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?

Milwaukee Journal Sentinel files Mahal, an orangutan, is pictured in March 2010 with his surrogate mother, MJ, at the Milwaukee County Zoo. The young orangutan, 3 years old in the photo, was transferred to Milwaukee from a zoo in Colorado after his birth mother rejected him. 2007 Series: Ambassador Mahal Part One: Holding on to life Part Two: A nature-nurture divide Part Three: At last, a sight to behold

Milwaukee Journal Sentinel files Mahal, an orangutan, is pictured in March 2010 with his surrogate mother, MJ, at the Milwaukee County Zoo. The young orangutan, 3 years old in the photo, was transferred to Milwaukee from a zoo in Colorado after his birth mother rejected him.

“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% 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.”

* * *

Like all great apes, orangutans face health challenges in captivity. Males are more susceptible to urinary tract infections than they are in the wild; it’s uncertain why. Many great apes also suffer from heart disease.

Mark Hoffman / Milwaukee Journal Sentinel Annette Gendron, who performed the necropsy on Mahal, at UW-Madison. She is director of pathology and laboratory services for the university's Research Animal Resources Center and is the senior pathologist for the Milwaukee County Zoo.

Mark Hoffman / Milwaukee Journal Sentinel Annette Gendron, who performed the necropsy on Mahal, at UW-Madison. She is director of pathology and laboratory services for the university's Research Animal Resources Center and is the senior pathologist for the Milwaukee County Zoo.

Neither illness seemed likely given Mahal’s youth.

The orangutan’s body arrived at the UW lab of Annette Gendron, a veterinary pathologist who’d worked in Kenya for a year and later at the Fresno Zoo before coming to Madison in 1983. In more than 30 years as a pathologist, she’d performed necropsies on everything from great apes and elephants to snakes and giraffes, “almost anything you can think of.”

Following protocol, Gendron and zoo pathology resident Kathleen Deering each dressed in a protective gown, a double set of gloves, a mask and face shield. Necropsies — autopsies performed on animals — take hours and it is all too easy for pathologists to spatter themselves with material from the dead animal.

Once, 40 years ago, Gendron was accidentally exposed to the fungal lung infection Valley fever, while helping veterinarians try to resuscitate an orangutan before it died. On another occasion, one of her gloves leaked while examining the intestines of an antelope and she wound up with an infection under her fingernail.

Although she never became seriously ill, these incidents reinforced her cautious nature.

Gendron began her examination of Mahal by taking measurements, noting the size of the orangutan’s pupils and searching for skin lesions, evidence of trauma and discoloration of the membranes of the mouth.

Then she cut open the orangutan’s body. There was no fluid in the abdomen. Fluid is often a sign of inflammation, which in turn raises a host of possibilities, from tumors to liver or kidney failure.

Provided by Research Animal Resource Center, UW-Madison This 10x magnification image shows parasitic larval cysts of a tapeworm found on the liver of Mahal, the Milwaukee County Zoo's famous orangatan that died in 2012. The large and small white areas are the larval cysts.

Provided by Research Animal Resource Center, UW-Madison This 10x magnification image shows parasitic larval cysts of a tapeworm found on the liver of Mahal, the Milwaukee County Zoo's famous orangatan that died in 2012. The large and small white areas are the larval cysts.

One by one, Gendron looked at the organs. First the spleen, then the liver, kidneys, adrenal glands, heart, lungs and trachea. She removed and weighed each organ and looked for scarring and abnormalities. She placed tissues from all of the organs in formalin, a preservative that would allow her to view them later under the microscope.

Although the pathologist worked methodically, she had noticed something strange from the very beginning.

Mahal had an enlarged spleen and liver. Both organs were overrun with cysts, small gray bubbles.

“There were enormous numbers inside the liver,” Gendron says. “We’d never seen anything like this.”

Whatever it was, the pathogen had also clogged Mahal’s lungs. The official cause of death was acute respiratory distress syndrome; what it meant was that the orangutan had essentially drowned in his own blood.

The necropsy took between four and five hours. When she was finished, Gendron made microscope slides of tissue from the nerves, heart, lungs, brain and other organs.

Over the next few weeks she sent zoo pathologists around the country a digital photo of one of the cysts from Mahal’s liver. She hoped specialists who’d examined great apes might have run into something similar.

No one had.

* * *

Aedes aegypti mosquito 4 to 7 millimeters Diseases carried Yellow fever, dengue fever, chikungunya and Zika virus. Geographic distribution One of the most widespread mosquito species. Found in Africa and Asia and more recently reported in South America, Europe and the southern half of the United States. Transmission method Bites victims for blood meals. Source: World Health Organization

Aedes aegypti mosquito 4 to 7 millimeters Diseases carried Yellow fever, dengue fever, chikungunya and Zika virus. Geographic distribution One of the most widespread mosquito species. Found in Africa and Asia and more recently reported in South America, Europe and the southern half of the United States. Transmission method Bites victims for blood meals. Source: World Health Organization

One of the colleagues Gendron contacted was Goldberg, a fellow member of UW’s faculty of veterinary medicine.

Goldberg, who had started out studying hummingbird behavior at Amherst College in Massachusetts, had gone on to focus on primates while earning his PhD at Harvard. He joined the UW faculty in 2008.

Over the last decade, Goldberg had flown to Uganda twice a year for up to a month of fieldwork examining the interactions of people, primates and other animals in and around Kibale National Park.

The fieldwork often reminds him how much the world is changing, and how quickly new health threats can cross from species to species, continent to continent. For much of the world, the Zika virus, named for the forest in Uganda where it was discovered, stands as the prime example.

Spread by the Aedes aegypti mosquito and first discovered in a rhesus macaque in 1947, the virus appeared in humans in Uganda and Tanzania in 1952. However, in the last two years, the virus suddenly spread through large sections of Central and South America. In July, the U.S. recorded its first homegrown cases of the virus in Florida. By December, Zika had been identified in 75 countries and territories.

But Goldberg can point to a more personal example of today’s rapid-transit pathogens. In the summer of 2012, just six months before Mahal died, Goldberg had returned to Madison after working in western Uganda. He’d been home only three days when the dull ache he’d detected in his nose flared into a searing pain, impossible to ignore.

By angling a mirror and contorting his body, the scientist was able to peer inside his nostrils. There it was, just as he’d suspected: the pale, fat back end of a fully engorged tick.

“It took all of my willpower not to claw off my face,” he would recall.

Instead, Goldberg gently extended a pair of forceps until they surrounded the tick’s mouth, then he pulled firmly, but carefully. Out came a creature about the “diameter of a pencil eraser,” he recalls, “really nothing more than a distended stomach.”

DNA testing revealed it was a new species of tick, a discovery that, in the scientist’s view, far outweighed his discomfort. Somehow the tick had crawled onto Goldberg’s body in the Ugandan forest and stowed away inside his nostril through the plane trip to the U.S. The scientist’s reward was a paper he co-authored entitled, “Coincident Tick Infestations in the Nostrils of Wild Chimpanzees and a Human in Uganda,” published in the American Journal of Tropical Medicine and Hygiene.

But it wasn’t Goldberg’s tick discovery that made Gendron think of him for the Mahal investigation. She recalled his recent paper on hepatocystis, a single-cell parasite transmitted by midges.

Could the cysts inside Mahal be hepatocystis?

Major outbreaks Zoonotic diseases, those that go from animals to people, cause an estimated 2.5 billion cases of human illness and 2.7 million deaths each year. It is hard to quantify the number of outbreaks in a given year, but the World Health Organization tracks a few dozen each year. The following are some of the worst outbreaks of the last century: 2016 – Zika virus Zika virus, first isolated from a rhesus macaque in Uganda. Between Jan. 3 and April 2 alone, Brazil reported more than 91,000 suspected cases of the mosquito-borne virus which has been linked to severe birth defects. Through March 1, 2017, the United States has reported 5,074 cases of Zika, 221 from local mosquitoes. Puerto Rico has reported more than 36,000 cases. While many experience mild or no symptoms, the virus has been linked to microcephaly, a birth defect in which a baby’s brain stops growing or does not grow properly.

2014 to 2016 – Ebola Ebola, passed to humans from wild animals, possibly infected monkeys or fruit bats. West Africa recorded 28,652 suspected, probable and confirmed cases resulting in 11,325 deaths. The U.S. saw four Ebola patients; the lone fatality was a man who traveled from Liberia to Dallas, developed symptoms four days after arrival, was treated and subsequently died.

2010 to 2013 – Cholera Cholera, an often fatal bacterial disease transmitted through water or food. In coastal areas, the disease often follows algal blooms. Ten months after a Haitian earthquake, an outbreak infected 665,000 people, killing 8,183. A November 2011 study determined the U.S. saw 23 cholera cases, all but one were travelers from Hispaniola, the island that includes Haiti. The other reportedly resulted from consumption of seafood from Haiti.

2009 – Swine flu, or H1N1 The pandemic of a respiratory disease found in pigs began in Mexico and is estimated to have killed some 284,500 people worldwide. The CDC estimated that 59 million Americans contracted swine flu, 265,000 were hospitalized and 12,000 died.

2008 to 2009 – Cholera An outbreak in Zimbabwe killed 4,293.

2002 to 2003 – SARS An outbreak of severe acute respiratory syndrome, a viral respiratory disease believed to have originated in bats, killed 774 people and sickened 8,098 worldwide. The outbreak began in Guangdong Province, China. The disease is spread person to person, largely by droplets when a person coughs or sneezes.

1981 to 2011 – HIV/AIDS HIV/AIDS, which is closely related to a disease in monkeys called simian immunodeficiency. HIV may have crossed from monkeys to humans as a result of a human killing and eating a chimp. At least 60 million people were infected by AIDS, resulting in 25 million deaths. In 2008, about 1.2 million Americans had HIV, while sub-Saharan Africa had almost 23 million cases. The virus is spread through blood and bodily fluids.

1968 to 1969 – Hong Kong flu Hong Kong flu, an influenza A virus. Aquatic birds are a natural reservoir, or long-term host, for this kind of virus. This influenza is believed to have killed about 1 million people. Scientists suspect that it evolved from a strain of the 1957 Asian flu.

1957 to 1958 – Asian flu This influenza killed about 2 million people.

1918 – Spanish flu As many as 50 million people worldwide are believed to have died.

Major outbreaks Zoonotic diseases, those that go from animals to people, cause an estimated 2.5 billion cases of human illness and 2.7 million deaths each year. It is hard to quantify the number of outbreaks in a given year, but the World Health Organization tracks a few dozen each year. The following are some of the worst outbreaks of the last century: 2016 – Zika virus Zika virus, first isolated from a rhesus macaque in Uganda. Between Jan. 3 and April 2 alone, Brazil reported more than 91,000 suspected cases of the mosquito-borne virus which has been linked to severe birth defects. Through March 1, 2017, the United States has reported 5,074 cases of Zika, 221 from local mosquitoes. Puerto Rico has reported more than 36,000 cases. While many experience mild or no symptoms, the virus has been linked to microcephaly, a birth defect in which a baby’s brain stops growing or does not grow properly.

2014 to 2016 – Ebola Ebola, passed to humans from wild animals, possibly infected monkeys or fruit bats. West Africa recorded 28,652 suspected, probable and confirmed cases resulting in 11,325 deaths. The U.S. saw four Ebola patients; the lone fatality was a man who traveled from Liberia to Dallas, developed symptoms four days after arrival, was treated and subsequently died.

2010 to 2013 – Cholera Cholera, an often fatal bacterial disease transmitted through water or food. In coastal areas, the disease often follows algal blooms. Ten months after a Haitian earthquake, an outbreak infected 665,000 people, killing 8,183. A November 2011 study determined the U.S. saw 23 cholera cases, all but one were travelers from Hispaniola, the island that includes Haiti. The other reportedly resulted from consumption of seafood from Haiti.

2009 – Swine flu, or H1N1 The pandemic of a respiratory disease found in pigs began in Mexico and is estimated to have killed some 284,500 people worldwide. The CDC estimated that 59 million Americans contracted swine flu, 265,000 were hospitalized and 12,000 died.

2008 to 2009 – Cholera An outbreak in Zimbabwe killed 4,293.

2002 to 2003 – SARS An outbreak of severe acute respiratory syndrome, a viral respiratory disease believed to have originated in bats, killed 774 people and sickened 8,098 worldwide. The outbreak began in Guangdong Province, China. The disease is spread person to person, largely by droplets when a person coughs or sneezes.

1981 to 2011 – HIV/AIDS HIV/AIDS, which is closely related to a disease in monkeys called simian immunodeficiency. HIV may have crossed from monkeys to humans as a result of a human killing and eating a chimp. At least 60 million people were infected by AIDS, resulting in 25 million deaths. In 2008, about 1.2 million Americans had HIV, while sub-Saharan Africa had almost 23 million cases. The virus is spread through blood and bodily fluids.

1968 to 1969 – Hong Kong flu Hong Kong flu, an influenza A virus. Aquatic birds are a natural reservoir, or long-term host, for this kind of virus. This influenza is believed to have killed about 1 million people. Scientists suspect that it evolved from a strain of the 1957 Asian flu.

1957 to 1958 – Asian flu This influenza killed about 2 million people.

1918 – Spanish flu As many as 50 million people worldwide are believed to have died.

* * *

There’s just no love for the lowly parasite.

The shark is more frightening, the naked mole rat uglier, the black mamba snake deadlier. Yet few creatures provoke a more visceral shudder of disgust than lice, tapeworms, mites and the various other organisms known as parasites.

After his close encounter with the tick, Goldberg understood why.

Still, years of research had tempered his reaction. “You wind up having a lot of respect for parasites,” he says. “They are very clever.”

They have to be. To keep living, parasites must depend upon another organism, or host. They employ finely-tuned machinery to infect and stay in the host without being detected and killed off by the host’s immune system.

Parasites are master survivors. They have been around much longer than humans, likely as long as there have been living organisms, according to Eric Hoberg, former curator of the 120-year-old U.S. National Parasite Collection, among the world’s largest repositories of different parasites.

Although it’s not known how many parasites share the earth with us, one estimate puts the number of different helminths, or intestinal worms, at between 75,000 and 300,000. All told, more than half of the organisms on the planet are parasites, from the microscopic bacterial cell to the multicellular tapeworm.

Many have evolved clever tricks to find the right host, survive and spread. Take the wiley Toxoplasma gondii, a single-cell parasite that starts by infecting a mouse but must enter a cat in order to reproduce. To go from mouse to cat, Toxoplasma gondii disables the rodent’s ability to smell cat urine, essentially leading the mouse into the jaws of its enemy.

Of greater concern to humans is the Guinea worm. The worm’s larvae develop after being consumed by water fleas; they then infect humans who drink the flea-infested water.

Once inside the human body, the worms grow to two or three feet in length, until the female worm causes a burning sensation so extreme it drives the human to seek relief by submerging the infected area in water. The worm’s offspring are then released from a skin blister into the water to continue their life cycle.

The sheer number of parasites can make identifying them difficult, and as Mahal’s death would show, there is still much we don’t know.

* * *

Goldberg crossed busy Campus Drive and proceeded to Gendron’s UW lab to collect samples of Mahal’s liver, lungs and spleen. All were securely stored inside biosafety bags. Goldberg then spent the afternoon performing a series of molecular tests on the tissue samples. He found no sign of hepatocystis.

He spent the rest of the week conducting more tests, checking whether something might be interfering with the chemistry and skewing his results. Still negative.

In his black lab notebook, he noted that ahead of him lay “a huge task.”

Not that the challenge displeased him. He loved a good mystery.

Goldberg and David O’Connor, a colleague at UW’s Primate Center, compared Mahal’s genetic sequence with that of another orangutan, obtained from a massive DNA repository known as GenBank.

West Nile A mosquito-borne disease capable of causing neurological problems and sometimes death in humans. Transmission Bites from female mosquitoes. Birds are a host for the disease. Geographic Regions Commonly found in Africa, Europe, the Middle East, North America and West Asia. Symptoms Fever, headache, fatigue, muscle pain or aches, nausea, loss of appetite, swollen lymph glands, vomiting, rash. Severe Symptoms Disorientation, coma, tremors, seizures, paralysis. Can cause encephalitis or meningitis. Cases and Death There are no reliable worldwide statistics. Among people who develop severe symptoms, death rates range from 3% to 15% and are highest among the elderly. In the U.S. According to the U.S. Centers for Disease Control and Prevention, nearly 44,000 cases have been reported in the U.S. since 1999, with more than 1,900 deaths. There were 2,175 reported cases in 2015. Treatment No vaccine for humans exists. Most people who are infected receive hospitalization, intravenous fluids, respiratory support and prevention of secondary infections. Source: World Health Organization

West Nile A mosquito-borne disease capable of causing neurological problems and sometimes death in humans. Transmission Bites from female mosquitoes. Birds are a host for the disease. Geographic Regions Commonly found in Africa, Europe, the Middle East, North America and West Asia. Symptoms Fever, headache, fatigue, muscle pain or aches, nausea, loss of appetite, swollen lymph glands, vomiting, rash. Severe Symptoms Disorientation, coma, tremors, seizures, paralysis. Can cause encephalitis or meningitis. Cases and Death There are no reliable worldwide statistics. Among people who develop severe symptoms, death rates range from 3% to 15% and are highest among the elderly. In the U.S. According to the U.S. Centers for Disease Control and Prevention, nearly 44,000 cases have been reported in the U.S. since 1999, with more than 1,900 deaths. There were 2,175 reported cases in 2015. Treatment No vaccine for humans exists. Most people who are infected receive hospitalization, intravenous fluids, respiratory support and prevention of secondary infections. Source: World Health Organization

The scientists took Mahal’s sequence, then subtracted the other orangutan’s. What remained, Goldberg reasoned, would contain the killer’s genetic signature.

They performed the analysis using a technique known as shotgun sequencing. Long strands of DNA are broken into millions of smaller, easier-to-read pieces and then reassembled by computer into the longer sequence. They felt compelled to work quickly.

“We really rushed it,” Goldberg recalls. “We were really curious and we were afraid there would be an outbreak at the zoo.”

He kept thinking about the story of Tracey McNamara, then head pathologist at the Bronx Zoo. In 1999, McNamara noticed that a disturbing number of crows near the zoo were becoming ill and dying. Around the same time, New York City health officials discovered that a small number of people had become deathly ill with what appeared to be St. Louis encephalitis. Mosquitoes pick up the virus from birds, then pass it to humans.

Tests showed, however, that the virus making the crows and humans sick wasn’t St. Louis encephalitis. It was West Nile virus. The virus had been discovered in Uganda in 1937, but the sick crows at the Bronx Zoo marked its first appearance in the U.S.

By the time Goldberg began working on the Mahal case, West Nile had spread across the continental U.S., sickening almost 2,500 people and causing 119 deaths in 2013 alone. Early on in his investigation of the orangutan’s death, he had no way to know whether he might be dealing with another West Nile virus.

A few weeks of genetic testing provided Goldberg with an answer — of sorts.

The killer’s DNA appeared to be that of a cestode, or tapeworm.

Except the sequence didn’t match any known tapeworm.

It was something new.

* * *

A tapeworm would not have topped the list of likely suspects. Although orangutans experience roundworm infections, it is much rarer to find one suffering from tapeworms.

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In Mahal’s case, the tapeworms were still in the larval stage; that explained the little gray bubbles. A tapeworm’s life cycle begins when eggs are excreted in the feces of a host animal, let’s say a cat, and wind up in the soil. There, they are ingested by a different animal, a secondary host, such as a mouse. The eggs hatch inside the mouse and wait to be ingested back into the cat, where they reach adulthood and reproduce, so that new generations of eggs can be excreted back into the soil.

Mahal appeared to have played the role of “accidental mouse.” But what species of tapeworm did the larvae come from? How did they get inside an orangutan? And how did they survive the anti-worming medication the Milwaukee zoo administers to its orangutans every three weeks?

The puzzle of Mahal’s death had hooked Goldberg.

“This is the thing you dream about,” he says. “An unusual infection caused by something unknown.”

Mark Hoffman / Milwaukee Journal Sentinel UW-Madison scientist Tony Goldberg in the Kibale National Park in Uganda.

Mark Hoffman / Milwaukee Journal Sentinel UW-Madison scientist Tony Goldberg in the Kibale National Park in Uganda.

Although researchers discover new tapeworm species on a regular basis, Goldberg was dying to run into the offices of his UW colleagues exclaiming: “Look what I found!”

Science, however, demands the opposite response: silence and self-discipline. First come weeks or months of rigorous confirmation work. Only then is a discovery announced.

“There’s this delicious little time,” Goldberg says, “when you have it all to yourself.”

One answer did emerge rather quickly, though it would ultimately raise new questions.

On the very day Goldberg received the sequencing information from the machine, a scientific paper appeared in The International Journal for Parasitology naming a new genus, or group, of tapeworms. Scientists discovered the new group inside the bodies of weasels in Japan and Finland.

Features that distinguished the new tapeworm from the two previously known groups included a retractable conelike structure armed with hooks used to attach itself to the host’s intestinal wall; a small head, or scolex; and suckers. Because the tapeworms found in Mahal were only larval, they displayed none of these features.

Rather, what linked the new group, Versteria, to the larvae inside Mahal was a similar genetic signature. Although the two were not an exact match, they appeared to be different species within the same group.

And from what Goldberg could tell, the tapeworms inside Mahal were a long way from home.

For one thing, they should not have been inside the body of an orangutan, but rather a vole or some other variety of weasel prey. For another, they had been found previously in Finland and Japan, half a world away from Wisconsin.

* * *

Where and how Mahal ingested the tapeworm larvae remained a mystery. Still, Goldberg felt a need to write up what he knew so far.

“You want to publish because it could be important,” he says. “You want to get it out there and raise the alarm.

“This could be a threat to orangutans and other apes — such as people.”

There was another reason to publish soon. Khan and the rest of the staff at the Milwaukee County Zoo remained in mourning for Mahal.

“We needed answers,” Khan says, “as does anyone suffering a big loss.”

They needed to know whether what had killed Mahal might pose a threat to the orangutan’s surrogate mother, MJ, or to any of the zoo’s other primates. Another question was harder and more personal: Could they have done something to have saved Mahal?