Dewormers: Why resistance is rising

Dear Daktari,

Thank you for your excellent work in educating us about livestock health. I keep some goats, sheep, and cows, and I deworm them every three months. However, I still see worm eggs in the faeces. Are worms developing resistance? What can I do to ensure I kill all the worms in my livestock?

Samuel Lonyagapuo, Kapenguria

Thank you very much, Samuel, for your question. Yes, just as disease-causing microorganisms are developing resistance, worms and ticks are also gradually becoming resistant to anthelmintics and acaricides, respectively. Today, I will focus on anthelmintics, or dewormers, as they are commonly called.

Many farmers recognise the negative effects of internal worms on livestock productivity and growth rates.

To counteract this, they often use drugs called anthelmintics or dewormers.

Although cattle farmers have good intentions when treating their animals for intestinal parasites, regular and frequent use of anthelmintics probably contributes to resistance.

With current practices of routine dewormer administration to all animals on the farm multiple times yearly, it’s likely that farmers are deworming animals that do not actually need it or using products that are ineffective in their herd.

Both scenarios lead to economic losses and promote the development of resistance.

Similar to antibiotic resistance, overusing dewormers inadvertently fosters drug-resistant parasites.

High worm burdens are commonly seen in younger animals, especially weaned calves and replacement heifers. Adults typically develop immunity to worms.

Worms cause anorexia and reduce the animal’s efficiency in converting forage into milk and muscle.

Anthelmintic resistance refers to a worm’s ability to survive a deworming dose of the chemical due to a genetic change (mutation).

Surviving parasites can reproduce and pass on their “resistance gene” to their offspring, further spreading resistance.

How do you measure resistance?

A faecal egg count reduction test determines dewormer effectiveness by counting parasite eggs in manure before and after treatment.

A dewormer is effective if it causes at least a 95 per cent reduction in eggs.

Farmers interested in conducting this test should contact their veterinarian or local extension officer for further guidance.

What can farmers do to reduce resistance?

Limiting unnecessary dewormer use is crucial. While deworming every three months is common, it should be done strategically and only when necessary.

Farmers must ensure the dewormers used are effective and that treatments are carried out correctly.

Avoid under-dosing.

Underdosing often occurs when animals are not weighed before treatment or are incorrectly weighed, resulting in lower doses and reduced efficacy. To avoid this, farmers should weigh animals accurately to determine the correct dosage.

Implement good grazing practices. Overstocking and overgrazing force animals to graze close to manure piles, increasing the risk of ingesting larvae. Rotational grazing and allowing paddocks to rest for a month can help break the parasite life cycle and reduce exposure.

Exposing pasture to sunlight kills larvae; hence, clipping pastures maximises sunlight exposure, lowering larval density.

Also, avoid grazing animals early in the morning when grass dew is present, as moisture facilitates larval movement.

[Dr Othieno is a veterinary surgeon and currently the head of communications at the Food and Agriculture Organisation of the United Nations (FAO) Kenya. The views expressed here are not necessarily FAO’s but his own]