Hello Elmar,
this is very interesting and I can only confirm. I am also in the south of france (would love to get in touch with your sister ???? ), and we just took over a flock this year. They were quite heavily invested in parasites. What we did is :
Started holistic grazing and moved to pasture that wasn’t grazed for 20 years. We also used garlic which worked very well for us.
We have a lot of pine and chestnuts so that probably helped as well.
Three month later we did another test and the parasite load went well down.
With genetics we also observe the same: there are some sheep that just seem to attract them. So a good breeding program will probably help in the long term.
There is an interesting herbal book called:
The complete herbal handbook for farm and stable, from Juliette de Bairacli Levy.
A lot of interesting information, she also stated garlic as the best remedy.