ANTHELMINTIC RESISTANCE

INTRODUCTION

Anthelmintic resistance occurs when worms survive a dose of dewormer that would normally be effective. These surviving worms pass their resistant traits on to their offspring. Over time, certain management practices—especially frequent deworming—can increase the risk of resistance becoming a serious problem on a farm.

Each time animals are treated, susceptible worms are killed while resistant worms survive and continue to lay eggs. This gradually increases the proportion of resistant worms in the population. The more frequently dewormers are used, the faster resistance can develop.

Once worms become resistant to a specific class of dewormers, all products within that class become less effective. Therefore, it is important to regularly test whether the dewormers used on the farm are still working. If reduced effectiveness is detected, rotational deworming between different classes at approximately three-month intervals is recommended.

Anthelmintic Classes

1. Benzimidazoles and Probenzimidazoles (Ashialben 10% and Fenacure 10%)
2. Levamisole (Nilzash S forte)
3. Macrocyclic lactones (Ashiver forte)

Once resistance develops to one drug within a class, worms are generally resistant to all drugs within that same class

Factors Influencing Anthelmintic Resistance (AR)

The rate at which AR develops is influenced by three main categories of factors:

1. Genetic Factors

• Some worms naturally carry genes that allow them to survive treatment.
• These resistant worms pass resistance traits to their offspring.

2. Biological Factors

• Worm species differ in their ability to develop resistance.
• The worm life cycle and environmental survival can influence how quickly resistance spreads.

3. Management Factors

Management practices play a major role in accelerating resistance:

Frequency of Treatment

• AR develops more rapidly in regions where animals are dewormed frequently.
• Short intervals between treatments create strong selection pressure.
• After treatment, resistant parasites have a reproductive advantage for approximately 2–3 weeks.

Incorrect Dosing

• Visual weight estimation is commonly used but often inaccurate.
• Underdosing allows partially resistant worms to survive and reproduce.
• This significantly accelerates resistance development.

Continuous Use of One Drug Class

• Frequent and repeated use of the same class without rotation increases resistance.
• Resistance develops to the entire drug class, not just a single product.

Minimising the Risk of Anthelmintic Resistance on the Farm

1. Quarantine and Treatment Protocols

• Quarantine newly purchased animals away from the main herd.
• Treat bought-in animals appropriately before mixing to reduce introduction of resistant worms.

2. Develop a Farm Parasite Control Plan

• Treat animals only when necessary.
• Select appropriate products based on the farming system.
• Plan correct timing of treatments.
• Avoid unnecessary dosing.

3. Pasture and Grazing Management

• Reduce reliance on dewormers by using grazing strategies.
• Graze young, high-risk animals on low-risk pastures (e.g., reseeded pasture or silage fields).
• Rotate grazing areas to reduce parasite burden.

4. Correct Dosing Technique

• Use proper dosing equipment.
• Calibrate drench guns regularly.
• Dose according to the weight of the heaviest animal in the group to avoid underdosing.

Painting the picture on the Ugandan market

Albendazole, levamisole, Ivermectin, Levamisole plus oxyclosanide are the most used dewormers on farms. Albendazole plus Ivermectin, Ivermectin plus Clorsulon are also gaining popularity for the effectiveness on Ectoparasites since tick resistance has become a problem. Nitroxynil 34 valued for its effectiveness against parasites that have developed resistance to other common classes of dewormers especially liver flukes, Barber’s pole worms, Oesophagostomum radiatum and Bunostomum phlebotomum in cattle and sheep nasal Bot (Oestrus ovis). Fenbendazole has also been looked at as new solution even when it’s from the same group as albendazole especially for its safety in pregnancy. Farmers just walk in the drugstore to buy a dewormer even when the dewormer may already be resistant

At ALS we are here to enrich you with a variety of deworming solutions for your livestock that allows you to rotate while eliminating helminths from your herd; Ashialben 10%, Nilzash s forte, Fenacure 10%, Ashiver forte and Albectin. With these 5 bullets we can ensure better productivity from your herd.

DR JOSEPH BAGUMA

KAMPALA UGANDA