How different worm control programs in a winter rainfall area can influence re-infection with worms following summer drenches

Research Need

In the high winter rainfall areas of SE-Australia, a 1- or 2- summer treatment program is generally quite effective at controlling worms. However, in areas with a longer growing season and consistent summer rainfall, rapid re-infection after the summer drenches can compromise worm control. This leads to high worm egg counts and additional drenching the following winter. For example, summer re-infection meant a pre-lambing drench was needed for ewes on an IPM-s demonstration farm near Ballarat – see the figure below.

Worm egg counts of ewes given 2 summer drenches (SD1 & SD2) and a prelambing drench

Key research questions

Does the use of a long-acting drench during the previous summer or winter reduce subsequent worm infections when compared to the standard 2-summer drench program.

Methods

We compared re-infection when sheep grazed paddocks contaminated with different levels of worm eggs over the late spring and summer.

There were 3 treatments:

• a standard 2-summer drench program
• a long-acting treatment that reduced contamination over summer to near zero, and
• no summer treatments, but a long-acting treatment the previous winter (Year 1) or spring (Year 2).

Conclusions

This study has not been completed. In the first year there was a lot of summer rainfall in Dec & Feb, and this would have allowed any differences in spring contamination, due to drench treatment, to be detected.

However, preliminary analysis indicated high worm re-infection occurred after each treatment and that there was no benefit of using long-acting treatments. In contrast, in Year 2 there were differences in spring contamination caused by drench treatment, but little rain and less pickup of larvae followed.

Consequently, no differences between the egg counts of each treatment were obvious. This study should give additional insights into what rainfall events determine re-infection after each summer treatment.

Personnel

Dr Norman Anderson and Dr John Larsen
The Mackinnon Project, University of Melbourne, Faculty of Veterinary Science, Werribee

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