Anthelmintic Resistance & Use

Question 1

what are cattle helminths

Answer 1

Ostertagia ostertagi*

Cooperia oncophora*

Dictyocaulus viviparus

Fasciola hepatica*

Calicophoron daubneyi

Question 2

what are sheep and goat helminths

Answer 2

Haemonchus contortus*

Teladorsagia circumcincta*

Trichostrongylus spp*

Nematodirus battus*

Fasciola hepatica*

Calicophoron daubneyi

Question 3

what are horse helminths

Answer 3

Small strongyles*

Parascaris equorum*

Strongyloides westeri

Anoplocephala spp

Oxyuris equi*

Question 4

what is the most common cause of PGE in sheep

Answer 4

Teladorsagia circumcincta

Haemonchus contortus

Question 5

what is the lifecycle of haemonchus contortus

Answer 5

Adult worms in abomasum, blood feeders

Mate and produce eggs

Eggs develop to L1, L2 and L3 is infective stage and is ingested

L3 travels to abomasum where it moults to L4 which berries into mucosa and can arrest or emerge as adult worms

Question 6

what are the anthelmintic groups (5)

Answer 6

Benzimidazoles (1-BZ)

Levamisole (2-LV)

Macrocyclic lactones (3-ML)

Amino-acetonitrile derivatives (4-AD)

Paraherquamide derivatives (5-SI0

Question 7

what are the mechanisms of action of benzimidazoles

Answer 7

bind to B-tubulin

Question 8

what are the mechanisms of action of levamisole

Answer 8

nicotinic acetylcholine receptors agonists

Question 9

what is the mechanism of action of macrocyclic lactones

Answer 9

glutamate-gated chloride channel agonists

Question 10

what amino acetonitrile derivatives

Answer 10

deg3 acetylcholine receptor agonists

Question 11

what is the mechanism of action of paraherquamide derivatives

Answer 11

nicotinic acetylcholine receptor antagonists

Question 12

what is anthelmintic resistance

Answer 12

A heritable reduction in the sensitivity of a parasite population to the action of a drug

Question 13

how does resistance arise

Answer 13

Mutations conferring anthelmintic resistance are found in untreated parasite populations at a low frequency

Anthelmintic treatment exerts a strong selection pressure on the parasite population

Only resistant parasite survive to reproduce and resistance mutations are passed on to the next generation

• A direct and unavoidable consequence of using anthelmintics to control worm populations is selection for individuals that are resistant to chemicals used

Animal movement facilities spread of resistance

Question 14

how are parasites highly adaptable

Answer 14

Parasite populations are large and genetically diverse

Mutations arise frequently and (if beneficial) spread rapidly

Haemonchus contortus very fecund!

Question 15

how many mutations can H. contortus produce every day

Answer 15

2000 eggs per female worm per day

500 female worms per sheep

500 sheep per flock

= 500 million eggs onto pasture per day

Mutation rate of 1-2 per genome per generation

Question 16

how can you measure resistance

Answer 16

1. fecal egg count reduction test (FECRT)
2. lab bioassays
3. molecular tests

Question 17

how can fecal egg count reduction tests be used to measure resistance

Answer 17

FEC before and after treatment (efficacy <95% = resistance)

Most commonly used test, simple to perform

Drug type and timing important (7 days post treatment for 2-LV, 14 days for 1-BZ and 4-AD, 14+ days for 3-ML)

Question 18

how can lab bioassays be used to measure resistance

Answer 18

Expose parasites to different anthelmintics and measure response

Ex. larval development = assay ‘DrenchRite’ commercially available in Australia and USA

All current tests are labour intensive and insensitive

Question 19

why is there a need for molecular tests

Answer 19

Early stage:

• Can be managed, but difficult to diagnose

Late stage:

• Easy to diagnose, but too late!

Molecular tests are needed to allow sensitive diagnosis, to monitor treatment and to plan control strategies

Question 20

what are the molecular tests for macrocylic lactones

Answer 20

Point mutations in gene encoding B-tubulin (drug target)

Phe (TCC) to Tyr (TAC) substitution at codon 200 (F200Y) of B-tubulin isotope 1 confers resistance

F200y mutation is a reliable marker and can be detected with PCR or pyrosequencing

Question 21

what are potential mechanisms of resistance

Answer 21

1. reduced sensitivity to drug
2. reduced uptake
3. removal of drug

Question 22

how can there be reduced sensitivity to a drug

Answer 22

Change in target binding site

Altered expression/composition of ion channel

Question 23

how does reduced uptake to drugs occur

Answer 23

Change in tegument/cuticle or amphid

Question 24

how does removal of a drug cause resistance

Answer 24

Increased expression of ABC transporters (P-glycoproteins)

Question 25

how do we control parasites and resistance generally

Answer 25

Ideally, aim for a reduced reliance on anthelmintics * Resistance * Residues * Ecotoxicity

Question 26

what are alternative methods of control of parasites

Answer 26

Resistant or resilient sheep (genetic selection) H. contortus killed vaccine licensed in Australia and South Africa T. circumcincta recombinant vaccine at research stage Grazing strategies (rotation, stocking density, mature ewes)

Question 27

what are the SCOPs guidelines to reduce resistance and control parasites (8)

Answer 27

1. work out a control strategy with your vet 2. use effective quarantine strategy to prevent importation of resistant worms in introduced sheep or goats 3. test for anthelmintic resistance 4. admin anthelmintics effectively 5. use anthelmintics only when necessary 6. select the appropriate anthelmintic for the task 7. adopt strategies to preserve susceptible worms on farm 8. reduce dependence on anthelmintics

Question 28

how do you quarantine animals according to SCOPs to prevent intro of resistant worms

Answer 28

Use of both new compounds (4-AD and 5-SI) recommended = SCOPs gold standard Or one new compound plus moxidectin (3-ML) = SCOPs silver standard 1. Yard all new sheep fro 24-48 hours 2. Treat with recommended products 3. Isolate on contaminated pasture (has been grazed by sheep this season for 3 weeks Also considered scab (injectable moxidectin or OP dip)

Question 29

what are common causes of anthelmintic failure (4)

Answer 29

1. Resistance 2. Dosing with insufficient anthelmintic * Underestimation of animal’s weight * Poorly maintained dosing equipment * Poor technique 3. Failure to follow manufacturer’s instructions * Not storing products correctly * Using products past their use by date * Mixing anthelmintics with other products 4. Use of the incorrect drug and/or misdiagnosis

Question 30

what are whole flock treatments

Answer 30

Common practice Treat all animals of a certain age simultaneously

Question 31

what are targeted selective treatment (TST)

Answer 31

The preferred option Only proportion of animals are treated (80% of worms are found in 20-30% of the hosts) Happy factor = animals performing as expected don’t require an anthelmintic

Question 32

how is performance measured to use a targeted selective treatment (TST)

Answer 32

FMACHA H. contortus only (mucous membrane colour) BCS lumbar vertebrae AUS/NZ Growth rate UK (automated weigh pens)

Question 33

what are the benefits of using a targeted selective treatment

Answer 33

No negative effect on pasture contamination Reduces reliance on anthelmintics Promotes ‘refugia’ Slows development of resistance

Question 34

what is refugia

Answer 34

Free-living stages of parasite on pasture as either eggs or larvae and parasites in untreated sheep Not exposed to anthelmintic Maintaining the population ‘in refugia’ offers means of maintaining genetic susceptibility Dose and move no longer recommended

Question 35

how can new groups of anthelmintics be used strategically

Answer 35

Amino-acetonitrile derivatives (4-AD) monepantel ‘Zolvix’ = initially effective against most worms resistant to other classes of anthelmintic but resistance emerges rapidly Paraherquamide derivatives (5-SI) derquantel ‘startect’ = licensed in combo with Abamectin (3-ML)

Question 36

what is the latest advice for moxidectin (3-ML)

Answer 36

Persistent action, particularly 2% injectable Sheep scab Often used in ewes at lambing to control ‘spring rise’ Latest advice from SCOPs is use moxidectin a maximum of once per year (if used for scab, don’t use in ewes at lambing)

Question 37

why is there major concerns for ML resistance

Answer 37

Major concerns for resistance: Long half life = underdosing Passing to lambs in milk = underdosing Used for sheep soap = off target selection

Question 38

why is there ML resistance in cattle

Answer 38

Persistence Ectoparasites: mites, lice, flies Eprinomectin zero milk withdrawal most commonly used

Question 39

what is the drug of choice to kill liver fluke

Answer 39

Triclabendazole (TCBZ) is drug of choice

Question 40

what stage does Triclabendazole (TCBZ) act on in liver flukes

Answer 40

active against both adults and juveniles

Question 41

what alternatives can be used to

Answer 41

closantel or nitroxynil for chronic infection with adult flukes in spring