Nematodes: gutworms (effective against adult, mucosal larvae and hypobiotic larvae), and lungworms Limited activity against trematodes, cestodes, and protozoa (some active against Giardia).
Pharmacokinetics of Benzimidazoles
BDZs kill worms very slowly
Triclabendazole spectrum of activity
Unique! High activity against Fluke, no activity against nematodes.
Potency of Benzimidazoles
1. Multiple low doses are more potent than a single large dose 2. Potency greater in the ruminant and horse than in the dog/cat (rumen/cecum acts as a reservoir) 3. Potency greater in ruminantes if starved for 12 to 24 hours prior to dosing
Imidazothiazoles/tetrahypropyrimazines mode of action
Cholinergic agonists; causes a rapid and reversible spastic paralysis
Target Parasites for Levamisole
Nematodes: gutworms (not effective against hypobiotic larvae) and lungworms
Pharmacokinetics of levamisole
Most potent is pour-on, followed by oral (reaches highest plasma concentration at 8 hours), and injection has shortest half-life in the body.
Types of macrocyclic lactones
Mode of action of macrocyclic lactones
Open invertebrate specific glutamate-chloride channels in postsynaptic membrane leading to flaccid paralysis
Causes tegument destruction (specifically at the proliferation zone). Increased membrane permeability for calcium ions, loss of intracellular calcium, leading to spastic paralysis and malabsorption.
Types of trematocides
AKA flukicides; salicylanilides (closantel, oxyclozanide) and subsituted phenols (nitroxynil)
Pharmacokindetics of salicylanilides
Bind to plasma proteins, giving them a prolonged plasma half-life.
When a parasite can tolerate anthelmintic doses which are normally lethal; the ability to do so is heritable.
The ability of a parasite population to tolerate doses of a drug that would prove lethal to the majority of individuals within the population
Selection for resistance
Occurs on exposure to anthelmintic and has a genetic basis
The intensity of the selection process
Resistance to compounds with the same mode of action
Resistance to two or more compounds
Return to susceptibility
reversion hastened by use of alternative mode of action compound
How does resistance appear?
Resistance alleles pre-exist in most worm populations; when anthelmintic is used, the very few worms with resistance are favored. As allele frequency increases, so does resistance.
What factors influence the rate of AR development?
1. Relative size of the in-refugia population 2. Frequency of treatment 3. Rate of re-infection after dosing 4. Dose rates
If there is a large in-refugia population, will AR develop faster or slower?
When an entire group of sheep is treated prior to a move to a low-contamination pasture, the in-refugia population is relatively ___1___ and the selection pressure __2__.
1. small 2. high
What will more frequent treatment do to AR?
Selects faster for AR
When is treatment particularly selective?
When frequency approaches the pre-patent period (i.e. 3 weeks), treatment becomes particularly selective.
What is the effect of under-dosing?
Encourages a rapid appearance of AR Allows heterozygous parasites to survive
What can be done to delay AR?
Rotations of anthelmintics Combinations of anthelmintics Prevent the entry of resistant worms onto farms from other farms
How is resistance measured?
In vivo: drench test, faecal egg count reduction test In vitro: laboratory-based larval development assays
Quick indicator of anthelmintic efficacy; FEC on 10 fecal saples post treatment. The time after treatment depends on the anthelmintic used (LV 7 days, BZ 10-14 days, ML 14-16 days).
Fecal Egg Count Reduction Test
Estimation of efficacy by coparing FEC before and after treatment with untreated controls. Resistance indicated if treatment does not reduce FEC by >95%.
Larval Development Test
Assess effects of drugs at different concentrations on the development of L1 to L3.
Anthelmintic resistance in Sheep
Southern Hemisphere: Haemonchus worms involved Australia and New Zealand: multiple resistance common in Teladorsagia and Trichostrongylus GB: BZ resistance now widespread, increasing reports of resistant Teladorsagia circumcincta.
Anthelmintic resistance in goats
Nematodes commonly reported as resistant in many countries. Also, NO licensed anthelmintics for goats, all off-license use. Sub-optimal dosing therfore common and selects heavily for resistance.
Anthelmintic resistance in cattle
Much less common than with small ruminants, reports mainly from Australia and New Zealand. Cooperia and some Ostertagia reports. Few reports of ML-resistant Cooperia in GB
Anthelmintic resistance in horses
Benzimidazole resistance no widespread in cyathostomins both in GB and worldwide. Resistance to pyrantel and macrocyclic lactones still uncommon.