The 5-year-old orangutan named Mahal died mysteriously in December 2012. Zoo personnel speculated at the time that he may have had pneumonia.
But DNA tests done by a team of scientists at the Wisconsin National Primate Research Center AIDS Vaccine Research Laboratory determined Mahal was struck by tapeworm larvae that spread throughout his body in an unusual fashion. By the time he showed symptoms, it was too late to save him, lead researcher Tony Goldberg said.
"Mahal's death was a tragedy in the community of Milwaukee," Goldberg said. "He was a symbol of hope."
The orangutan was less than a year old when he was flown to Milwaukee to live with a surrogate mother because his biological mother at a zoo in Colorado had rejected him. He gained fame as the subject of a Milwaukee Journal Sentinel series, and zoo spokeswoman Jennifer Diliberti-Shea described him as a favorite among visitors. It is not unusual for zoo births and young animals to boost attendance.
Scientists believe the tapeworm that killed Mahal is native to North America but previously unknown. Goldberg, a veterinarian and epidemiologist at the University of Wisconsin-Madison, described its sudden appearance as worrisome.
"People are apes, and orangutans are fairly close relatives to us ... so any new infectious disease that affects primates is a concern," he said.
Also, the tapeworm's sudden appearance could mean it is increasing in the environment, Goldberg said.
Most Americans are familiar with tapeworms that develop in their pets' intestines, but the parasite has a complex life cycle that includes several stages, Goldberg said. Its eggs are consumed and carried by hosts such as mice or rats. Only when those animals are eaten by others do the tapeworms typically progress to adulthood.
For example, a cat may eat a mouse infected with larvae and then develop the type of long, white tapeworm feared by pet owners.
Mahal's tapeworms remained in larval form as they spread throughout his body, indicating he consumed the parasite's eggs himself, Goldberg said. It's not clear where or when this might have happened, although Goldberg said one possibility would be from eating soil, which is a common orangutan behavior.
Goldberg dismissed the idea that the parasite could have been in Mahal's food or water, saying the zoo was well aware of animal health threats and took appropriate precautions.
"Most likely it's the environment," he said.
The tapeworm is related to two others found in small carnivores and could be carried and spread by raccoons or weasels living on the zoo grounds, Goldberg said. Researchers will be looking into that possibility, although, he said, "we could be wrong. Tapeworms are tricky."