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Lecture 1

Parasites are eukaryotic organisms that live in or on a host organism and cause harm to a host without immediately killing it

Parasites have the capacity to exploit every possible niche on earth

Can be vectors
Could have neurological control

Life Cycles

All parasites need a definitive host; without it they won’t be able to sexually reproduce

Direct life-cyles are ideal as you have the one host

Indirect life-cycles are less ideal as the parasite needs multiple hosts and it could therefore be harder to track the parasite

Prepatent periods are different for all parasites, which can change depending on the environment as well

Key Questions

Where and when is the parasite present
How does the parasite get in
Where is it in/on the host
How does it reproduce
What does it feed on
How does it avoid the immune response
How does the parasite get out
How does it get its progeny back in

C(ontrol) = M + E + T

Focus on how to break the parasite’s lifecycle to prevent infection in the first place/prevent intermediate host (if present) from being eaten by the definitive host.

Management: Clean, healthy animals in a controlled, clean environment less likely to encounter/be infested by parasites in the first place (e.g., remove breeding grounds for parasites [dung], stabling animals at night, using deworming/antiparasitic compounds).
Epidemiology: Specific for each parasite. Ecto/endoparasites? How are they transmitted? Do they thrive/die in a certain temperature? Are they vectors/carried by vectors?
Treatment: Chemical compounds used to prevent/treat a parasitic infestation. Proceed with caution as parasites develop resistance very quickly. Treatment shouldn’t be relied on solely for parasite control, and chemical compounds should be alternated when in use.

Arthropods

The name = Arthros - joint; podos - foot = jointed legs; originally 1 pair of appendages/segment

Classifications

Crustaceans >5 pairs of legs
Insects = 3 pairs of legs
Arachnids = 4 pairs of legs
- Baby stages = 3 pairs of legs

Common characteristics

Chitinous exoskeleton (called the cuticle)→ impervious to many chemicals

Internal muscles attach to the exoskelton

Open circulatory system (haemocoel)

Closed digestive system

Nervous system (important for drugs) consists of ganglia or brain surrounding oesophagus, longitudinal nerves with a ganglion in each segment

They have a bundle of ganglia with 2 nerve cords which run ventrally and dorsally

They breathe via diffusion of O2 through cuticle, gills or a tracheal system opening through spiracles (or stigma(ta))

Life cycle → instars (stages) with moults between instars

Metamorphoses are common

Chemical Control

Antiparasitics act selectively by inhibiting a parasite biochemical target while not affecting the host

Routes of administration

Internal administration: oral liquids/pastes/gels/tablets & injection
External delivery: dipping, immersion, backline, back spray, spot on, body spray, high pressure jetting, dusting applicators, chemical impregnated ear tags/collars

Repellents

Mainly based on DEET (N,N - Diethyl - m - toluamide) but can also see dibutyl phthalate, dipropyl isocinchomeronate, piperonyl butoxide (synergist), citronella oil and eucalyptus oil

Acting on Nervous System

Chlorinated Hydrocarbons Interfere with ion transport across the membrane of axons and produce hyper-excitability

Super stable → can affect non arthropods or host

See resistance in many arthropods

BANNED

Organophopshates

Interfere with acetylcholine-esterase activity (muscarinic effects of Ac-Ch) = Acetylcholine esterase inhibitors (irreversible effect)

Causes spontaneous muscle contractions followed by paralysis

Can be toxic to host/humans so must wear PPE

Broad spectrum insecticide (non-specific)

Rapidly broken down in environment - short period of activity (24 - 48h)

Resistance has occurred to OPs in many parasites

Prevent with atropine

Carbamates

Bind to and inactivate Ac-Ch esterase; main effect is in ganglia but the binding is reversible

“Safer” than organophosphates since carbamates are rapidly detoxified from animal tissue

Duration of effects longer than OP’s → can be 4-6 weeks

Prevent with atropine

Many arthropod parasites are resistant to carbamates

Formamidines

Octopamine receptor agonist that raises metabolic rate → leads to hyperexcitability

Specific receptor found in arachnids therefore highly effective against ticks and mites

Pyrethrins and Pyrethroids

Pyrethrins are derived from flower of Chrysanthemum cinerariaefolium (Compositae)

Pyrethroids are synthetic pyrethrum-like compounds (majority of commercial household insecticides)

Both disrupt Na+ and K+ fluxes in neurons causing paralysis

Toxicity is low to vertebrates but toxic to aquatic organisms, bees and cats !!!

Macrocyclic Lactones

Secreted by soil-inhabiting Streptomyces spp.

Act on glutamate-gated chloride channels → results in paralysis

Avermectincs include selamectin and aprinomectin

Milbemycins include moxidectin

Extremely potent and fast acting → effective against arthrppods

Neonicotinoids

Synthetic insecticides designed specifically for the nicotinic receptors in arthropods

Extremely low toxicity to mammals (high risk to bees)

Translocated in skin

May persist in shed keratin and kill arthropods in the environment

Period of activity 2-4 weeks

Inhibit cholinergic transmission = paralysis / death

Oxadiazines

Block voltage-gated sodium ion channels

Pro-insecticide → metabolises in the insect to a more active form

Isoxazolines

Novel mode of action – block arthropod ligand-gated chloride channels

Phenylpyrazoles

Broad spectrum - fleas, ticks, mites, lice

Causes CNS disruption and hyperexcitability in arthropod

Cause hyper-excitability

Spinosyns

Novel mode of action - target different binding sites on nicotinic acetylcholine receptors (compared to neonicotinoids)

Causes tremors and leads to death

Growth Regulators

Don’t cause hyperexcitability/paralysis/death much like the NS chemicals

Interfere with growth and moulting – doesn’t kill immediately

Two types:

Hormonal insect growth regulators mimic/inhibit the juvenile hormone, causing premature moulting of juvenile stages or disrupting larval development
Chitin synthesis inhibitor work by preventing the formation of chitin

Definitions

Mutualism - clear benefit to both organisms involved

Commensalism - where one organism benefits, but the other is not harmed/not affected

Parasitism - where one organism clearly benefits at the expense of the other (one benefits, one is harmed)

Definitive host - an organism which supports the adult or sexually reproductive form of a parasite

Intermediate host - an organism that supports the immature or non-reproductive forms of a parasite

Paratenic host- an organism acting as a substitute intermediate host of a parasite, usually having acquired the parasite by ingestion of the original host; no development of the parasite takes place but the phenomenon aids in the transmission of infection. Also called a transfer or transport host.

Prepatent period - the period between infection of the parasite and demonstration of the parasite in the host body

Permanent parasite - remain on the host for their entire life cycle (e.g. lice)

Temporary parasite - only associated with the host briefly during its life cycle (e.g. mosquitoes)

Obligate parasites - organisms whose existence is totally dependent upon the host (e.g. fleas)

Facultative parasites - can multiply without a host but under some circumstances can become parasitic (e.g. blowflies whose maggots normally develop in decaying organic matter may also develop in wounds on animals)