THE RELAPSE PHENOMENON IN THE LEUCOCYTOZOON SIMONDI INFECTION OF THE DOMESTIC DUCK1

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Abstract
Following the primary parasitemia, which results from infection in mid-summer (in northern Michigan) the parasites rapidly disappear from the peripheral circulation. During the fall and winter only an occasional parasite may be seen and most of the blood smears appear to be negative. Concurrently with the development of sexual activity in the spring there is a recurrence of parasites in the peripheral blood. Since the birds used in these studies were maintained within the laboratory in Baltimore, outside the endemic area, this recurrence of parasites can hardly result from reinfection. It appears to be a relapse due to renewed activity of the underlying asexual cycle (schizogony) in visceral organs. The relapse is characterized first by the appearance of mature gametocytes and thereafter, usually, by the secondary appearance of various numbers of immature gametocytes. The peculiarities of this sequence of events are discussed but not wholly resolved. Although the parasites during relapse are much fewer than during the primary parasitemia (ratio of about 11000), the relapse continues much longer than does the primary attack; in some cases it continued all summer. While both male and female ducks relapsed when they became sexually active, the relationship could be more sharply detd. in the females. In most cases relapse occurred within 10 days before or after the first egg was laid. By artificially increasing the amt. of light per day to 16 hrs., egg laying was induced as early as Oct.'' and relapse of Leucocytozoon occurred at that time. That light per se did not induce relapse directly is shown by the fact that it did not occur in immature ducks exposed to 16 hrs. of light daily for many months. That relapse is not specially associated with egg-laying but with sexual activity is shown by the fact that drakes relapsed at about the same time as the females. Thus it seems evident that the relapsing mature ducks provide the source of infection for black flies at the time of egg laying and the flies in turn infect the newly hatched ducklings. The early season infections in the ducklings is, thus, comparatively light. However, the heavier pool of gametocytes provided by the primary infections in the first crop of ducklings insures the heavier and highly fatal infections which occur during midsummer.