January 29, 2009

Drench resistant worms in sheep - resilience and resistance

Sheep, farming, husbandry, disease, animal health, worms, internal parasites, resilience, resistance

By Dr Clive Dalton

Resilience & resistance
  • These terms appeared only recently on the farming scene from the research world, and without a lot of explanation.
  • They sneaked up on us so it’s not surprising that some farmers are a bit skeptical that the issue has been made overly complicated by academics.
  • The two terms are different concepts and it’s important to understand the difference between them to farm sheep sustainably.
  • Veterinarians, consultants and ram breeders are certainly using these terms now – and hopefully they all agree on their meaning. It doesn't always seem to be the case.

There are three parts to the story. You can have:

  1. Resilient animals
  2. Resistant animals
  3. Resistant worms
1. Resilient animals
  • The term ‘resilience’ is straight forward as it only applies to the sheep. It’s sometimes called ‘host resilience’ but ignore this as there is enough confusion.
  • The main thing is that resilient animals can tolerate a worm burden and still keep on producing well. They need less drench but they still contaminate pasture with worm eggs. This is very important.
  • Note that resilient animals are not themselves resistant to worms – they only tolerate them without any ill effects.
  • It’s when the worms in resilient animals become resistant to chemicals that things get difficult, as they are still producing eggs in large numbers.
  • You may see the word ‘tolerance’ used to describe resilience. Don’t use it either to avoid more confusion.
  • Geneticists state that resilience as a genetic trait is complex and weakly inherited. Heritability estimates of 5-15% have been published.
  • Key point: Note that a so-called resilient animal can carry both resistant worms and non-resistant (susceptible) worms, and the only way to find out which is present is to do a ‘Faecal Egg Count Reduction Test’ (FECRT) (see later).

2. Resistant animals (host resistance)

  • Resistant animals have developed immunity to their parasites so have lower worm burdens and hence they have lowered FECs.
  • Resistant animals do two things. They are able to prevent the establishment of worms in their system, and then evict any that do establish. They pass fewer and less-viable worm eggs on to the pasture, and this helps even more to break the parasite life cycle.
  • Worm resistance as a genetic trait has been shown to have a heritability of 23-25%. This is in the medium range of traits but is high enough to say that good progress will be made if you generate enough selection pressure (Selection Differential) on both the male and female side. Traits with high heritability are lamb growth (38-48%), fleece weight and FE resistance (30-40), and fleece weight (50-80%).
  • The other good thing about resistance is that it also has a high ‘repeatability’. This means that once you measure resistance and find animals that are high, they should always remain high. This is especially useful for early identification of replacement breeding stock.
  • Animals with high genetic resistance to worms are said to produce more dags, because it is stated they produce excess mucus to flush the parasites out of the intestine. Be wary of this statement.
  • Key point: Use the term ‘host resistance’ to describe these resistant animals, then you won’t get confused with resistant worms.

3. Resistant worms
  • These are simply the worms that through a ‘reproductive advantage’ or ‘survival of the fittest’ have multiplied to large numbers to dominate the population.
  • Research so far shows that once they become resistant to drench chemicals they stay resistant.

Changing from resilience to resistance

There’s a lot of conjecture about this. Here’s some points you hear discussed.
  • We know that milk lambs can handle quite high worm burdens (above 10,000 epg) while still on the ewes, but then problems occur once they are weaned.
  • So from this you hear comments that lambs start out being resilient and then once their immunity develops (and their FEC drops to below 500 epg), they can be considered to be resistant.
  • If a lamb didn’t get below 500 epg and remained at 1000 epg, then it would remain resilient - in good health and happy to live with it’s worm burden.
  • All this is a nice concept but things are probably much more complex. Resilience and resistance could be the two ends of the same thing, but don’t hold your breath until it’s sorted out as I doubt if it’s even on researchers’ ‘to-do’ list.

Why would anyone want resilience?

  • Resilience is a daft goal.
  • Who would want sheep that maybe could live with a worm burden, but polluted pasture with larvae that we now know can live for long periods and even go down to 10-15cm in the soil when things get tough up top?
  • And the chances are also very high that they’d be great producers of dags.
  • With a resilient flock, imagine the words you’d need to instruct the auctioneer before he asks for an opening bid on your lambs in future.
  • Or worse still, what are you going to tell the prospective buyer of your farm when the day comes to sell and he/she asks about the drench resistance status.
  • And if you don’t sell and pass the farm on to a family member, what a great heirloom resilient worms will be to deal with. It would justify a plaque in the woolshed where the dags are laid out to dry commemorating your generosity!
  • So don’t complicate life by worrying about resilience. Ignore it.

Resilience as a breeding objective
  • Resilience as a breeding objective is daft!
  • Why would anyone want to farm sheep that maybe perform OK with a gut full of worms, but pollute the farm with larvae that are hard to kill in the environment?
  • Add to that the chances that their progeny for the whole five years in the flock will need to be dagged at least once every year, if not twice.
  • Breeding for resistance is the only way to go.
  • I suspect the difference between resilience and resistance has been made in to a big issue by academics, and not by sheep or their worms.
Disclaimer This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.

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