Awareness of Drench Resistance Slowly Gaining Momentum

by Barry Easton


Worm resistance to a shrinking arsenal of drenches is here to stay and dairy farmers should make judicious use of existing knowledge and resources.

That’s the claim from Inglewood veterinarian, Doctor Jonathan Spencer, a year on from a ground-breaking study on the Inglewood property of New Zealand Grazing Company managing director, Ian Wickham, which focused on the issue of worm control in dairy heifers.

That study, which was conducted by Jonathan Spencer and fellow veterinarian, Peter Benn, in the autumn of 2006, turned conventional thinking related to worm control on its head.

It concluded:-

  • That when stock are being drenched, a percentage of the mob (about 10%) should be left undrenched
  • That after drenching stock should not be returned to “clean” pasture but instead, to a worm contaminated pasture
  • That no drench kills 100% of worms
  • That combination drenches should be used
  • That stock should not be drenched religiously every month, but only when necessary. Ideally this should be when faecal egg counts reach 250 epg (eggs per gram) or when cattle show reduced weight gain

 “The situation (the conclusions) have been confirmed both on Ian’s property, on our other farms (in the Inglewood area) and throughout New Zealand,” said Dr Spencer in a recent interview which focused on feedback and subsequent findings since the 2006 study.

Accumulating Evidence Confirms Study

“I gave a presentation based on this study at the Ancare Conference in Auckland. It was made to a large group of veterinarians and they certainly didn’t come up with any dissenting voices. I think the feeling was that the points made in the study were confirmed, and more evidence has certainly been accumulating.

 “Actually, at the same time that we were doing our study on Ian Wickham’s farm, something similar was being done, I think, in the Ashburton area. This study demonstrated similar findings.”

Count and Culture

Faecal egg counting and the culturing of worm eggs is becoming increasingly important in combating worm resistance, says Dr Spencer.

 “This is because when you look down a microscope, with one exception, you actually can’t differentiate between the eggs of the different worm species,” he says.

 “This is now regarded as increasingly important so that you can actually say whether you have got resistance with drench to every worm species or just some of the worm species.

 “I did have a very good illustration of this just a few weeks ago with a drench test which I did on a sheep flock. The rule of thumb for whether we have got resistance is whether or not the drench will kill or reduce 95% or more of the worm egg count.

 “In this particular case we did have a faecal egg count reduction with a particular drench of exactly 95%. In spite of that being the cut off, we decided we would do a larval culture of those 5% of worms which remained and when we did that we discovered that the breakdown of worms was such that the reduction for trichostongylus using this particular drench was only in the high 80%s and it wasn’t as high as 95%. Therefore with some of the worm species we had resistance and with others, we didn’t have resistance.

 “Whereas superficially, when we looked at the faecal egg count alone, it looked as though this particular drench had passed the test.

That really illustrates well the importance of larval culture.

Long Term Problem

“On another farm the farmer concerned felt that he had encountered drench resistance to white drench in his sheep flock 20 years before and he hadn’t used white drenches for 20 years.

 “When we did a larval culture just a few weeks ago, we discovered that resistance is still there, 20 years later. This fits in with the information which scientists are feeding back to us. Once you have got drench resistance, it won’t disappear for many, many years. It is so important that we try to delay the onset of resistance.”

Quarantine Drench

Quarantine drenching, stresses Dr Spencer, is also becoming increasingly important.

 “It is particularly important on properties like Ian’s, with animals coming from several different properties. They can bring with them different levels of resistance – different worm genetics which will bring resistance on to the property,” he says.

 “It is really important that when these animals come on to the property that they are drenched with a combination drench and really, it should be the best combination drench available, which is going to be something like ‘Eclipse.’

 “Similarly, when animals leave Ian’s property, it is in the interests of the dairy farmer that he drench them back on to his own property so that any resistant worms which may have been picked up on Ian’s property, are not taken back to his own property.

 “This is something which is slowly gathering momentum, but there are still a lot of dairy farmers in particular out there who don’t have an enormous grasp of this resistance.”

Economic Argument?

There are differing viewpoints, swayed by arguments of economic advantages, in drenching adult dairy cows, says Dr Spencer.

 “From the point of view of the clinical health of the animal, certainly older cows don’t need drenching. Even some of the younger cows don’t need it. If they have got an infection, it’s a sub-clinical infection, but because there is an economic argument there is a strong push from some of the animal health companies to drench all of the adult cows in the herd.

 “This goes right against what we are trying to do to limit blanket drench usage and to limit lowered resistance. We are fighting against the economic argument to get people to appreciate that a small economic gain now could mean a huge economic loss further down the track.”

Faecal egg counting, stresses Dr Spencer, is an important tool in determining animal health problems other than worms.

Dirty Tails? Worms or-

 “It may be worms which are causing an animal to have loose faeces or diarrhoea, but the thing to do is find out if it is worms by doing a faecal egg count. If the animal doesn’t have a high worm egg count then the chances are that the problem isn’t due to worms.

 “In addition to dietary causes there are other disease-causing entities like ‘Yersiniosis.’ This is a bacterial condition which can be quite serious. We see a lot of it in Taranaki and it requires anti-biotic treatment to fix it.

 “Having said that, many animals have ‘Yersiniosis’ sub-clinically, and get over it by themselves.

 “In addition to ‘Yersiniosis’ a much smaller number of animals can get ‘Coccidiosis.’ Coxidia are also an environmental bug and if they reach significant levels then they will cause disease too.

 “Again, you need to take dung samples in order to find out if this is the cause, much as you do with ‘Yersinia’ and with worms.”