Massey University expert queries bull system sustainability

This article originally appeared in Country-Wide Magazine. Article by Sarah McVerry.

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

The Cooperia challenge on many intensive bull-beef finishing systems that buy in weaners and sell them as 18-month bulls could render them unsustainable in the future.

Massey University senior lecturer in veterinary parasitology, Bill Pomroy says resistance to ivermectin-type anthelmintics is now common and on many bull beef units levamisole drenches are the only remaining means of controlling Cooperia.

Pomroy and colleagues at Massey University have been studying anthelmintic resistance on the University’s bull beef unit, Tuapaka for more than 10 years.

Massey University first set up the unit on 111ha of flat country, at Aokautere in 1983. Since then it has run only bulls and steers. Each spring the unit buys its 250 (100kg) weaner bulls from three local contract rearers who source the calves from Massey University’s three dairy farm units. The bulls spend one winter on the unit and are finished to a target liveweight of about 500kg at 18 months.

Ostertagia remains the number one parasite threat to cattle, but fortunately is yet to develop resistance to anthelmintics. The Cooperia parasite is unlikely to kill cattle, but can be a production cost in cattle under 18 months of age (see following story). In large numbers it causes scouring and slows growth rates in the first autumn/winter in young cattle.

Pomroy’s suspicions that ivermectin had lost its efficacy in controlling Cooperia at Tuapaka were confirmed in 1993. Faecal egg counts were reduced by only 92%, 14 days after ivermectin injectable was administered.

“Anything less than 95% indicates that resistant parasites are not being killed and are still producing eggs which will then accumulate on pasture.”

In 1995 they compared the effectiveness of a range of injectable ivermectin-type products on the market. Ivermectin injectable, moxidectin injectable and doramectin injectable had allowed eggs to be shed within their claimed protection periods indicating leakage from resistant parasites.

“The point you can make here with injectables is there is very little difference between ivermectin and moxidectin, both were similarly ineffective in controlling Cooperia.”

Pomroy says the result for doramectin of 100% reduction in faecal egg counts at day 7 is an aberration caused by the temporary paralysis, rather than death of parasites in the gut, which means they begin to lay eggs again within 21 days.

“So you have resistance to all three ivermectin-type drenches at about the same level and moxidectin was not superior to ivermectin as an injectable formulation.”

In contrast the ivermectin bolus was effective against Cooperia, although Pomroy has since conducted another trial where resistance was found (see following story). Levamisole was 100% effective.

In 1998 Pomroy decided to trial emerging pour-on drench products.

“I wouldn’t say anymore than eprinex didn’t work. There was clearly resistance.”
In the meantime Pomroy’s parasitology colleagues in the United States confirmed that Cooperia at Tuapaka were also resistant to the benzimidazole (BZ) family of drenches.
“The results from those laboratory tests showed that 49% of the Cooperia parasites tested were homozygous for the gene for BZ resistance and most others were carrying one copy of the gene indicating a very high level of resistance genes in this population.”

This was demonstrated by the results of drenching with Oxfendazole in 2001 which reduced faecal egg counts by only 19%. However a combination of the Ostertagia targeting drench, albendazole and levamisole for Cooperia control resulted in 100% faecal egg count reduction.
“So the only thing left on this farm that is still killing Cooperia is levamisole. But I don’t think that’s unique. There will be many bull beef units in the same position.”

Pomroy says the Tuapaka data raises the issue of sustainability, because of the heavy reliance on anthelmintics in systems running young bulls.

“They’re not developing a fully mature immune response until 18 months, so in the meantime you’re running a monoculture of susceptible animals that require help to control worms. And how long will it be before levamisole falls over?”

Levamisole resistance in sheep parasites is now common, indicating it is highly possible in cattle, says Pomroy.

Pomroy says a survey of bull beef farmers to establish the extent of anthelmintic resistance is needed as the present understanding is largely dependent on anecdotal reports. Regardless, farmers need to consider strategies to avoid resistance to levamisole from developing such as integrating bull beef and sheep or specialist crops.

“Don’t run bulls on the same paddocks all the time and use the older ones that are more tolerant to follow up behind young bulls.”

He suggests farmers should be considering where the “refuge” populations of anthelmintic susceptible parasites is on their farm such as older cattle which aren’t regularly drenched.

Institute of Veterinary, Animal and Biomedical Sciences
Massey University