Sustainable parasite control is crucial

Understanding resistance

Anthelmintic resistance in parasites is a natural, inherited characteristic, where a genetic ‘trait’ is passed from one generation to the next via the worm genome.

When worms come into contact with an anthelmintic treatment, susceptible worms die, while less sensitive worms and those carrying resistance genes will survive.

Those which survive will reproduce, increasing the percentage of the population carrying resistant genes, until a population may be entirely resistant to that anthelmintic.

This process of selecting for anthelmintic-resistance genes in worms can be sped up by repeated use of the same wormer compound, and by treating cattle and immediately moving them to clean pasture.

There are a number of techniques which can be used to slow down the rate resistance develops in a worm population. This is achieved by keeping a source of anthelmintic-sensitive parasites in the population, and worms are referred to as being in ‘refugia’, essentially ‘in refuge’ from the wormer.

Anthelmintic-sensitive worms can exist in untreated cattle within the herd, as eggs and larva on the pasture at treatment time and in cattle, but at development stages which are unaffected by the particular treatment.

Once you know where these anthelmintic-sensitive populations exist, you can target your treatment plans and grazing strategies accordingly.

Monitor, test, treat

An essential part of sustainable parasite control is the monitoring of parasite infection levels.

Records of stock performance over the previous grazing season and comparison to expectations or targets can help identify parasite issues.

Cattle should be weighed regularly to track progress. Growth rates in calves and youngstock in their first and second grazing season are useful indicators of parasite burden, since cattle which are not meeting growth targets may have a high parasite challenge.

Treating those which are behind target can result in significant improvements in weight gain at the end of the grazing season.

This will also reduce future pasture contamination by removing egg-producing adult worms. At housing or during an autumn stock check, cattle should be weighed and individuals identified for targeted worm treatment.

Diagnostic tests, such as faecal egg count tests can be useful to help inform the decision. Your vet or animal health adviser can provide these tests. Leaving some individuals untreated will preserve some worms in refugia.

Any new stock coming onto the farm should be quarantined and treated with an appropriate anthelmintic.

This is vital since new animals may bring anthelmintic-resistant worms with them, which if untreated, will contaminate the pasture and infect the home herd.

After quarantine, new cattle should be grazed on pasture contaminated with the resident parasite population to ensure any resistant worms are mixed with those with susceptible genes.

Ms Timothy says: “These simple measures can be taken by all dairy producers. It is vital we take action now to protect our ability to treat parasites in the future.”