Article Index "Drench Losing Its Power Against Parasites" Article Index

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By Dr. Clive Dalton, 2002
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There's a potential disaster on farms - the zillions of internal parasites (worms) sucking the blood and blocking the guts of sheep, goats and cattle have already developed, or are developing, a resistance to the chemicals used in drenches to kill them.

Goats are worst affected and some farmers have had to put them indoors. Cattle are showing the early stages of the problem.

Over time, and it's been a very short time, worms have developed a resistance to the three main chemical families used in drench.

So the drenches don't work as well and worms multiply at even faster rates. There are no new chemical families coming up from the research labs anywhere in the world.

It's happened in worms as in many other bugs - if some of them are not killed by treatment, and get only half clobbered, they survive and breed the next generation and resistance develops.

Now it's not difficult to work out how this has happened and will continue. There are, of course, folk who say the problem is exaggerated but many reasonable farmers are worried.

Drenching is a basic chore on the farm. Indeed it saves many a boss from exposing what a disorganised manager he really is.

When the worker asks the boss what jobs are lined up, he's on the spot! After rolling a smoke, the suggestion to go and drench the hoggets always saves his bacon.

Generations of hoggets have been drenched because the boss couldn't think of anything better to do. It's a great job, and at the end you feel that you've slaughtered millions of parasites now writhing in agony and heading for the sheep's rear portal.

Accountants are also to blame. Farmers hate paying tax so they were advised by generations of accountants to buy product at the end of the financial year.

I remember woolsheds stacked high with drench that ended up every month through hoggets, needed or not. It even became fashionable to drench ewes two or three times a year.

Then there are the pharmaceutical companies who come out with new sexy names for the same old chemical drench families, sounding like new drenches.

The pour-on was developed and was even more fun to apply. You just pour it on with much less hassle than trying to hit throats with the gun. Many farmers thought the pour-ons were the answer and didn't read the active ingredients.

A new product called anendectocide appeared. It is a drench that kills parasites on the inside and on the outside at the same time. The limited chemical family problem remained.

Veterinary clinics became mini-supermarkets with specials to encourage sales. Vets after all are in business to make profits.

These specials, if you buy the larger quantities, include every gimmick you can think of from Swiss army knives to Bledisloe Cup tickets. Why don't they reduce the price of drench instead?

But some people have stopped to think. One enterprising farmer has developed a kit with which farmers can check their sheep's faecal egg count to see if they have a high load of internal parasite eggs before wasting money on drench. This has been taken up with great enthusiasm by British farmers, who have the same sheep problems as we do.

Why don't pharmaceutical companies give away some free faecal egg count kits?

Some farmers are selecting sheep that have a natural resistance to worms, and this is progressing at a surprisingly good rate.

Others are even taking an organic approach and using home brews containing cider vinegar.

Emphasis has also returned to better pasture management and moving lambs on to clean pastures.

About the author: Dr. Clive Dalton is a former scientist and Waikato Polytech tutor, and technical editor of


Agricultural Research Service

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