anthelmentic resistance

Question 1

what is anthelmintic resistance

Answer 1

• whenever a anthelmintic is used, some nematodes will have mechanisms to survive
• may be single gene or multiple genes
• initial frequency very low
• increases after “lag” period
• may not be a case on a single closed
property

Question 2

what is double resistance

Answer 2

one genus is resistant to one class of anthelmintic while another genus is resistant to another class of anthelmintic 
-need to treat with both drugs to remove parasites

Question 3

what is multiple resistance

Answer 3

single genus is resistant to both levamizole and benzimidazoles
- uncommon situation

Question 4

what is side resistance

Answer 4

when parasites develop resistance to one benzimidazole they are now resistant to other benzimidazoles too
efficacy: FBZ>MBZ>OBZ>ABZ»OFZ>ABZSO>TBZ

Question 5

what is resistance mainly a problem in

Answer 5

• Resistance mainly a problem in nematodes of sheep and goats
• Main parasite genera involved are Trichostrongylus, Ostertagia and
Haemonchus

Question 6

what are factors that influence the development of resistance

Answer 6

• polymorphism in nematode population
• initial frequency of resistance alleles
• number of genes involved
• fecundity & length of life cycle
• resistance genes dominant or recessive
• treatment frequency
• refugia
• pharmacokinetic profile of drug (“tail selection”)

Question 7

what is the mechanism of resistance in BZ

Answer 7

a single substitution of A for T on the ß tubulin gene, converting a single phenylaniline to a tyrosine.

Question 8

what are the five drug groups used and rotated for parasite control

Answer 8

BZ
LEV
OP
ML
Closantel

Question 9

what are tests for anthelmintic resistance

Answer 9

• egg hatching inhibition using benzimidazoles
• tubulin binding assay using benzimidazoles
• larval paralysis test using levamisole/ML
• larval development test - Drenchrite test

Question 10

what is the drenchite test

Answer 10

• send 1 kg of faeces to lab
• eggs extracted, test in 96-well microtitration plate
• larval differentiation to identify genera
• cheap
• determines level of resistance for BZ and LEV only
• depends on access to lab
• cannot test other mixtures

Question 11

what is a field test for resistance

Answer 11

FECRT
need groups of 10 and include a control
check feces prior to treatment
treat then collect feces about a week later
resistance present if less than 95% reduction

Question 12

what are management steps against resistance (6)

Answer 12

1. Check timing and administration of drenches (weight of sheep, calibration of drenching gun)- no underdose
2. Check management practices - timing of treatments, pasture management, etc.
3. Double doses of anthelmintic??? • No use in case of BZ’s
   • Temporarily effective with LEV
4. Use of rotations & mixtures
   • Mixtures preferable but increases costs
5. Quarantine drenches for all introduced sheep
6. Use of monepantel