Anthelmintic resistance Flashcards
Main nematodes causing sheep PGE - 5
Teladorsagia cicumcincta Trichostrongylus spp Nematodirus battus Haemonchus contortus Cooperia spp
Other genera causing sheep PGE 5
Other nematodirus spp Oesophagostomum venulosum Chartia ovina Bunostonum trigonocephalum Strongyloides papillosus
Main cattle nematode causing PGE
Ostertagia ostertagii
Other genera causing cattle PGE
Cooperia spp, Trichostrongylus spp, Nematodirus helvetianus
Lungworm - cattle
Dictyocaulus vivparus
Most important horse worms
Strongyles (redworms)
How are strongyles (horses) categorised?
Basis of size and life cycle into two groups:
MIGRATORY (large) - Strongylus vulgaris/edentatus/equinus
NON-MIGRATORY: Triodontophorus spp (large),
-Cyathostomins (or trichonemes; small)
-Posteriostomum, Oesophagodontus small)
Why do we need sustainable worm control?
- decrease complacency
- decrease opportunity for treatment failure
- decrease anthelmintic resistance (AR).
Causes of treatment failure
Under-dosing:
- underestimation of body weight
- inadequate maintenance of equipment
- poor treatment techniques
- failure to follow manufacturers instructions
- use of incorrect drug for target worms
- re-introduction of animals onto heavily contaminated pasture
- AR
What is AR?
Anthelmintic resistance
the parasite can tolerate anthelmintic doses which are normally lethal and this ability is heritable
How does AR appear?
Resistance alleles pre-esixt in most worm populations even more anthelmintics are ever used. Then, when the anthelmintic is used, the very few worms with resistance are favoured, resistance develops slowly at first, then more rapidly.
Desribe AR to sheep nematodes - 5
Mutliple resistance now widely reported
In southern hemisphrere countries - Haemonchus worms often invovled
Multiple ‘triple’ resistance common in Australia/NZ
GB: BZ resistance now widespread, increasing reports of multiple/triple resistant Teladorsagia circumcincta
What is a very common AR?
BZ AR
Describe AR in cattle nematodes
Reports of resistance much less common than with sheep nematodes
- most reports of ML-resistance are in Cooperia spp
- Britain (few reports)
- recent study suggested many treatment failures were due to under dosing with pour on products
Desribe AR in horses
- BZ resistance now widespread in cyathostomins (britain/worldwide)
- resistance to pyrantel and ML still uncommon)
- large strongyles (Strongylus spp) are now rarely found and so little or no AR reported.
At what frequency is the resistance allele when AR becomes detectable?
> 25% (the frequency of resistan parasites is 5%)
At what frequency is the resistance allele when AR causes clinical failure of the anthelmintic?
Unlikely to be clinical failure of the anthelmintic until the allele frequency approaches 50% and the frequency of resistant parasites is >20%
How is resistance measured? 6
IN VIVO: drench test (DT), Wormer test (WT), faecal egg count reduction test (FECRT)
IN VITRO: laboratory-based larval development assays - larval development test (LDT), egg hatch assay (EHT), larval migration inhibition test (LMIT)
How does the drench test work?
- Quick indicator of anthelmintic efficacy
- FEC on 10 faecal samples post-treatment
- Time after treatment depends on anthelmintic used (LV -7d, BZ =10-14d, ML 14-16d)
On what principle does the FECRT work?
Estimation of efficacy by comparing FEW before and after treatment with untreated controls. Resistance indicated if treatment doesn’t reduce FEC by >95%.
What are SCOPS and COWS?
UK guidelines on sustainable worm control
Define SCOPS
Sustainable Control of Parasites in Sheep
Define COWS
Control of Worms Sustainably (in cattle)
SCOPS guidelines - 8
- Work out a control strategy with vet/advisor
- Use effective quarantine strategies
- Test for AR on your farm
- Administer anthelmintics effectively
- Use anthelmintics only when necessary
- Select the appropriate anthemintic for the task
- Adopt strategies to provide susceptible worms
- Reduce dependence on anthelmintics
COWS guidelines - 8
- have a strategy
- treat cattle in quarantine (treat with two products in different groups sequentially, hold off pasture for 24-48 hours, turn out onto dirty pasture)
- test for AR on your farm (especially for pour-on products, use either WT or FECRT, larval identification important for species involved)
- Administer wormers effectively (weigh or does for the heaviest animal, check dosing/pour-on gun, correct techniques)
- Dose only when necessary (adult cow doing not normally required, indoor calves usually worm free at turnout, monitor by FEC to optimise timing of treatments)
- select the appropriate wormer (use narrow spectrum products where possible, avoid ‘off-taget’ use such as combination products, rotate products where appropriate)
- preserve a susceptible worm population (reduce selection pressure by leaving a percentage of stock unwormed, dose on contaminated pasture and delay the move to clean pasture)
- reduce dependence on wormers (use grazing management instead to reduce the parasite challenge, alternation of cattle and sheep grazing, use of new leys and aftermaths
What is an in-refugia population?
When you leave some animals untreated with anthelmintics with the aim of reducing AR selection pressure. Ultimately the in-refugia population will have a greater proportion of susceptible parasites than the exposed population in treated cattle.
