

[BULB 2] What new ideas did “Silent Spring” present in the public mind?

• Spraying chemicals to control insect populations can also kill birds that feed on dead or dying insects.

• Chemicals travel through food chains.

• Chemicals don’t outrightly kill but can accumulate in fat tissues causing medical problems later.

• Chemicals can be transferred generationally
  from mothers to their young.

[BULB 2] When “Silent Spring” was published, it was met with what kind of public interest and criticism?

• Her research became the central testimony at two congressional hearings and a Presidential Science Advisory Committee report on pesticides in 1963.

• Carson and “Silent Spring” were the catalysts that inspired the environmental movement that began in the 1960s and gained international momentum by the 1970s.

[BULB 2] Years after Silent Spring was published, the U.S. Environmental Protection Agency (EPA) was established, and various environmental laws were enacted.

What were some of the effects of this?

• The domestic use of DDT was banned in 1972 due to its widespread overuse and harmful impact on the environment.

• Since the publication of “Silent Spring,” the chemistry discipline now includes green chemistry.

• Scientists, policy makers, and the public recognize the trade-offs of new technologies.
  

[BULB 2] What are some attributes of DDT?

• Accumulates in fatty tissues

• Is very persistent in the environment

• Can travel long distances in the upper atmosphere

[BULB 2] What happened after the use of DDT was discontinued in the US?

Its concentration in the environment and animals has decreased, but because of its persistence, residues of concern from historical
use remain.

[BULB 2] What are some insecticide controversies in the case of DDT?

• Since 1996, the EPA has been participating in international negotiations to control the use of DDT around the world.

• The Stockholm Convention on POPs includes a limited exemption for the use of DDT to control mosquitoes that transmit malaria.

• In September 2006, the WHO declared its support for the indoor use of DDT in African countries where malaria remains a major health problem.

• DDT is one of 12 pesticides recommended by the WHO for indoor residual spray programs.

[BULB 2] DDT has been used since _ as an effective method of _ control.

1940, malaria

[BULB 2] What are some facts about DDT and malaria control?

• On a global scale, DDT production is decreasing.

• Many countries still rely on DDT for malaria control.

• South Africa reintroduced DDT for indoor spraying in 2000 and has kept cases and deaths from malaria at all time lows since then and is moving towards elimination of malaria.