Carlos E. Medina and Ryan W. Miller

Ocala Star-Banner and USA TODAY

SILVER SPRINGS, Fla. – An invasive species of herpes-carrying monkeys is growing in size, raising the possibility of confrontations with humans and expansion of the non-native primates' range beyond an idyllic river in Florida.

On the banks of the Silver River near Ocala, the rhesus macaques shriek, grunt and squeal as they fight and defend their space in an increasingly crowded habitat not their own.

By 2022, the monkey population on the Silver River is estimated to double, according to research co-authored by Steve Johnson, a University of Florida professor of wildlife ecology and conservation.

That could mean somewhere near 400 monkeys prowling the marshy banks of the river.

"Unless there is some management action by the state to curtail their numbers, it's going to create a situation where they will be forced to take more drastic action due to a serious incident," Johnson said.

Adding to the fears: About a quarter of the monkeys carry the herpes B virus.

Johnson said the chances of a monkey infecting a human are slim, but they are not zero. The only verified transmissions of herpes B from monkeys to humans came in lab settings. The last fatal infection was in 1997, with 21 fatalities listed since 1932, according to the Centers for Disease Control and Prevention.

The CDC also advises that infections within humans are rare.

"Hundreds of bites and scratches occur every year in monkey facilities in the United States, but people rarely get infected with B virus," the CDC says.

Still, more monkeys also mean more chances of conflict with people. Not long ago, the state temporarily closed portions of the park due to aggressive monkeys, including one incident where a monkey chased a family.

In late June 2017, the park shut two areas because of aggressive monkey behavior towards visitors. One family shot a video that showed monkeys approaching them, though no contact was made with those people or anyone else, park officials said at the time.

"I understand why they don't want to address the issue. It's a lose-lose situation. But if they do nothing, they are potentially opening up a barrel of monkeys, so to speak," Johnson said. "It's not an issue if it's catching pythons. No one cares about snakes. When it's a furry, charismatic animal, it makes it different."

Monkeys came for cruise boat company, not 'Tarzan' movies

Contrary to local legend, the monkeys were not part of the multiple "Tarzan" movies that were shot at Silver Springs during the 1930s and '40s.

The species is native to parts of Afghanistan, Pakistan, India, China and other areas of southeast Asia. But they are easily adaptable and do well in captivity, which made them popular as research animals and for entertainment at roadside attractions.

It was their popularity as attractions that brought them to Silver Springs almost 90 years ago. Colonel Tooey – Colonel was his first name, not a rank – operated a jungle cruise boat on the river in the 1930s.

Tooey wanted to jungle-up his cruises, so he bought six rhesus macaques to Ocala. He let them loose on a small island on the river, still known as Monkey Island, and almost as soon as they were on dry land, the monkeys swam away.

That small troop prospered and multiplied as they found a cornucopia of tasty plants, insects and bird eggs.

From 1984 to 2012, about 1,000 monkeys were removed or sterilized through permits issued by the state. But since 2012, the Department of Environmental Protection stopped issuing removal permits when it became known that trapped monkeys wound up at research facilities.

It was about the same time that the DEP’s Florida Park Service took full control of Silver Springs from private operators. For decades, the Silver River’s headwaters were part of a theme park attraction.

Since then, the monkeys have continued to reproduce unchecked. The only state action came from the Florida Fish and Wildlife Conservation Commission, which in 2018 prohibited the feeding of wild monkeys.

In 2015, an estimated 176 monkeys lived on the Silver River, according to the University of Florida study published last year in the Journal of Wildlife Management.

Capt. Nick Bozman, who runs tours up and down the river, says he has noticed the monkey population increase.

"I've seen six different troops around here in one day," he said. "The population, I'm pretty sure, is growing."