Anthelmintic Resistance

Question 1

What are the main NEMATODE causes of ovine PGE in Britain? (most common)

Answer 1

• H eamonchus contortus
• O stertagia [Teladorsagia] Circumcincta
• T richostrongylus Spp
• C ooperia spp
• Nematodirus Battus

Question 2

Which species other than nematodes may cause ovine PGE in Britain? (less common)

Answer 2

• Nematodirus spp
• Oesophagostamum venulosum
• Chabertia ovina
• Bunostomum trigonocephalum
• Strongaloides papillosus

Question 3

What are the main NEMATODE causes of bovine PGE in Britain?

Answer 3

Ostertagia ostertagi

Question 4

Which species other than nematodes may cause bovine PGE in Britain? (less important)

Answer 4

• Cooperia spp
• Trichostrongylus spp
• Nematodirus helvetianus
  > Lungworm also important (Dictyocaulus viviparous)

Question 5

Give the main lungworm of cattle

Answer 5

Dictyocaulus viviparous

Question 6

What are the most important horse worms?

Answer 6

Strongyles (redworms)

Question 7

How are strongyles grouped?

Answer 7

Based on SIZE and LIFECYCLE
> Migratory (Large)
- S vulgaris, S edentates, S equinus 
> Non-migratory (Large) 
- Triodontophorus spp
> Non-migratory (Small) 
- Cyathostomins (=trichonemes) [4 genera, 40+ spp]
- Poteriostomum, oesophagodontus

Question 8

Why are worm control STRATEGIES necessary?

Answer 8

• treatment fialures due to innapropriate product use and application
• resistance increasing

Question 9

Why may worming treatments fail?

Answer 9

• Underdosing (estimating body weight, maintainence of equipment, poor technique, failing to follow manufacturers instructions)
• Incorrect drug for target worms
• Re-introducing animals to heavily contaminiated pasture
• Resistance

Question 10

How is anthelmintic resistance spread through a population?

Answer 10

• Individual able to tolerate doses which are noramllyl lethal due to resistance alleles pre-existant in worm population
• this is heritable
• resistance develops slowly at first than more rapidly as allele frequency ^

Question 11

What drench is most commonly resistant?

Answer 11

BZ (White)

Question 12

Which broad spec anthelmintics have least resistance?

Answer 12

Aminoacetonitrite derivatives [4] and spiroindoles [5]

Question 13

Which species has most reports of resistnace?

Answer 13

Sheep

Cows not so much

Question 14

Which species are most resistant to ML?

Answer 14

Cooperia

Question 15

What is the most likely cause of resistnece developing?

Answer 15

Underdosing pour on treatments

Question 16

What drug most commonly shows resistance in horses?

Answer 16

Benzimidazole for cyathostomins (UK and wordwide)

Question 17

Which horse worms are rarely seen nowadays?

Answer 17

Large strongyles

Question 18

Why is the clinical onset of resistance an indicator of worse problems underlying the population of parasites?

Answer 18

Alleles are recessive so by the time a clinical problem is detected it is likely that resistnace genes are wide spread in the population

Question 19

At what stage of allele frequency is there likely to be clinical failure of the anthelmintic?

Answer 19

allele freq 50% (resistant parasites >25%)

Question 20

Give 2 in vivo tests that may measure resistence

Answer 20

DT (Drench test) or WT (Wormer test)

FECRT (Feacal Egg Count Reduction test)

Question 21

Give 3 in vitro tests that may measure resistance

Answer 21

Lab-based larval development assays eg.

• LDT (larval development test)
• EHT (egg hatch test)
• LMIT (larval migration inhibition test)

Question 22

Are in vivo or in vitro tests more common?

Answer 22

> In vivo

X in vitro = time and money X

Question 23

Outline the drench test

Answer 23

• FEC 10 faecal samples post anthelmintic treatment
• time after treatment depends on drench used
  > LV: 7d
  > BZ: 10-14d
  > ML: 14-16d
• snapshot of how many worms have survived the treatment; no way of knowing whether initial worm numbers were high, with small amount of resistance or low, with high resistance

Question 24

Outline the faecal egg count reduction test

Answer 24

• estimate efficacy by comparing FEC before and after, with UNTREATED controls
• resistnace indicated if FEC not reduced by >95%
• 5 animials per group
• all given different treatments
  > allows you to prioritise use of specific successful treatment
  > more expensive and time consuming

Question 25

What are SCOPS and COWS?

Answer 25

SCOPS - industry led working group for sustainable sheep parasite control
COWS - sustainable cattle parasite control

Question 26

What are the 8 SCOPS/COWS guidelines for sustainable parasite control?

Answer 26

1. work out control strategy with vet (knows local area)
2. effective quarantine strategies (treat with 2 product dequentially, turn out onto clean pasture)
3. test for resistance on farm (esp. with pour ons, WT/DT/FECRT and larval culture to ID L3)
4. administer anthelmintics correctly (weigh or dose to heaviest in group)
5. use only when necessary (adult cows usually do not need anthelmintic treatment as become immune; indoor calves usually worm free at turnout - provide low risk pasture beginning of grazing season, move mid season to low risk pasture; monitor FEC)
6. select appropriate anthelmintic (narrow spec if poss, avoid combination, rotate products yearly)
7. preserve susceptible worms (leave some undosed, dose on contaminated pasture and delay move to clean pasture, refugia population where suscpetable worms compete with resistant ones to keep resistant population down)
8. reduce dependence on anthelmintics (try to use grazing management to reduce parasite challenge - cattle/sheep alternation, new leys and aftermaths)

Question 27

Can worming protocols be developed for particular areas of the country etc?

Answer 27

NO must be farm specific