4.2 Worm control and anthelmintic resistance

4 reasons for anthelmintic treatment failure

• Underdosing

• Incorrect drug useage

• Reintroduction of animals on heavily contaminated pasture

• Anthelmintic resistance

Most common resistant worm species in cows

Cooperia (oncophora) - macrocyclic lactone resistance

• Often attributed to underdosing - not true resistance?

Drug with widespread resistance in sheep worms in UK

Benzimidazoles (but still not entirely useless)

What % of allele frequency has to be reached for clinical failure of anthelmintic to be seen?

50% allele frequency

• Resistant parasites >20% of population

• Resistance gene is recessive → only homozygous recessive worms resistant

Most common mutation for beta tubulin leading to benzimidazole resistance

Phenylalanine replacement with tyrosine at amino acid position 200

How is a drench test done? (2)

• Administer group of animals with anthelmintic

  • No untreated control unlike FECRT

• Faecal egg count before and after drug has taken effect

What are the 3 drug classes tested for in a drench test or faecal egg count reduction test, and how many days after drug admin should the faecal samples be taken?

• Benzimidazoles - 14 days (10-14)

• Levamisole - 7 days

  • Only kills adults - measure earlier in case new wave matures

• Macrocyclic lactones - 14 days (14-16)

• (untreated control group in FECRT only - 7 and 14 days)

Describe the steps of a faecal egg count reduction test. What counts as resistance? (5)

• Faecal egg count 4 groups of sheep

• Then treat 3 with benzimidazoles, levamisole, macrocyclic lactones leaving 1 group untreated (control)

• Check untreated and levamisole egg counts at day 7

• Check untreated, benzimidazoles and macrocyclic lactones at day 14

• Resistance = treatment does not reduce faecal egg count by >95%

7 SCOPS guidelines for sheep

Sustainable control of parasites:

• Effective administration

• Preserve refugia - susceptible worm population

• Quarantine

• Reduce worming dependence

• Select appropriate wormer

• Test for resistance

• Only use anthelmintics when necessary

How do you select refugia sheep? Why does this work?

• Measure body condition score (and FAMACHA if haemonchus) - leave some healthy sheep undosed

• Lowers selection pressure for worms to evolve resistance 

  • resistance has a fitness cost when worms 

2 ways of administering dewormers in quarantining animals

• Alternating dewormers used in sequential quarantining cohorts

  • Not together - otherwise multiple resistance

• Spiroindole (group 5) - resistance is rare

Why are quarantined animals turned out onto dirty pasture?

Dirty pasture (hopefully) has nonresistant worms - if new animals shed resistant worms then may get outcompeted