First of all, what molecular studies you would like to do is not clear. Whether you want to localize the protein or mRNA in the tissue?

For fixative, you can use 4% Paraformaldehyde in PBS (see protocol for 8% PFA stock solution preparation in internet) instead of Bouin's fixation.

Once you fixed the tissue, it is not good to proceed with that for DNA or RNA isolation. So it's better to take fresh or snap frozen tissue for DNA isolation rather than the fixed one.

Dear Raju,

Thank you for your answer.

What i mean by molecular studies is that i want to do DNA extraction from the tissue, not exactly the tissue itself, but the parasite which attached on the tissue.

I see, so probably it is better to use two different preservation method for microscopy and molecular study. In case getting the sample is difficult, how long does frozen parasite can preserve DNA in a good condition?

Thank you

You should be able to preserve the tissue with the parasite in it under - 20°C  for DNA extraction after sample collection. As long as they are kept under this temperature, there is no problem. When tranporting them abroad, they should be packed in dry ice. However, if you desire to do microscopy otherwise fix part of each sample in 4% paraformaldehyde in PBS. Do not crush the kidney tissues (in the name of making smear) if you want sections. I do hope this will help you.

Good luck.