M. and A. Leger[193] (1914) propose to classify Leucocytozoa, provisionally, according as the host cells are fusiform or rounded.
(4) The Hæmogregarina type. Included herein are many parasites of red blood corpuscles, with a few (the leucocytogregarines) parasitic in the white cells of certain mammals and a few birds. They are not amœboid but gregarine-like, vermicular or sausage-shaped (fig. 78). They do not produce pigment. They are widely distributed among the vertebrata, but are most numerous in cold-blooded vertebrates (fishes, amphibia and reptiles). The hæmogregarines of aquatic hosts are transmitted by leeches, those of terrestrial hosts by arthropods.
The nucleus of hæmogregarines is usually near the middle of the parasite, but may be situated nearer one end. The body of the parasite may be lodged in a capsule (“cytocyst”). There is much variation in size and appearance among hæmogregarines. Some are small (Lankesterella); some attack the nucleus of the host cell (Karyolysus); others have full grown vermicules larger than the containing host corpuscle, and so the hæmogregarines bend on themselves in the form of U (fig. 78, b). Schizogony often occurs in the internal organs of the host, sometimes in the circulating blood.
The hæmogregarines occurring in the white cells (mononuclears or polymorphonuclears) of mammals have been referred to a separate genus, Leucocytogregarina (Porter) or Hepatozoön (Miller). Such leucocytogregarines are known in the dog (fig. 79), rat, mouse, palm-squirrel, rabbit, cat, etc. Schizogony of these forms occurs in the internal organs, such as the liver, lung and bone-marrow of the hosts. They are apparently transmitted by ectoparasitic arthropods, such as ticks, mites, and lice.
Fig. 78.—Hæmogregarines from lizards, a, H. schaudinni, var. africana, from Lacerta ocellata; b, H. nobrei from Lacerta muralis; c, H. marceaui in cytocyst, from Lacerta muralis. (After França.)
A few hæmogregarines are known to be parasitic in the red blood corpuscles of mammals. Such are H. gerbilli in the Indian field rat, Gerbillus indicus; H. balfouri (jaculi) in the jerboa, Jaculus jaculus, and a few species briefly described from marsupials. These parasites do not form pigment.
Strict leucocytic gregarines have been described from a few birds by Aragão and by Todd.