Combination Doses Better
By Dr Mackie Hobson BSc(Agric),BVSc

Thursday, 1st June 2017

As the SAMGA vet I regularly come across roundworm resistance when analysing faecal samples, doing faecal egg count reduction tests (FECRT) or conducting Angora goat post mortems.

Roundworms are by far the biggest health problem facing the Angora goat farmer, a problem which is unfortunately likely to become worse over future decades.

Anthelmintic resistance develops much faster in Angora goats than in sheep as goats metabolise anthelmintics (dose) at a faster rate than sheep. This is the reason why vets will off licence often advise using 1.5 to 2 times the dose rate of sheep in goats. It is important to note that Levamizole, Rafoxanide, Closantel and Ivermectin have a narrower toxic range compared to most anthelmintics and so should be used with care in Angora goats (especially kids and goats in poor condition).

It is no longer acceptable for farmers to view parasite control in terms of a “deworming program” and roundworm management strategies need to be implemented to reduce roundworm resistance to doses. For articles on Roundworm management  to maintain susceptible roundworm populations (‘refugia’) and Breeding worm resistant Angora goats can also see our website

Vets in the past advocated that farmers use a different dose (active ingredient) each time they dosed their goats. This is changing and it has now been proved that the use of combination doses is better at slowing down parasite resistance.

How and why do combination treatments work?

Research done in New Zealand has convincingly shown that the best approach is to use several different de-wormers all at one time as a combination treatment.

There are 2 major benefits to using drugs in combination:

  1. You get an additive effect with each drug used, thus the efficacy of the treatment increases with each additional drug given.
  2. By achieving a higher efficacy, there are fewer resistant worms that survive the treatment, thus there is a greater dilution of resistant worms by the susceptible portion of the population.

The sooner you start using a combination; the better off you will be, since the more drugs that are used in combination, the greater the efficacy of treatment will be.

To explain why combinations help reduce the development of resistance, but rotation of de-wormers does not:

  • If two drugs each with 90% efficacy are used in rotation, then each time the goats are dosed 10% of the worms survive (the resistant ones).
  • If these same two drugs are used in combination at the same time, then the efficacy increases to 99%.

This calculation involves a simple additive function; the first drug kills 90%, and the second drug kills 90% of the remaining 10% [90% + (90% x 10%) = 99%].  Thus the efficacy achieved is now 10X greater and this then yields 10X fewer resistant survivors.

Because fewer resistant worms survive at each treatment, there is a greater dilution of the resistant worms among the majority of worms – ‘refugia’.

In contrast, if using a rotation of drugs, you would get 10X as many resistant worms surviving each time you treat.

Having resistant roundworms in your Angora goat flock does not mean that all the worms are resistant. Once efficacy of a dose falls below 95% it usually worsens over time as more and more doses of that drug are given.

To see which dose combinations are available see our website and under the vet’s corner see the section under drugs and anthelmintic drug list.


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