THE EPIZOOTIOLOGY OF LEUCOCYTOZOON SIMONDI INFECTIONS IN DOMESTIC DUCKS IN NORTHERN MICHIGAN1

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Abstract
Three successive summer surveys disclosed that L. simondi infections among farm-raised ducks were comparatively uncommon late in June or early in July. However, surveys late in July or early in Aug. demonstrated that very few ducks escaped infection during the intervening weeks. Successive test groups of young ducklings (6 weeks of age) were exposed to natural infection under ordinary farm conditions for 8-day periods during the summer of 1950 with the following results: (1) Ducks exposed in late June or early July did not become infected rapidly, nor did all birds become infected; of those that did acquire infections none died. (2) In birds of 3 successive test groups exposed July 11-Aug. 4, infections were very rapidly acquired; group attack rates ranged from 90 to 100 % and the case fatality rates by group varied from 14 to 83 % (3) By mid-Aug. the epizootic had so declined that it was no longer measurable by the technique employed; at this time, none of a test group of 10 birds acquired infections as a result of farm exposure. The birds introduced as transients for brief periods during the course of the 1949 and 1950 epizootic seasons suffered 3-5 times the mortality rate of the indigenous farm birds, despite the fact that the latter were exposed to infection during the entire summer. Perhaps indigenous birds, having been exposed very early in the summer, received mild infections which rendered them immune to the heavier, mortality producing infections reflected in the data for the transient groups during mid-summer. In terms of flock management, the evidence seems to warrant the conclusion that if young birds are introduced sufficiently early in the season (before the end of June) they would probably survive the epizootic although they would almost certainly become infected. In the case of these young birds the protection against fatal issue resulting from infection would seem to lie in an acquired immunity rather than in one accountable to age. However, considerably older birds (18 weeks of age) seem to enjoy greater chances of surviving primary infections acquired during the midepizootic season. The sex of the host appears to be of no consequence in susceptibility to L. simondi infection, nor does it mediate in the outcome of these infections.