Anthelmintic Resistance

Question 1

What are the main nematode species causing disease (PGE) in British sheep?

Answer 1

Teladorsagia circumcincta
Trichostrongylus spp.
Nematodirus battus
Haemonchus contortus
Cooperia spp.

Question 2

What is the main nematode causes PGE in cattle?

Answer 2

Ostertagia ostertagi

Question 3

What is the main lungworm in cattle?

Answer 3

Dictyocaulus viviparus

Question 4

What are the most important horse worms?

Answer 4

Strongyles (redworms)
Migratory:
Strongylus vulgaris
Strongylus edentatus
Strongylus equinus

Non-migratory:
Triodontophorus spp.
Cyathostomins
Poteriostomum

Question 5

What are the potential issues with using highly effect anthelmintics?

Answer 5

Complacency in product use and application

Resistance

Question 6

What mistakes are used with anthelmintic use?

Answer 6

Underdosing
Use of incorrect drug for target worms
Re-introduction of animals onto heavily contaminated pasture

Question 7

Causes of under-dosing in anthelmintics…

Answer 7

Underestimation of BW
Inadequate maintenance of equipment
Poor treatment techniques
Failure to follow manufacture’s instruction

Question 8

What is anthelmintic resistance?

Answer 8

When a parasite can tolerate anthelmintic doses which are normally lethal

Question 9

How does anthelmintic resistance appear?

Answer 9

1. Resistance alleles pre-exist in mort worm populations before anthelmintic is ever used
2. When it is used, few worms with resistance alleles favoured
3. Resistance develops slowly at first, then more rapidly as allele frequency increases

Question 10

Is resistance in sheep nematodes common?

Answer 10

Yes - multiple resistance no reported worldwide

Question 11

Which sheep nematodes have resistance in the UK?

Answer 11

Benzamidazoles resistance widespread

Increasing reports of multiple resistant Teladorsagia circumcincta

Question 12

Which broad spectrum groups don’t have anthelmintic resistance?

Answer 12

Group 4 and 5

Amino acetonitrile and spiroindoles

Question 13

Is resistance in cow nematodes common?

Answer 13

Much less common than sheep

Question 14

Which cow nematode are there most issues with resistance?

Answer 14

Cooperia spp.

Question 15

Describe anthelmintic resistance in horses.

Answer 15

Benzimidazole resistance widespread in cyathostomins

Resistance to pyrantel and marcocyclic lactones still uncommon

Large stongyles now rarely found = no/little resistance reported

Question 16

Describe the development of anthelmintic resistance…

Answer 16

1. In earliest phase resistant alleles are rare and develops slowly
2. Lag phase between increase in resistant alleles and resistant phenotypes
3. By the time it is detectable using tests allele frequency => 25% phenotype = >5%
4. Clinical failure of anthelmintic allele frequency = 50%, phenotype = >20%

Question 17

How is anthelmintic resistance measured in vivo?

Answer 17

Drench test or Wormer test

Faecal egg count reduction test

Question 18

How is anthelmintic resistance measured in vitro?

Answer 18

Laboratory based larval development assays

Egg hatch assay

Larval migration inhibition test

Question 19

What is a drench test?

Answer 19

Drench animal then take faecal sample (at least 10 individuals) and count egg.s

Time after treatment depends on anthelmintic used:
LV = 7 days
BZ= 10-14 days
ML = 14-16 days

Question 20

What is a faecal egg count reduction test?

Answer 20

Estimation of efficacy by comparing faecal egg count before and after treatment

Resistance indicated if treatment doesn’t reduce faecal egg count by >95%

Question 21

What UK guidelines on sustainable worm control are there?

Answer 21

SCOPS = sustainable worm control stragegies for sheep

COWS = control on worm sustainability

Question 22

What is SCOPS?

Answer 22

Group for sheep parasite control providing advice on sustainable worm control

Question 23

What are the SCOPS guidelines?

Answer 23

1. Work out control strategy with vet/advisor
2. Use effective quarantine strategies
3. Test for AR on your farm
4. Administer anthelmintics effectively
5. Use anthelmintics only when necessary
6. Select appropriate anthelmintic
7. Adopt strategies to preserve susceptible worms
8. Reduce dependence on anthelmintics

Question 24

Expand on ‘treat cattle in quarantine’ guideline for anthelmintics..

Answer 24

Treat with…
2 different products
In different cohorts
At different times

Hold off pasture for 24-48 hours (prevent shedding)

Turn out onto dirty pasture = susceptible population dilutes resistance genes

Question 25

How would you administer wormers effectively?

Answer 25

Weight or dose for the heaviest in the group
Check dosing/pour on gun
Use correct techniques

Question 26

When is dosing of anthelmintics not necessary?

Answer 26

Adult cows usually
Indoor calves at trunout (provide low risk pasture, strategic preventation treatments and move mid season)

MONITOR

Question 27

How would you select the appropriate wormer?

Answer 27

Use narrow spectrum products where ever possible
Avoid off target use (combi products)
Rotate product where appropriate

Question 28

Why would you want to preserve a susceptible worm population?

Answer 28

Allow survival of susceptible worms
Resistant worms have ‘fitness cost’ => reduced fitness
Susceptible parasites left will compete with resistant worms and win
Selection pressure for resistant parasites therefore is reduced
Reduced frequency of resistance allele in the population

Question 29

How would you preserve a susceptible worm population?

Answer 29

Leave some calves undosed

Dose on contaminated pasture and delay move to clean pasture

Question 30

How would you reduced dependence on wormers?

Answer 30

Alternation of cattle and sheep grazing

Use new lays and aftermaths