Drench resistant Cooperia build up rapidly

This article originally appeared in Country-Wide Magazine.

BIll Pomoroy
Institute of Veterinary, Animal and Biomedical Sciences
Massey University

A trial designed to investigate the production cost of Cooperia in cattle indicates large populations of ivermectin-resistant Cooperia can build up rapidly.

Bill Pomroy says the trial was set up on a property with known ivermectin- resistant Cooperia in order to observe the effect of ‘survivor’ Cooperia on one group of animals, compared to another group effectively treated with anthelmintics.

There were four replicates of two different drench treatments. In total eight paddocks with 140 weaner bulls split evenly between them were observed from January 1 until July 1 2001. Four paddocks of the weaner bulls were treated with Ivomec pour-on at four weekly intervals from January 1. The other four paddocks of bulls were treated with a combination benzimidazole (BZ) and levamisole drench at four weekly intervals and a 130-day Ivomec bolus. Faecal egg counts were taken monthly from each of the eight mobs until July.

The mean faecal egg counts for cattle treated with Ivomec pour-on increased steadily from about 90 FEC (eggs/g) in February to about 350 FEC in April, but then began to drop steadily until the end of the trial.

Bill Pomroy says although 350 FEC would be considered a “time to drench” level of parasite infection, it indicated only modest worm burdens in the bulls treated with pour-on. The decline in egg count in the second half of the study indicates the usual development of immunity to this particular worm and resulting lowered FEC.

The mean faecal egg counts for cattle treated with the combination drench and bolus hovered just under 50 FEC until May when it began a gradual increase, ending the trial in July at about 100 FEC.

Pomroy says this indicates the bolus, which produced a 100% reduction in faecal egg counts in previous studies at Massey University’s bull beef unit, was not effective.

“I was planning for that combination drench and bolus to produce zero FEC, whereas I was expecting leakage egg counts increasing in the other mobs treated with Ivomec pouron.”
The liveweight of each of the eight mobs of cattle was monitored at the same intervals as the faecal egg counts. The mean weights indicated the four mobs treated with the combination drench and bolus were 14kg heavier at the end of the study, but Bill Pomroy says this was not statistically significant.

The most telling aspect of the trial were the worm counts of tracer calves, which grazed the eight paddocks for two weeks at the end of the trial. These were 21,000 Cooperia in tracer calves that had grazed where bulls treated with Ivomec pour-on had spent the previous six months. In contrast, tracer calves that grazed where bulls had been treated with the combination drench and bolus only had an average of 1300 Cooperia. Pomroy says this difference is statistically significant and raises the question of the production impact from grazing those areas for a second season.

“The main message is that Cooperia didn’t cause a significant production effect in the first season, but you would have to query what would happen if the trial was continue for another year.”

He says the trial results mimic the situation where a farmer uses an ivermectin-type drench in the presence of ivermectin-resistant Cooperia.

Institute of Veterinary, Animal and Biomedical Sciences
Massey University