for observation, it is advisable to make systematic examination of the blood of the inhabitants of some district in which filariasis is endemic. If this be done, the observer is sure to come across, sooner or later, cases in which microfilaria bancrofti abounds, and also of the diseases to which it gives rise.
Technique.—I recommend the following procedure as likely to supply the investigator not only with material, but also with much useful information. Let him visit, late in the evening, some hospital, or prison, or other establishment where he can have an opportunity of examining the inmates, and let him procure slides of the blood of, say, 100 individuals. The slides are conveniently prepared by pricking the finger of each person in turn, and transferring large drops of the blood so obtained to ordinary microscope slips by simply dabbing the centre of the slip on the blood. The blood is then spread out with a needle so as to cover in a moderately thin film about a square inch of one surface of each slip. Each slip, so soon as the blood is spread, should be laid on its uncharged surface on a smooth, level surface until dry; it is then labelled and put aside. One preparation of this description may be made from each person, who should be selected simply as a representative of the general population, and therefore irrespectively of his being physically sound, or of his being the subject of any particular type of disease.
The slides may be examined in various ways, either at once or, if more convenient, weeks or months afterwards; if kept dry and away from cockroaches, ants, etc., they do not spoil. A convenient plan is to dip the slides, without previous fixing, in a weak solution of fuchsin—about three or four drops or more of the saturated alcoholic solution to the ounce of water. They are left in the stain for about an hour, and then examined wet and without cover-glass. If the slides are old, they may stain too deeply; in this case they may be partially decolorized in weak acetic acid—two or three drops to the ounce of water—and afterwards washed. Recent slides, if placed in water for a few minutes until the hæmoglobin is discharged, show the microfilariæ very well; it is advisable, however, for the novice at this sort of work to use, in the first instance, the fuchsin method described above.
Another plan is to fix the blood-film with alcohol, and then to stain by running on a few drops of saturated watery solution of methylene blue, washing off the superfluous stain after half a minute, and, if necessary, decolorizing with weak acetic acid and washing; the wet slide is then examined with the microscope. Or, without previous fixing with alcohol, the slide, after it has dried, may be dipped for a few seconds in distilled water so as to wash out the hæmoglobin, dried, and then, with or without fixing, stained with methylene blue, logwood, or other suitable stain.
