47 
cycle of the former is known. Only the trophozoite has been observed, 
which, according to the description, is smaller than that of H. per- 
niciosum', and is without a cyto-cyst. It may be mentioned that 
there exists a marked similarity m form of the trophozoites of dif- 
ferent species of haemogregarines in both warm- and cold-blooded 
animals. 
Specimens of the type species of the new genus have been deposited 
in the museum of the division of zoology, hygienic laboratory, United 
States Public Health and Marine-Hospital Service. 
CONCLUSIONS. 
Hepatozoon peniiciosum is the only haemogregarine occurring in 
mammals which has been observed outside of India and northern 
Africa, both tropical regions. 
No other haemogregarine has been described which exhibits dis- 
tinct pathogenic properties or causes the death of the animal host. 
Other haemogregarines have been found only in wild animals, in 
which they appear to exert no harmful effect. It is well knovm that 
domesticated animals may succumb to infection by protozoa or 
bacteria, practically harmless to closely related vdld animals. Such 
may be the case as regards infection of white rats by H. perniciosum. 
The results thus far obtained in the experimental infection of wild 
rats {Mus rattus) with this parasite are not conclusive as regards 
pathogenicity. That the wild rat is susceptible to infection is clearly 
shown. As is the case with white rats, it is probable that the most 
pathogenic effect is exerted upon young animals. Whether or no 
this parasite can be turned to account in the destruction of wild rats 
is a matter for further investigation. The role assumed by wild rats 
as agents in the transmission of plague makes it a matter of great 
importance to exterminate them. To do this by spreading disease 
among them is generally regarded as the simplest and most desirable 
method. 
The Gamasid mite (Lelaps ecJiidninus) , which is the intermediate 
host, has frequently been found both upon white and wild rats and in 
widely separated localities. A study of the habits of rats taken in 
conjunction with the experimental infections already described, proves 
beyond question the mode of entry of Hepatozoon perniciosum into 
the body of the rat. That this is the sole manner in which infection 
takes place is not proved, but is extremely probable. 
The puncture produced by the pointed mouth parts of the mite 
before sucking the blood of the host apparently causes some irritation. 
Rats infested with mites may frequently be seen to bite at the skin 
on various parts of the body and frequently they seem to catch and 
devour the offending mite. The behavior of dogs when disturbed by 
fleas is more familiar. 
