Anthelmintic resistance

Question 1

What should be considered when using an anthelmintic

Answer 1

Spectrum of activity - species, pharmacokinetics, parasite stage
Egg reappearance period
Dose and frequency
Resistance

Question 2

What are the 3 dosing strategies to control parasitic infections?

Answer 2

Interval dosing (regular intervals based on egg reappearance period)
Strategic dosing (dose when parasite number highest to disrupt cycle of transmission)
Targeted dosing (treat when infection present)

Question 3

What is anthelmintic resistance?

Answer 3

When parasites usually killed by a specific drug at a specific dose are no longer killed

Question 4

Is anthelmintic resistance heritable?

Answer 4

Yes

Question 5

Nematodes are a large, diverse population. How much resistance is natural? How is resistance acquired?

Answer 5

Very little is natural

Most is mutations

Question 6

How much resistance is present in older groups of anthelmintics?

Answer 6

Lots in benzomidazoles
Some in tetrahydropyramidines
Some in MLs

Question 7

When should you suspect anthelmintic resistance?

Answer 7

If faecal egg count remains high after Tx

If clinical signs persist after Tx

Question 8

It is difficult to know how to prevent resistance when we are unsure how parasites have become resistant. What are 3 ways they could develop this resistance?

Answer 8

Change target site
Increase efflux of drug
Alter drug metabolism

Question 9

What are 3 ways to manage anthelmintic resistance?

Answer 9

Faecal egg count reduction test
Refugia based control
Targeted selective Tx

Question 10

What is a faecal egg count reduction test?

Answer 10

Give group of animals Tx and do faecal egg count 2 weeks after
Compare with control group
<90% killed = resistance 
90-95% killed = uncertain
>95% killed = effective Tx

Question 11

What is refugia based control?

Answer 11

Have a proportion of the population unexposed to the anthelmintics
To allow some non-resistance in gene pool

Question 12

What is targeted selective treatment?

Answer 12

Only give treatment to affected animals?

Question 13

Apart from resistance, what may be other reasons as to why a drug may not be effective?

Answer 13

Inadequate dose
Parasite stage/species not susceptible
Drug activity reduced