Within grass-based farming systems, young stock are widely exposed to nematode parasites such as lungworms and stomach worms. Depending on weather conditions and parasite control practices, worm burden can build up to high levels within a few weeks after turnout. Infected young stock show symptoms such as diarrhoea and reduced liveweight gain, with studies recording a 30kg to 40kg weight reduction in untreated calves compared with treated calves after a few months of grazing.

Similar to any other pathogen infection, the immune system is the animal’s own worm control tool. Full immunity is usually achieved at the end of the second grazing season. In the meantime, worm infections are controlled by combining two main tools: grazing management and anthelmintic products (worm doses). Specific grazing management practices (such as strategic rotational grazing and alternate grazing of young stock and adult cattle) can slow down the build-up of worms on pasture and considerably reduce the challenge faced by young stock. In addition, anthelmintics are typically used to treat and prevent worm infections, as a complement to grazing management.

Given their high efficiency and convenience, anthelmintics have been widely used in the last decades, for purposes of treatment or prevention. This intensive use has led to development of resistance in many areas, with unusual numbers of worms surviving treatment. Resistance is defined by a less than 95% reduction in the number of worm eggs excreted in faeces after correct treatment (dose rate, dosing equipment and product storage etc).

In practice, resistance is observed where young stock continue to suffer from worm-related diarrhoea and/or reduced weight gain despite correct anthelmintic treatment. The evidence of anthelmintic resistance worldwide is growing.

Other options

To preserve anthelmintic efficacy, new practices are already being promoted. These include weighing animals to avoid under-dosing, enhancing the role of grazing management and improving the timing of anthelmintic treatment. For example, it is now recognised that the dose and move grazing strategy (dosing the animals and moving them immediately to a new pasture) leads to a build-up of resistant worm larvae on young stock pastures.

To maintain the effectiveness of anthelmintics, recent studies have attempted to reduce the use of such drugs by identifying and selectively targeting treatment to animals based on growth rate and evidence of worm burden. Reducing anthelmintic use has two major benefits: it slows down the progression of resistance and it enables young stock to become immune as quickly as possible.

Targeted selective treatment (TST) strategies, in which treatments are selectively given to only part of the herd, at targeted times over the grazing season, are a promising solution.

Targeted selective treatment (TST)

As TST strategies are based on leaving some animals untreated, we need highly efficient indicators to decide which animals require treatment. Previous studies have used both parasitological measurements (faecal worm egg counts) and/or performance indicators (liveweight gain).

Three previous Irish studies have reported significant reductions in anthelmintic use while recording similar weight gain between treatment and control groups. They involved parasitological analysis, which can become expensive, time-consuming and impractical for many farmers. Performance indicators can be more practical and the use of liveweight gain (LWG) is an attractive alternative. In New Zealand and Sweden, two trials implemented LWG-based strategies in calves. Both studies showed promising results. Calves in TST groups recorded only slight reductions in LWG and similar faecal worm egg counts to calves treated monthly.

Recent Irish TST study

A study was conducted last year at AGRIC, Teagasc, Moorepark, to evaluate if a LWG-based TST strategy could be successfully implemented in Irish dairy calves. We defined four objectives for the study. It had to:

(i) Limit worm infections.

(ii) Maintain calf LWG.

(iii) Reduce anthelmintic use.

(iv) Be practical for farmers to implement.

Between 6 May and 7 August, we compared the performance of a TST group (50 calves, individually treated if they satisfied the criteria outlined below) to two other groups – no treatment (10 calves, all left untreated to evaluate the overall worm challenge) and group treatment (50 calves, all treated with a long-acting anthelmintic to serve as the reference point). Calves were mixed and rotationally grazed in groups.

The TST calves were individually dosed within a week if they met one of three criteria:

Main results and conclusions

Overall, 29 of the 50 TST calves were eventually dosed. In each of these cases, calves were treated due to poorer LWG criteria rather than scour assessment. Thus, our LWG-based TST strategy reduced anthelmintic use by 40%. This reduction did not impair calves’ health. There was no significant difference in weight, LWG, or faecal worm egg count between TST and the treated group at the end of the study. The results of this study indicate that worm infections of Irish dairy calves could be controlled using a TST strategy based on liveweight gain without any significant effect on calf performance. When adapted to specific on-farm conditions, implementing such a strategy has two main benefits: it helps young stock acquire immunity as quickly as possible and it slows down the progression of anthelmintic resistance. The TST strategy, however, is more time-consuming, requiring more regular calf monitoring to detect clinical signs of diarrhoea or coughing, and frequent weighing of stock to select calves in need of treatment. It should be noted that this study was conducted over a single year and although mean temperatures were similar to the 10-year average, total rainfall was unusually variable for the duration of the study, which may have affected the development of larvae on pastures.

Conclusion

If practical, efficient and, economical methods of determining the requirement for anthelmintic treatment in calves are further researched and promoted, the levels of dosing required on farms can be significantly reduced.

Katell Delaby is a French vet. She graduated from Nantes veterinary school in December 2015. In her last year of studies, she specialised in large animal practice and spent 6 months in Teagasc Moorepark to study new strategies for worm control in calves.