Anthelmintic resistance

Question 1

Main nematodes causing sheep PGE - 5

Answer 1

Teladorsagia cicumcincta
Trichostrongylus spp
Nematodirus battus
Haemonchus contortus
Cooperia spp

Question 2

Other genera causing sheep PGE 5

Answer 2

Other nematodirus spp
Oesophagostomum venulosum
Chartia ovina
Bunostonum trigonocephalum
Strongyloides papillosus

Question 3

Main cattle nematode causing PGE

Answer 3

Ostertagia ostertagii

Question 4

Other genera causing cattle PGE

Answer 4

Cooperia spp, Trichostrongylus spp, Nematodirus helvetianus

Question 5

Lungworm - cattle

Answer 5

Dictyocaulus vivparus

Question 6

Most important horse worms

Answer 6

Strongyles (redworms)

Question 7

How are strongyles (horses) categorised?

Answer 7

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)

Question 8

Why do we need sustainable worm control?

Answer 8

• decrease complacency
• decrease opportunity for treatment failure
• decrease anthelmintic resistance (AR).

Question 9

Causes of treatment failure

Answer 9

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

Question 10

What is AR?

Answer 10

Anthelmintic resistance

the parasite can tolerate anthelmintic doses which are normally lethal and this ability is heritable

Question 11

How does AR appear?

Answer 11

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.

Question 12

Desribe AR to sheep nematodes - 5

Answer 12

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

Question 13

What is a very common AR?

Answer 13

BZ AR

Question 14

Describe AR in cattle nematodes

Answer 14

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

Question 15

Desribe AR in horses

Answer 15

• 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.

Question 16

At what frequency is the resistance allele when AR becomes detectable?

Answer 16

> 25% (the frequency of resistan parasites is 5%)

Question 17

At what frequency is the resistance allele when AR causes clinical failure of the anthelmintic?

Answer 17

Unlikely to be clinical failure of the anthelmintic until the allele frequency approaches 50% and the frequency of resistant parasites is >20%

Question 18

How is resistance measured? 6

Answer 18

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)

Question 19

How does the drench test work?

Answer 19

• 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)

Question 20

On what principle does the FECRT work?

Answer 20

Estimation of efficacy by comparing FEW before and after treatment with untreated controls. Resistance indicated if treatment doesn’t reduce FEC by >95%.

Question 21

What are SCOPS and COWS?

Answer 21

UK guidelines on sustainable worm control

Question 22

Define SCOPS

Answer 22

Sustainable Control of Parasites in Sheep

Question 23

Define COWS

Answer 23

Control of Worms Sustainably (in cattle)

Question 24

SCOPS guidelines - 8

Answer 24

• 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

Question 25

COWS guidelines - 8

Answer 25

• 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

Question 26

What is an in-refugia population?

Answer 26

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.