Irish livestock production systems are mostly based on grazing grass, and an inevitable consequence of this is infection with parasitic nematodes such as lung and gut worms.

The big problem is, recent Irish studies have shown treatment failures involving ivermectin on dairy farms, and varying levels of resistance to all commonly available wormer (anthelmintic) classes in gut worms.

We only have three classes of broad-spectrum wormers: the white drenches (benzimidazoles), the yellow drenches (levamisole) and the clear liquids (macrocyclic lactones), which include the ivermectin-type products.

There isn’t a conveyer belt of new wormers coming on stream — not for over 30 years in fact. Anything new is just a different version or combination of what is already there.

Anthelmintic resistance is found globally, having first been reported in Argentina in early 2000s. These products are vital to the success of pasture-based farming, but poor practices around product usage are often the main contributing factor to the worms developing resistance.

In 2019, I surveyed 249 dairy farmers in 2019 around various animal health practices on farm. Over 90pc of farmers were blanket-treating every animal in a group when dosing for worms.

The problem is that when a group is blanket-treated, all susceptible worms are killed off and a tiny number of resistant ones remain; and these are the ones that produce eggs that eventually seed pasture when next grazing.

Over time, blanket group treatments slowly shift the worm population towards more resistant types.

But if, for example, we decided to not treat some animals when worm dosing based on live weight performance, or on faecal egg counts in dung, we would preserve susceptible worms within our herd. 

This counter-intuitive concept is known as ‘refugia’ — the idea is that you provided shelter or protection to the gut parasites from worming products, after years of being told that continuous, whole-herd dosing regimes was the most effective way to manage them.

On our farm, targeted selective treatment is carried out based on live weight gain of calves in their first grazing season, aiming to strike the balance between allowing them enough exposure to develop immunity to roundworms but not so much as to cause ill thrift.

The curve ball in Irish grazing herds is the more unpredictable issue of lung worms. High summer rainfall can be favourable for the development of lungworm larva on pasture; these can cause infection of varying severity often known as Hoose.

The best way of preventing this in first-season grazing calves is by vaccine. Two doses are given, at eight and 12 weeks of age, with immunity developing over the following two weeks.

This is not a common practice in Ireland as some herds turn out calves to pasture before this time-frame, but in herds where automatic feeders are used and calves are housed anyway, it is a good option.

Teagasc research has shown that once calves were vaccinated against lungworm, a reduction in wormer treatments could be achieved without an effect on animal performance.

Overall, the measures to slow the development of anthelmintic resistance involve selective, targeted usage of wormer products; keeping that ‘refugia’ population of nematodes; and preventing further introduction of resistant worms through quarantine management of bought-in animals.

Each of these steps requires a considerable change away from how we do things now, and discussion with your vet to draw up a farm-specific strategy.

Change is required on farms today because once resistance appears, the clock cannot be turned back easily.

Gillian O’Sullivan farms with her husband Neil near Dungarvan, Co Waterford