MILWAUKEE _ On a frigid night a few days after Christmas 2012, Trish Khan drove back to the Milwaukee County Zoo to check on the star attraction, a playful, wildly popular 5-year-old orangutan named Mahal. It was almost 11 p.m.
Khan, the zoo's primary orangutan keeper, was off on medical leave. Yet she'd come in earlier in the day, as soon as she heard something was wrong with Mahal.
Raised on a horse farm in Wisconsin, Khan has a passion for animals, especially primates and most especially orangutans, a great ape found in Asian rainforests and admired for its intelligence.
Even so, her deep affection for Mahal was unique. She had flown to Colorado to pick him up from the Cheyenne Mountain Zoo after the orangutan had been rejected by his mother. Khan had accompanied Mahal to Milwaukee, and when he settled in at the county zoo, she bottle-fed him for the first year.
When Mahal was sick, she would move a mattress into the enclosure and stay with him, allowing him to nest beside her and sleep until morning.
Four years later, as she examined the orangutan on that winter evening, Khan noted how much he resembled a sick human child: no energy, no appetite. After an unsuccessful attempt to administer antibiotics, Khan and one of the zoo veterinarians decided to move the orangutan into a smaller room and wait until morning to anesthetize him and take blood samples.
The next morning, a zookeeper found Mahal lying motionless on the floor. He was dead at just 5 years of age; the typical orangutan lives 35 or 40 years in the wild and sometimes more than 50 in captivity. In his short life, Mahal had been the subject of a newspaper series and a children's book.
Stunned as they were by the loss, Khan and her colleagues now faced a mystery with implications for both animals and humans: What killed Mahal?
"Is it something that could affect our other orangutans or other animals?" Khan remembers wondering. "Is it something that could affect our keepers?"
The similarities between humans and other primates are well-known. It is the reason monkeys have long been used in medical tests as proxies for humans. It's also the reason the zoo regularly consults with doctors at Froedtert Hospital on health problems involving its great apes.
But the close biological relationship between the two species takes another form as well. Lethal diseases, including Ebola and HIV, have jumped from apes to humans. Others, such as influenza and polio, have gone the opposite route, passing from humans to apes.
"When you're dealing with the great apes," says Khan, "pretty much anything they get, we can get."
And therein lies the problem. The zoo could not simply mourn the loss of Mahal and dispose of his remains. Other lives were at stake, including those of the zoo's 45 other primates and the seven employees who cared for them.
Of the 400 or so emerging infectious diseases identified since 1940, more than 60 percent have been zoonotic, meaning they have passed from animals to humans.
A 2012 report by researchers in Britain, Kenya and Vietnam found that each year zoonotic diseases account for 2.5 billion cases of human illness and about 2.7 million deaths. A separate study published the same year put the direct costs from these diseases at more than $20 billion over the previous decade.
Zoo officials in Milwaukee were not taking any chances. Within hours of his death, Mahal's body lay in a cooler packed in ice, bound for a pathologist's lab at the University of Wisconsin-Madison.
So began an investigation that would span more than three years and lead to the discovery of a new species of pathogen. The search for Mahal's killer also would illustrate the links between diseases and some of the most powerful forces on the planet: evolution, glacial periods and the Earth's orbital patterns, known as Milankovitch cycles.
"The fact that we share so many diseases with primates tells us about evolution," explains Tony Goldberg, the UW professor of epidemiology who led the investigation into Mahal's death.
"There are an awful lot of primate pathogens that don't really care whether they're in a human or a chimpanzee or an orangutan."
