In the 1930s and ‘40s, the captain of a glass-bottom boat released a dozen or so rhesus macaques on an island in Florida’s Silver River, which snakes through Marion county in the center of the state. The idea was that the monkeys, native to Asia, would be a laugh for tourists passing by. But it seems the monkeys may be the ones to get the last laugh.

For one thing, macaques are excellent swimmers and promptly got themselves off the island. In the decades since, their population has exploded to upward of 800 in the surrounding Silver Spring State Park and nearby Ocala National Forest. A new study, out in the February issue of Emerging Infectious Diseases, reveals that the population is also spreading a dangerous type of herpes. The virus—macacine herpesvirus 1 (McHV-1), aka herpes B or monkey B virus—is common and causes mild infections in macaques. But in humans, it can lead to severe, often lethal, illnesses.

The study authors, led by Samantha Wisely of the University of Florida, Gainesville, concluded that the monkeys must be considered a public health concern and "adequate public health measures should be taken."

For the study, Wisely and her colleagues analyzed blood data collected from 317 monkeys by private trappers between 2000 and 2012. The researchers also examined 121 oral swabs, 23 fecal samples, and 10 soil samples that they collected between 2015 and 2016.

Based on the blood tests, the researchers estimated that about 25 percent of the population carried the virus annually between 2000 and 2012. From the saliva swabs, about four to 14 percent of the infected were actively shedding the virus during the stressful fall mating season of 2015.

Primate plague

Like the herpes viruses that infect humans (HSV-1 and HSV-2), McHV-1 infects nerves and can go dormant. If the animals are stressed or have a weakened immune system, the virus can reemerge and seep from the mucus membranes of the mouth, nose, or genitals. This is similar to how HSV-1 and HSV-2 periodically erupt in humans, typically around the mouth and genitals, respectively.

But when McHV-1 gets into humans, it can cause serious problems in the central nervous system. The virus can be spread to humans by monkey bites and scratches, as well as infectious fluids/feces getting splashed into the eyes (which happened once). Depending on the route of infection and the number of virus particles transferred, the infection in humans can progress from flu-like symptoms to neurological problems. These include double vision, lack of voluntary control of muscle movements, and paralysis. If neurological symptoms develop, the infected person will likely die even with antiviral therapy.

Since McHV-1 was identified in 1932, researchers have only documented 50 cases of human infections, all from captive macaques. Of those cases, 21 resulted in death.

So far, there have been no documented cases of humans getting McHV-1 from wild monkeys, in Florida or elsewhere. This, the researchers speculate, may be because captive monkeys are more stressed and therefore shed more virus, and/or that humans have contracted McHV-1 from wild monkeys but those cases were unreported or misdiagnosed.

Wisely and colleagues say the threat of transmission in Florida is real. They note that between 1977 and 1984, the Florida Fish and Wildlife Commission recorded at least 23 cases of monkeys biting humans in the Silver Spring State Park. After private trapping collected hundreds of monkeys between 2000 and 2012, the population is lower now, with an estimated 175 monkeys in Silver Spring State Park in 2015.

Still, the researchers note, the primates have "high reproductive capacity" and are accustomed to getting close to and interacting with humans. The bottom line is that "these macaques can shed McHV-1, putting humans at risk for exposure to this potentially fatal pathogen," the researchers conclude.

Emerging Infectious Diseases, 2017. DOI: 10.3201/eid2402.171439 (About DOIs).