Anthelmintics Flashcards
Define anthelmintics and chemotherapy
Anthelmintic
- Chemotherapeutic agent
- Used to control helminth infections
Chemotherapy
- Use of drugs to injure an invading organism
- Without injury to the host
What are the two classes of anthelmintic based on spectrum of activity?
Broad - wide range of activity
Narrow - specific target
What are the three classes of anthelmintics based on chemical group/mode of action?
Benzimidazoles (fenbendazole)
Imidazothiazoles (levamisole)
Macrocyclic Lactones (ivermectin)
What are the three classes of anthelmintics based on target parasites?
Nematocide
Cestodicide
Trematodicide
What are the three classes of anthelmintic based on method of delivery/formulation?
Oral
Prenteral
Topical
What are the three main groups of anthelmintics that are broad spectrum?
Benzimidazoles (white drenches)
Imidazothiazoles (yellow drenches)
Macrocyclic lactones (clear drenches)
How does each of the five groups of broad spectrum anthelmintics work?
Group 1
- Benzimidazoles
- Beta-tubulin binding
Group 2
- Imidazothiazoles
- ACh agonist
Group 3
- Macrocyclic lactones
- Modulate glutamate-gated chloride channels
Group 4
- Amino-acetonitrile derivatives
- Targets specific Hco-MPTL receptors
Group 5
- Spiroindole
- Ach antagonist
How do benzimidazoles work as anthelmintics?
Bind to parasite tubulin
Lead to:
- Inhibition of glucose uptake
- Glycogen depletion
- Death
What determines the potency of benzimidazoles?
Duration of exposure
In which animals do benzimidazoles have a greater potency?
Ruminant and horse
Rumen/caecum acts as reservoir
How can the potency be increased in ruminants?
Starve them for 12-24 hours
Slows rate of passage of ingesta
What do imidazothiazoles/tetrahydropyrimidines cause in parasites?
Rapid and reversible spastic paralysis
What are imidazothiazoles now only available as?
Oral drenches
Which parasites does levamisole target?
Gutworms
Lungworms
How is the half-life of levamisole varied?
Route of administration
How long is the half-life of levamisole in the body?
Very short
How do macrocyclic lactones work?
Open invertebrate spcific glutamate-chloride channels in post-synaptic membrane
Leads to flaccid paralysis
How can you tell a drug is a macrocyclic lactone?
Drug usually ends in -ectin
Which parasites do macrocyclic lactones work on?
Gutworms
Lungworms
Arthropods
What is the difference between metaphylaxis and prophylaxis?
Metaphylaxis protects animals grazing contaminated pasture
Prophylaxis prevents pasture infection
What does the persistent effect and activity of macrocyclic lactones allow?
Period of protection against reinfection
Desgin of prophylactic worm control strategies
What should be used if anthelmintic resistance is suspected?
The smaller groups of broad spectrum anthelmintics
- Aminoacetile derivatives
- Spiroindoles
What can praziquantel be used for?
Echinococcus
How does praziquantel work?
Destroys tegument
Allows increased membrane permeability for calcium ions
Causes spastic paralysis and malabsorption
Disruption of tegument leads to digestion of tapeworm
How do flukicides work?
Uncouple oxidative phosphorylation
Decreases availability of high energy phosphate compounds
Starves parasites
What are the two types of flukicides?
Salicylanilides
Substituted phenols
What anthelmintics can’t be used in milking cattle or sheep?
Salicylanilides
What are the five methods of delivery via the oral route?
Drench
Paste
Tablet
In feed
Bolus
What are the main nematode species causing disease (PGE) in British sheep?
- Teladorsagia circumcincta*
- Trichostrongylus* spp.
- Nematodirus battus*
- Haemonchus contortus*
- Cooperia* spp.
What is the main nematode species causing PGE in cattle?
Ostertagia ostertagi
What are the most important horse worms?
Strongyles - redworms
How do treatment failures occur with anthelmintics?
Underdosing
- Underestimation of bodyweight
- Inadequae maintenance of equipment
- Poor technique
- Failure to follow instructions
Incorrect drug used
Re-introduction of animals onto contaminated pasture
Resistance
Describe anthelmintic resistance
Parasite can tolerate anthelmintic doses which are normally lethal
Ability is heritable
How does resistance appear?
Resistance alleles pre-exist in most worm populations
When anthelmintic used very few worms with resistance favoured
Develops slowly at first then more rapidly as allele frequency increases
Describe anthelmintic resistance in sheep nematodes
Multiple resistance now widely reported
Benzimidazole resistance now widespread
Reports of multiple resistant Teladorsagia circumcincta
In what are most macrocyclic lactone resistance reports in cattle?
Cooperia spp
Describe anthelmintic resistance in horses
Benzimidazole resistance widespread in small strongyles (cyathostomins)
Uncommon resistance to pyrantel and macrocyclic lactones
How can resistance be measured?
In vivo
- Drench test
- Wormer test
- Faecal egg count reduction test
In vitro
- Larval development assays
- Egg hatch assay
- Larval migration inhibition test
Describe the drench test
Give treatment
FEC on 10 faecal samples post treatment
Test for amount of larval eggs present
Describe a faecal egg count reduction test
Compare FEC before and after treatment with untreated controls
Resistance indicated if treatment does not reduce FEC by more than 95%
What are the 8 steps of the SCOPS outline for sheep?
- Work out control strategy with vet
- Use effective quarantine strategies
- Test for AR on farm
- Administer anthelmintics effectively
- Use anthelmintics only when necessary
- Select appropriate anthelmintic for the task
- Adopt strategies to preserve worms
- Reduce dependence on anthelmintics
How should cattle in quarantine be treated with anthelmintics?
Two products in different groups sequentially
Hold off pasture for 24-48 hours
Turn out on to dirty pasture
Which animal should we dose for with anthelmintics?
Heaviest in the group
