Mesnilium is a genus of parasitic protozoa of belonging to the phylum

Phylum

In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 Apicomplexia. Its vertebrate hosts are fish. The vectors are not presently known but are thought likely to be leech

Leech

Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways...

es.

Description

Merogony occurs in erythrocytes and reticulo-endothelial cells.

Gamogony occurs only in erythrocytes.

Pigment granules are present only in microgamonts and macrogametes.

Hosts

The only known host of this parasite is the freshwater murrel (Ophicephalus punctatus).