Anthelmintic resistance

Question 1

What are the main species that cause ostertagiosis in sheep in the UK?

Answer 1

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

Question 2

What are the main species that cause ostertagiosis in cattle in the UK?

Answer 2

Ostertagia ostertagi

Question 3

What are the most important horse worms?

Answer 3

‘Redworms’ Strongyles

Question 4

How are strongyles divided?

Answer 4

Size (small vs. large)
Life cycle (migratory vs. non-migratory)

Question 5

What are the main reasons for anthelmintic treatment failures?

Answer 5

Normally through user error. e.g.
UNDER DOSING
-Underestimation of BWT
-Equipment maintainance
-Poor dosing technique
-Failure to follow manufacturer instructions
INCORRECT DRUG USAGE
RE-INTRO ONTO CONTAMINATED PASTURE
ANTHELMINTIC RESISTANCE

Question 6

Is anthelmintic resistance conferred by environmental factors?

Answer 6

No it is heritable

Question 7

What is the state of anthelmintic resistance for sheep and cattle parasites in the UK?

Answer 7

Sheep - widespread resistance to BZs. Reports of ‘triple’ resistant Teladorsagia circumcincta.

Cattle - much less. treatment failures more likely due to under dosing.

Question 8

What is meant by triple resistance?

Answer 8

Parasite resistant to the 3 main anthelmintics.

Question 9

What is the state of resistance to anthelmintics in horse parasites?

Answer 9

BZ resistance widespread in cyathostomins
Pyrantal and ML resistance uncommon
Large strongyles are uncommon so resistance not really reported.

Question 10

When is clinical failure of the anthelmintic likely, with regard to allele frequency, and how does this relate to when resistance is detectable?

Answer 10

Clinical failure is only really detectable when a population has 20% resistance and allele frequency is >50%.

Testing can detect resistance when allele freq. is >25% and >5% of the population is resistant.

Question 11

What are the types of in vivo tests used to test for resistance?

Answer 11

The drench test and the faecal egg count reduction test.

Question 12

How long after treatment should faecal samples be taken for the drench test?

Answer 12

LV - 7 days

BZ & ML - 14 days

Question 13

When is resistance indicated by a FECRT?

Answer 13

When the egg count is not reduced by >95%

Question 14

What are the key SCOPs guidelines ?

Answer 14

Work out a control strategy with vet/advisor
Quarantine effectively
Test for AR on the farm
Effectively administer anthelmintics
Use only when necessary
Select appropriate anthelmintic
Preserve susceptible worms
Reduce dependence on AH.

Question 15

When should AR be tested for in particular?

Answer 15

Where pour-on products are used.

Question 16

How can wormers be delivered effectively?

Answer 16

Dose to heaviest animal in group

Use correct technique.

Question 17

How can indoor calves be prevented from becoming infected?

Answer 17

Provide low risk pasture at beginning of grazing season.
Use strategic prophylaxis is infectivity high risk
Move mid-season to low-risk pastures.

Question 18

How can selection pressures for AR be reduced?

Answer 18

By using narrow spec when possible
Avoid combination products
Rotate products where necessary

Question 19

How can a ‘refugia’ population of susceptible worms be preserved?

Answer 19

Leave some calves undosed
Dose on contaminated pasture and delay move to infected pasture to allow low level infection of susceptible pasture to develop.