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What are the two classes of ectoparasites?
Insecta and arachnida
List reasons why we might want to control ectoparasites
- Irritant to host
- Source of parasite infestation of others
- Cause of various disorders
- Vectors of disease
- Reduced milk and meat yields
- Downgrading of wool and leather
- Compromise welfare
Under what conditions might there be an increased risk of ectoparasitic challenge?
- Housing situation - increased density, poor hygiene, location e.g. ticks are found in long grass
List different methods of delivery of ectoparasiticides
- Ear tags, tapes, collars for farm animal species
- Pour on solutions for large and small animals
- Spot on solutions
- Collars for cats and dogs
- Dips for farm animal species
- Oral tablets or suspensions
What is selective toxicity?
When we use drugs that target a feature of the parasite that is different from the host
How the position of target ion channels differ between vertebrae and ectoparasites? And how does this help us target only ectoparasites?
Specific ion channels are found in the central nervous system of vertebrae but in the peripheral nervous system of ectoparasites. This means the drug will be unable to pass into the CNS of the vertebrae (protected by BBB and proteins that actively pump drugs out of the CNS) whereas the drug will pass easily into the peripheral ion channels of the ectoparasite.
Metabolism is faster/slower in insects than endotherms?
Slower - we can exploit this when using drugs to kill parasites
What is chitin?
A protein found in the exoskeleton of parasites that vertebrates lack, making it a good target of ectoparasiticides
Some drugs have a greater ___ for the equivalent ectoparasite receptor?
What is the mechanism of action of the organophosphates?
Selective inhibition of acetyl cholinesterase
What is the spectrum of activity of the organophosphates?
Flies, fleas, lice, ticks and mites - wide spectrum of activity
Why do organophosphates carry vital operator warnings?
As they have a limited toxicity safety margin for animals and humans
What is the most commonly used organophosphate in veterinary medicine?
Describe in detail the mechanism of action of organophosphates
Organophosphates inhibit the enzyme acetyl cholinesterase. If the enzyme is inhibited it does not break down acetylcholine in the synaptic cleft, so it continues to have its effect at the post synaptic junction. This causes problems for the insect.
What is the difference between pyrethrins and pyrethroids?
Pyrethrins are natural; they have been extracted from the Chrysanthemum flower. Whereas pyrethroids are entirely synthetic.
What is the mechanism of action of pyrethrins and pyrethroids?
They act on presynaptic sodium channels
What is the spectrum of activity of pyrethrins and pyrethroids?
Lice, fleas, flies and keds
What is the toxicity of the pyrethrins and pyrethroids?
Generally low if given orally, only fish and some aquatic invertebrates are sensitive
How are the pyrethrins and pyrethroids selectively toxic?
The rate of metabolism is more rapid in vertebrates
Describe in detail the mechanism of action of pyrethrins and pyrethroids
Pyrethrins and pyrethroids hold the sodium channel open so sodium continues to travel into the cell. The insect is paralysed, drops off the host and, if given a high enough dose, will actually die.
Pyrethroids are liposoluble so can get into the insect via ...?
What type of pyrethroid is toxic to cats?
Permethrin - can be used in flea collars. Never give a dog flea collar to a cat!
What does permethrin toxicity in cats cause? What is the treatment for this?
- Tremors, fasculations, hyperthermia, seizures
- Methocarbamol IV, Seizure control (barbiturates, diazepam, inhalant anaesthetics)
What is the mechanism of action of avermectins and milbemycins?
Act of glutamate and/or GABA gated chloride channels
What is the spectrum of activity of avermectins and milbemycins?
Sucking lice, some mange mites, warbles
What is one of the most commonly use avermectin/milbemycin?
Why is ivermectin contraindicated in some dog breeds? And what type of breeds are susceptible?
Certain breeds of dogs have an MDR1 mutation that means they do not pump Ivermectin back out of the CNS as they should and so the drug can act on chloride channels in the brain. This causes toxic symptoms which result in coma and death. Breeds affected include: collies, Australian shepherd dogs, shetland sheep dogs, etc.
What mechanism of action does Amitraz (an amidine) have? What effect does this have on ectoparasites?
It is an octopamine receptor antagonist and alpha 2 receptor agonist. It inhibits tick MAO. This leads to insect hyperactivity and detaching behaviour; reduced fecundity, inhibition of oviposition, and reduces egg hatchability.
What are the adverse effects of high doses of amitraz?
Depression and sedation, polyuria, bradycardia, GI stasis in horses (colic)
In what animals is amitraz contraindicated in?
Cats, Chihuahuas and dogs in heat stress
What is the treatment for an overdose of amitraz?
An alpha 2 antagonist
What is the mechanism of action of cyromazine and methoprene?
They are insect growth regulators. They effect deposition of chitin in the cuticle and juvenile hormone.
What are cyromazine and methoprene used for?
Prophylatic use for blowfly strike in domestic rabbits, sheep and lambs
What must be considered when using products containing cyromazine and methoprene in production animals?
Withdrawl period of three days for meat. Should not be used in sheep producing milk for human use.
What is the mechanism of action of a benzoyl urea derivative e.g. Lufenuron?
Inhibits chitin synthase in fleas. Also kills eggs and larvae.
Describe the pharmacokinetics of Lufernuron
Its absorption is improved in the presence of food. It is highly lipophilic and accumulates in adipose tissue.
How is Lufenuron administered?
Oral suspension or tablets (monthly with food). Or injectable (six monthly).
What is the mechanism of action of Fipronil e.g. Frontline?
An antagonist to GABA and glutamate gated chloride channels. Greater affinity for insect receptors.
What is the spectrum of activity of Fipronil?
Fleas, ticks, mites in cats and dogs
To which species is Fipronil toxic?
Toxic to some gallinaceous birds and some fish and aquatic invertebrates
What is the mechanism of action of Neonicotinoids?
Blocks nicotinic receptors. Inhibits cholinergic transmission causing paralysis and death.
What is the spectrum of activity of neonicotinoids?
Fleas in cats and dogs, rabbits and ferrets.
What is the mechanism of action of Metaflumizone?
Blocks voltage dependent sodium channels resulting in paralysis
What is the mechanism of action of Oxadiazine insecticides?
Sodium channel blockers. Needs bioactivation - flea enzymes activate insecticide.