As carefree as penguins might look, torpedoing through the water or rocketing into the air like a Poseidon missile, zoo penguins are stalked by an unrelenting killer: malaria.

“It’s probably the top cause of mortality for penguins exposed outdoors,” said Dr. Allison N. Wack, a veterinarian at the Maryland Zoo in Baltimore. The avian version is not a threat to humans because mosquitoes carrying malaria and the parasites are species-specific; mosquitoes that bite birds or reptiles tend not to bite mammals, said Dr. Paul P. Calle, chief veterinarian for the Wildlife Conservation Society, which runs New York City’s zoos. And avian malaria is caused by strains of the Plasmodium parasite that do not infect humans.

But for penguins in captivity, the threat is so great that many zoos dose their birds in summer with pills for malaria, said Dr. Richard Feachem, director of global health at the University of California, San Francisco.

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