Yes I agree Telford 2009 is the best source to go through, and taking blood from the caudal vein is probably the easiest. However, I dont think that is what is causing your "strange things" on your slide (unless the host reptile is being put under lots of stress?).
I dont think these are particles of cells, I think it could be some sort of virus or bacteria (I have found some of these, but only in the blood cells), have you tried sequencing any of your samples? It also could still be as J.C. Toriano suggested be a fungus, however this is unlikely (especially if you fixed them in ethanol directly after collection), and fungi spores usually look a bit different.
If I was you I would try sequencing them to see if you can pick anything up, and to see if you are getting the same results from other infected samples and nothing from samples not infected.
What is the percentage of slides positive to not infected?
Do you fix them and stain them the same day, or do you fix them in the field and stain them a few weeks later in the lab? I would use methanol to fix the slides instead of ethanol.
Make sure your reptiles are not under to much stress. Do you use traps, or do you collect by hand? If using traps how long are then left in the traps and how long do you keep them before taking the blood samples?