Parasite resistance vs. parasite tolerance

[Beekissed]

The other day I had read a conversation on here or another forum about breeding/culling for parasite resistance in hair breeds and I can't remember where I was reading it, so figured I'd open the discussion here.

I've been studying up on what folks are trying to accomplish in these hair breeds and how they are trying to breed for parasite resistance in them to preserve the breed characteristics of the same.

Then I think it was Bay who mentioned that her best ewe, with the best condition and with the best lambs, had the heaviest parasite load when she did fecals. That got me to thinking and doing research, as somewhere in my past research I had run across this as well. Something about the Soay sheep....then I found this article: https://askabiologist.asu.edu/plosable/parasites-wild-world

...and I will post more of the same when I get the chance to expand my research...gotta get out there and get busy! But I think we may be approaching the whole parasite issue a little one sided, whereas this article kind of outlines why. Instead of trying to only breed for animals that carry little to no loads of parasites, why not also breed for animals that can carry acceptable loads and still produce well?

Any given year or time in a flock's life, the parasites on the land could be heavier due to weather conditions, changes in flock dynamics, aging sheep, lambing, lactating, etc. could expose those sheep that are resistant to a state of having to be tolerant...and what if they could then not tolerate a load of parasites? Wouldn't it be better to look for a balance in that breeding wherein you breed for resistance AND tolerance at the same time and going less by the fecals and even the FAMACHA, and more by the conditioning and production of each individual sheep?

All questions I've always had in my chicken flock as well, as I don't deworm them and never have, but only cull by conditioning and production alone. I keep and breed the best and cull the rest, culling any that have aged out of good laying for their particular age, etc.

What did all those shepherds do since the beginning of time and before the advent of chemicals to aid them in keeping flocks parasite free? They culled by conditioning and performance, only keeping those that stayed fat and produced well without supplementing their feed and supporting their health with extra measures.

Thoughts?

 

[Beekissed]

Here's another good article about sheep and goat parasites and dewormers, selecting for tolerance, etc. It's a great little read.

https://www.thefencepost.com/news/mass-treatment-deworming-not-recommended-for-sheep/

“What we have left is selection criteria,” he said. The bottom line is sheep and goats will always have parasites. The key is selecting for animals that can tolerate them.

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In most cases, 20 percent of the animals in the group cause 80 percent of the problems. “That is why we need to select animals with some parasite tolerance,” he said. “Animals will never totally be void of parasites. They will always have some. Management-wise, I would keep a list and gradually take animals that I treat the most off that list by culling them.”

 

[Beekissed]

https://www.princeton.edu/news/2014/08/07/wild-sheep-show-benefits-putting-parasites

“This study shows that parasite tolerance can have a profound effect on animal health and breeding success,” said Schneider, who is familiar with the work but was not involved in it. “In the long term, this suggests that it could be profitable to invest in breeding tolerant livestock.”

https://www.sciencedirect.com/science/article/pii/S0944200616300368

https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001989

https://pdfs.semanticscholar.org/fbba/d2319efb6f4287c068fdd41571610c7370a5.pdf

https://www.asas.org/taking-stock/b...5/fighting-against-resistance-parasitic-worms

 

[mystang89]

why not also breed for animals that can carry acceptable loads and still produce well?

I'm not really in much of a position to argue either side but my first impression would be that breeding for parasite resistance would be superior to tolerance.

If you have sheep that are resistant to the parasite doesn't that mean the parasite can't live and simply does, ending the cycle? After some time you should be left with an area that has a relatively low parasite count.

Whereas with tolerance you still have all the parasites and you are really just increasing the total amount in the area. Eventually the animal will be overcome by whatever parasite they have if the load is great enough.

Another thing to think about is the type of parasite. Most parasites we think of are those that attack the stomach, however you must also think about those that attack the nervous system and heart. It doesn't take many of those too deal a lethal blow and if you breed for tolerance then this may not give you much time to react when they finally are overcome; symptoms may not exhibit until to late, which is already a common trait in many sheep.

Just my thought.

 

[Roving Jacobs]

There was a lecture at the jacob sheep meeting about parasite management that touched on this topic by a local extension agent. He brought up that some animals just don't seem that bothered by their parasite loads and he cautioned against using FECs and FAMACHA scores as your sole deciding factor for cull decisions. He said that merinos used to be really big here in OH and they were culled really strongly for parasite resistance. They ended up being super healthy animals in that regard but their fertility ended up being crap because that factor was forgotten when deciding on breeding animals.

He also mentioned that FECs are only a brief snapshot in time. Ewes tend to have the highest parasite loads about when most producers wean, a few months after lambing, but as soon as they aren't supporting a lamb anymore many are able to "self cure" and reduce their parasite load/increase FAMACHA score without intervention. Unless you are doing fecals regularly, especially after using a dewormer, you aren't getting the whole picture.

At least that's what I got out of the talk! I was very tired so hopefully it isn't too off base.

 

[Beekissed]

There was a lecture at the jacob sheep meeting about parasite management that touched on this topic by a local extension agent. He brought up that some animals just don't seem that bothered by their parasite loads and he cautioned against using FECs and FAMACHA scores as your sole deciding factor for cull decisions. He said that merinos used to be really big here in OH and they were culled really strongly for parasite resistance. They ended up being super healthy animals in that regard but their fertility ended up being crap because that factor was forgotten when deciding on breeding animals.

He also mentioned that FECs are only a brief snapshot in time. Ewes tend to have the highest parasite loads about when most producers wean, a few months after lambing, but as soon as they aren't supporting a lamb anymore many are able to "self cure" and reduce their parasite load/increase FAMACHA score without intervention. Unless you are doing fecals regularly, especially after using a dewormer, you aren't getting the whole picture.

At least that's what I got out of the talk! I was very tired so hopefully it isn't too off base.

That's what I've gathered from the posted article links also....that the whole parasite resistance movement is too one sided and true parasite control of any herd depends on many, many factors. If only focusing on one side of the issue, one is bound to lose the bigger picture.

The bigger picture is that the parasite population grows stronger throughout the world, while the population~both animals and people~have grown less resistant and/or tolerant due to the use of dewormers and, now, the focus seems entirely on resistance rather than also factoring in natural tolerance.

Like your speaker said, worm loads are not static in any given herd or flock and there are many factors as to why a sheep, at any given time, is carrying more than is considered acceptable.

I think, if the sheep are supported nutritionally(and I don't necessarily mean with grain supplements but with just good pasture, hay and browse) through periods of heavy infestation and they are maintaining good conditioning during those times on their own genetics, they are able to tolerate and move past those vulnerable times. Those that do not maintain good condition during those times, but seem to have a very light parasite load at the same time, would still be a cull for me. Sure, her poop may look fine but she does not...to me that separates the men from the boys when it comes to sheer hardiness in tough conditions.

Then that brings the whole parasite loads in the soil into question....will tolerant sheep(moving in and out of heavy loads at any given time)shed so many worm eggs so as to build this massive ticking time bomb of parasites in the soils? There too people are not thinking of parasites as something that can die or thrive depending on healthy or poor soils. Weather conditions, soil health, populations of good nematodes in those soils, etc. all factor into if sheep that are tolerant are also "filling up the land with parasites". Parasites have their own parasites, in a way....things that consume them but do not complete the parasite cycle, diseases that kill them, conditions that halt their development, etc.

Tunnel vision has developed over time when the whole issue is a much bigger, more multifaceted thing than what is currently in mode. I think some are starting to wake up to that, hence more speakers making noise about tolerance along with resistance and not just one or the other.

 

[mysunwolf]

...
Then that brings the whole parasite loads in the soil into question....will tolerant sheep(moving in and out of heavy loads at any given time)shed so many worm eggs so as to build this massive ticking time bomb of parasites in the soils? There too people are not thinking of parasites as something that can die or thrive depending on healthy or poor soils. Weather conditions, soil health, populations of good nematodes in those soils, etc. all factor into if sheep that are tolerant are also "filling up the land with parasites". Parasites have their own parasites, in a way....things that consume them but do not complete the parasite cycle, diseases that kill them, conditions that halt their development, etc.
...

YES. When you have a climate that is 50-80 degrees for 6 months out of the year with heavy rainfall like we do in the Southeast, you have a recipe for disaster in letting these tolerant, but not resistant, sheep hang out on your limited acreage. We have this problem every year and are still identifying and culling those animals. They also create a rough time for everyone's lambs each year, since lambs have less resistance.

 

[Beekissed]

YES. When you have a climate that is 50-80 degrees for 6 months out of the year with heavy rainfall like we do in the Southeast, you have a recipe for disaster in letting these tolerant, but not resistant, sheep hang out on your limited acreage. We have this problem every year and are still identifying and culling those animals. They also create a rough time for everyone's lambs each year, since lambs have less resistance.

I think, therein lies one of the facets of parasite loads on the land and in a herd. Should we be stocking so heavy on limited acreage that we create soil conditions that cannot handle the loads? At any given time of the year or in changing weather conditions(we are getting warmer and wetter overall), can the soils we have support the type and number of stock we have on it?

It's not all about the herd, but also about the soils. Soil has a health that needs maintained if the herd health is to be maintained normally, be it in whatever region. Stocking rates, resting of the soil, etc. all play into parasite loads, so focusing on merely resistant sheep~that at any given time can be LESS resistant, as is detailed in the many articles posted on this thread~is turning out to be a little short sighted and some leaders in farming of ruminants are waking up to this.

Also, in a reference to another really great thread on BYH by TOR~https://www.backyardherds.com/threa...the-landscape”-not-the-other-way-round.39681/~does the livestock fit the landscape?

Whereas sheep and goats do well in more arid and/or cooler places, do they do well in hot and humid regions? If so, what breeds do well there and is one utilizing that breed?

All of these things are great discussion fodder for the problem of parasites in ruminants.