Wormer resistance a growing concern
Wormer resistance is posing a threat to profitable cattle production, but there are
some simple steps that livestock farmers can make now, which will improve the
sustainability of worm control and safeguard the future of cattle production.
Parasites are estimated to be costing UK livestock producers around £270 million in
terms of lost production and treatment costs each year, with cases in cattle making
up the majority of this cost1. There are also concerns about increasing resistance to
wormers, making it harder to effectively treat production-limiting worm burdens.
According to Professor Eric Morgan, at the School of Biological Sciences at Queen’s
University Belfast, some commonly-used worming strategies are now understood to
Strategies that may drive wormer resistance
Treating youngstock with long-acting wormers early in the grazing season and
implementing whole group treatment, rather than selectively treating a proportion of
the group, are examples of strategies that may drive wormer resistance. Additionally,
these approaches may impact on the ability of youngstock to develop immunity to
Ecological effect of wormers
There is also an increasing focus on the ecological effect of wormers, since the
presence of certain active ingredients in cattle dung can have an impact on dung-
fauna, such as beetles and other insects.
Climate change impact on worm infections
Climate change is also having an impact on worm infection dynamics, which is
further compounding the potential for resistance. Hotter, drier summers, reduce the
survival of worm larvae, meaning that there is little or no refugia on the pasture. This
intensifies the selection pressure for resistance when animals are treated.
However, infective larvae may survive in dung pats, and if released en-masse during
wet weather in late summer, can cause sudden outbreaks of parasitic disease in
What is worm resistance
Resistance is defined as a faecal egg count reduction of under 95% following
treatment, but the benefits of treatment on production can still be seen even with
FEC reductions of only 70 and 80%. It’s not until efficacy reduces to 60% or 50% that a reduction in livestock productivity becomes evident, but by then the problem is
more difficult to address.
But Cattle producers can be reassured that making steps towards a more
sustainable approach to worming cattle is not as hard as it sounds, and the start of
the grazing season is the ideal time to put integrated worming strategies into action.
Future alternatives to wormers
Despite our reliance on anthelmintics for worm control, there are alternatives
emerging that can contribute to an integrated, sustainable approach in the future.
Vaccines are already available for lungworm, and others are in development.
However, optimising immunity to worms is just one component of an integrated
strategy to control parasites in a more sustainable way.
Biological control options are being developed, including fungal spores that trap and
kill the worms in the dung. These are more sensitive to environmental conditions, so
sometimes they work well, but other times they won’t be as effective, and there is a
need for further research and development in this area.
Grazing management techniques are important, and the use of mixed swards
containing bioactive forage species, and certain nutraceutical feed additives are
being investigated. Genetic improvements, with breeding lines selecting for
resistance to worm species is also in the mix for the longer term.
None of these methods alone will control worms to a level where they no longer
cause production loss, but combined with an increase in on farm monitoring, and
correct, targeted use of anthelmintics when necessary, they could provide a sustainable long term solution.
Target your treatments
Using a targeted selective treatment (TST) strategy is a more sustainable method of
treating worm burdens in cattle, and protecting production. It is also something that
can be introduced over time, testing and refining the technique so that it works for
The aim with TST is to focus treatment on animals carrying significant worm burdens,
avoiding whole group treatments, whilst maintaining health and productivity. This
approach slows the development of resistance by allowing low numbers of worms to
persist in untreated animals, so any resistant worms that survive in treated animals
are diluted by this mixed population.
When you’re targeting animals for treatment, remember the 80:20 rule. Around 20%
of animals carry 80% of the worms, so a proportion of healthy animals in good
condition can be left untreated without adverse impact on health or productivity.
Faecal egg counts (FEC) can be used to assess worm burdens in young animals in
their first few months at pasture. This becomes a poor measure of worm burden as
immunity develops, but sequential pooled FECs can still provide an indication of
pasture contamination as the season progresses.
There are also indirect indicators of which animals are likely to have a high worm
burden and may need treatment. Animals failing to meet target live weight gain can
be targeted for treatment, and regular weighing of animals can provide broader
insights into productivity. Alongside this, general monitoring of thrive can give an
early indication of parasite problems, which can then be investigated further.
Checking that your wormers are working properly is the final, important step. A
pooled FEC taken before and after a treatment (one week after treatment with a 2-LV
product, and 2 weeks after a 1-BZ or 3-ML) with an egg count reduction of less than
95% may be a sign of resistance. But, it’s important to remember that treatments can
be ineffective for other reasons, such as incorrect dosing or administration.
Click to watch the webinar
Find out more about this topic in our recent webinar with Professor Eric Morgan, and
Boehringer Ingelheim Animal Health’s Technical Services Manager, Sioned Timothy.
Watch on demand to discover more techniques to achieve sustainable parasite
control at pasture, and the impact that wormers are having on parasites, our cattle,
and the environment.
For advice on parasites and parasite treatments, speak to your vet or animal health advisor.
1. NFU Online (2020). New study reveals cost of worms to UK cattle farmers [online] Available at: https://www.nfuonline.com/sectors/animal-health/animal-health-news/new-study-reveals-cost-of-worms-to-uk-cattle-farmers/ [Accessed 4 March 2021]