help! parasites in sheep in the tropics

Hey guys, anyone have experience with raising sheep in the tropics? I have 5 right now. They started as 8 and even reached 12 at one point. I have treated them with standard medicine many times and without hesitation but their illnesses seem to come on extremely fast and sometimes the medicines just dont work. I feel terrible keeping them but others in my area who have sheep dont seem to do any better. They have a mineral/vitamin block available to them and we give them mineral salts as well. We do not rotate them and they use the same pen thats quite large for them and mucked every 2 days or so. They get both internal parasites and mites. We would love to raise sheep and want to add goats as well but morally cant do this until we work this out. I have heard rotating pens and pasture will help a lot so we are working on setting that up but i think they must still be deficient in something to be getting sick so often. When we successfully treat them it never lasts more than a couple weeks. I think I will also try mixing in some diatomaceous earth with their honey (byproduct of sugar production around here) and advice on how much per animal and how often would be great!

Are they on grass or penned up and getting 100% supplements?

They are on grass and because our fencing is generally for cows they can roam through several large pastures easily. The pastures have about 5 different grasses with a little bit in the way of legume options.

If you can afford it I'd get a vet to do an autopsy to find the exact reason they're dying.
If they're dipped correctly as long as they don't contact other sheep they shouldn't get lice again, seems strange they're just falling over.

Yes, DE is a good move!  The 3 keys to maintaining sheep (and other livestock) health is 1. rotational grazing 2. Diatomaceous Earth 3. garlic/molasses treatment.

For the DE, offer it free choice, mixed in with kelp meal.  Ratio of kelp meal to DE is 4:1.  The food grade DE is a fine powder and can cause irritation in the sheep's lungs in inhaled.  Mixing in with the kelp meal mitigates that and increases palatability.  

As for garlic treatment - it is a natural deworming alternative that is very effective if #1 and 2 above are also in place.  Per head: 1 Tbs garlic, 1 Tbs molasses, mix with a small amount of hot water until even consistency.  Use a drenching syringe and administer orally.  20 mL for a lamb, 40mL for a ewe, 60mL for a ram.

I hope this helps.