Could the Blood Parasite Leucocytozoon Deter Mallard Range Expansion?

Abstract

We investigated whether the blood parasite Leucocytozoon simondi could slow mallard (Anas platyrhynchos) population growth in the east that has been associated with American black duck (A. rubripes; hereafter black duck) population decline. Susceptibility to parasites was compared among F1 ducklings produced from crosses between mallard and black ducks from areas of Leucocytozoon endemicity (Ontario), and between mallards from an area free of Leucocytozoon (Saskatchewan). We produced 6 types of ducklings: Ontario black duck x Ontario black duck (OB x OB), Ontario black duck x Ontario mallard (OB x OM), Ontario black duck x Saskatchewan mallard (OB x SM), OM x OM, OM x SM, and SM x SM. We predicted that because of probable coevolution of black ducks and Leucocytozoon, black duck ducklings would have resistance to the parasite. We also predicted that Ontario genes would confer some resistance to ducklings because these ducklings' parents had survived exposure to Leucocytozoon. In contrast, we predicted that mallard and Saskatchewan genes would not confer resistance, i.e., OB x OB ducklings would have greatest resistance to Leucocytozoon, SM x SM ducklings would have least, and remaining duckling types would have intermediate resistance. Of 169 ducklings exposed in 2 years in 3 geographically separate locales, none died, showed noticeable symptoms, or otherwise behaved abnormally. Nonetheless, weekly blood smears indicated that 91% of ducklings became infected, and many developed intense parasitemias. However, infection intensities were not different among the 6 duckling types. In addition, hematocrits were not lowered by intense infections. These results suggest that the effects of Leucocytozoon on wild waterfowl populations have been overestimated, and that Leucocytozoon will not prevent further range expansion of mallards.