62 
MALARIA 
thick films. In heavier portions of thick 
films, particularly when the blood dries 
slowly, mature gametocytes assume a 
rounded shape, a change which would take 
place normally in the mosquito during the 
early stage of maturation. Usually typical 
forms will be found in the thinner edge of 
such films, however. 
Mixed infections. Mixed infections are 
undoubtedly more prevalent than reports 
would indicate, since the parasites of one 
species usually predominate and the other 
species is overlooked or not recognized in 
examinations. 
Fallacies and Puzzles in Thick Film 
Examinations 
The inexperienced microscopist may be 
confused in thick film examination by dirt 
or bacteria from the skin; by dust particles 
on the slide; by vegetable spores, yeast cells 
or fungi from the air; and by bacteria, 
molds and protozoa, or other contaminants 
from distilled water used in staining. 
Hence great cleanliness should be exercised 
to reduce these artefacts to a minimum. 
Artefacts which may deceive the inexperi¬ 
enced will be found frequently to lie above 
the blood plane, or they may be retractile 
and focus out of the field unevenly. Cocci 
from the skin may stain like chromatin dots 
and may resemble parasites if adjacent to 
blue-stained particles of fibrin or cellular 
substance. This rarely occurs more than 
once or twice on a single slide and numbers 
of free cocci will usually be found also; 
whereas in well stained smears free chro¬ 
matin dots are rarely found. Thick films 
should not be diagnosed as positive on what 
appears to be only one parasite. The para¬ 
sites should be unmistakable, or search 
should be continued until others are found. 
If this is impossible, later smears should be 
made. 
Possibility of Detection of Parasites 
by the Thick Film 
Many malariologists believe that the 
number of parasites necessary to produce 
symptoms of malaria as a general rule is 
more than the minimum which can ordi¬ 
narily be detected in thick films, unless the 
patient has been recently taking antima- 
larial treatment. However, in persons with 
extreme susceptibility symptoms occur be¬ 
fore parasites can be found in the blood 
stream. In these eases further subsequent 
smears should be made on successive days. 
