Anthelmintics

Question 1

What are Anthelmintics?

Answer 1

drugs that control helminths parasites and disease they cause (nematodes, platyhelminths)

Question 2

How do we group drugs?

How many classes of drug do horses, sheep and cattle have?

Answer 2

Classified based on their mode of action
Horses and cattle –> 3 classes of drug
Sheep –> 5 major classes

Question 3

Aim of “control”

Answer 3

reduce transmission and therefore limit the amount of parasites lambs are exposed to

Question 4

How do we control parasitic infections?

Answer 4

1. Prophylactic - PREVENTING
   Case of PGE is a FAIL of prophylaxis.
2. Therapeutic - when a case has been identified, how we treat
3. Non drug related parasite control strategies getting more important: faecal collection, grazing strategies

Question 5

With drugs what do we need to consider?

Answer 5

1. Spectrum of activity
2. Pharmacokinetics
3. Formulation and administration of drug
4. Drug efficacy
5. Toxicity and specificity Vs host
6. Cost

Question 6

What is a drug’s spectrum of activity?

Answer 6

1. gives idea of how broadly each class of anthelmintic works
2. Each drug has their own spectrum – can kill x y and z parasite and in what part of lifecycle (e.g L3 and adult telodorsagia for)
3. Parasite species and stage

Question 7

What comes under a drugs pharmacokinetics?

Answer 7

o How well a drug works - Absorption and excretion of drug
o Residual activity – drug given but persists in body
o Drug residues and withdrawal periods – meat and milk. For majority of A there is a withdrawal period

Question 8

What comes under a drug’s administration

Answer 8

1. Oral, pour on, spot on, injectable

Question 9

Drug efficacy

Answer 9

1. the maximum response that can be achieved with a drug

Question 10

Toxicity and specificity Vs host

Answer 10

o Careful not to overdose
o Some drugs only target molecules parasite has, not host. However some drugs target both host and parasite. So mustn’t harm host

Question 11

How does the cost of a drug affect things?

Answer 11

drives drug decision for most things. E.g. drug can be used for wrong purpose but cheap. May not be fit for purpose of intended

Question 12

What are the anthelmintic drug classes?

Answer 12

1. 1 B-Z (Benzimidazoles)
2. 2-LEV ( Imidazothiazoles, tetrahydropyrimidines)
3. 3- ML (Macrocyclic lactones)
   - ———2 new classes
4. 4-AD-Amino acetonitrile derivatives
5. 5-SI-Amino acetonitrile derivatives

Question 13

Examples of Benzimidazoles

Answer 13

1. Fenbendazole (in panacole, horses)
2. albendazole (often in sheep and human)
3. flubendazole (chicken),
4. mebendazole (most widely used drug in humans for threadworm
5. Triclabendazole - excpetional amongst BZs
6. Febantel is a 1BZ that is given and only becomes active inside body

Question 14

Examples of 2 LEV

Answer 14

Imidazothiazoles, tetrahydropyrimidines

1. pyrantel (PYR)
2. morantel (used to lesser extent in UK

Question 15

Examples of 3 ML

Answer 15

Macrocyclic lactones:
Avermectins e.g.
1. Ivermectin - toxic to long nosed dogs as crosses blood brain barrier
10 years later 3MLs Milbemycins e.g.
2. Moxidectin

Question 16

Examples of 4AD

Answer 16

Amino acetonitrile derivatives

1. monepantel - nematode control in sheep (zolvix)

Question 17

Examples of 5SI

Answer 17

5 - SpiroindoleAmino acetonitrile derivatives

1. Derquantel. To inc efficacy use with abamectin (a 3 ML)

Question 18

1 BZ spectrum of activity

Answer 18

Benzimidazoles
• Not well absorbed systemically, give orally
• Primarily target nematodes (wouldn’t use for tape or flat worms!)
• Ovicidal – even if eggs released kills eggs so reduces transmission as any eggs won’t develop
• Effective at targeting hypobiosed larvae

Question 19

2 LEV spectrum of activity

Answer 19

• Probs most limited
• Not well absorbed systemically
• Primarily targets nematodes
• Gut dwelling stages
• Short acting – give product as kill parasites and nematodes within gut but act and then gone
• Note: EXCEPTION – double dose of PYR effective vs tapeworm of horses
• Restricted to killing gut dwelling nematodes

Question 20

3 MLs spectrum of activity

Answer 20

Macrocyclic lactones
• Well absorbed systemically – given orally and absorbed so can target lots of stages of the parasite
• Effective against nematodes, as absorbed so will also target 4th, 5th larvae, adults, hypobiosed larvae – broad spectrum in terms of targeting multiple stages of nematodes
• Target multiple nematode stages
o Migrating larvae, hypobiotic larvae
• Residual activity
o e.g ivermectin residual activity for 2 weeks, moxidectin 2 weeks persistence at levels that will KILL parasites
• ENDECTOCIDE - target internal (endo) and external (ecto) parasites

Question 21

What are combination drugs?

Answer 21

• Combine two drugs from different drug classes with different spectrum of activity .
e.g. o Horses e.g moxidectin (nematodes) and praziquantel (tapeworms)
o Cattle e.g ivermectin (nematode) and triclabendazole (fluke)
o Sheep e.g levamisole (N) and triclabendazole (f)
o Dogs and cats e.g pyrantel (n) and praziquantel (tape worms)