MOSQUITO-borne diseases are on the rise. Disturbingly, the animal viruses they carry are finding ways to infect new human hosts.

A new instance of just such an inter-species leap has just been discovered in Florida.

Known as the Keystone virus, it is normally carried by mosquitoes between animals.

But a 16-yar-old boy who underwent urgent treatment for a severe fever and rash in North Central Florida in 2016 has been found to be suffering something new.

A recent edition of the Clinical Infectious Diseases journal reports the teen was initially suspected to be suffering a zika virus infection.

Fears were raised he could end up suffering a severe brain infection.

It was, after all, at the height of zika’s headlong-race through Florida. This new disease emerged in South America in recent decades, and has recently reached epidemic proportions in the southern United States as a warming climate enables the breeds of mosquitoes that are its host to range further north than ever before.

The virus can cause severe birth defects.

But tests for zika came back negative.

Researchers from the University of Florida became involved in the case in a bid to resolve the teen’s mystery condition.

“We couldn’t identify what was going on,” Doctor Glenn Morris, director of the university’s Emerging Pathogens Institute, told NPR. “We screened this with all the standard approaches and it literally took a year and a half of sort of dogged laboratory work to figure out what this virus was.”

Eventually, they discovered the Keystone virus in cultures grown from samples taken from the patient.

This was unexpected.

It’s not supposed to be capable of living in humans.

Now researchers believe the virus may be far more widespread than previously believed.

“Although the virus has never previously been found in humans, the infection may actually be fairly common in North Florida,” Doctor Morris said in a statement. “It’s one of these instances where if you don’t know to look for something, you don’t find it.”

A 1972 study discovered antibodies which could have been caused by the Keystone virus in about 20 per cent of the Tampa Bay region’s population.

The Keystone virus was first identified among animals living in the Tampa Bay area in 1964. It has since been discovered living in a band between Texas and Chesapeake Bay.

Two close relatives of the Keystone virus, known as the Jameston Canyon virus and La Crosse encephalitis virus, can cause damaging brain inflammation.

“All sorts of viruses are being transmitted by mosquitoes, yet we don’t fully understand the rate of disease transmission,” Dr Morris said. “Additional research into the spread of vector-borne diseases will help us shine a light on the pathogens that are of greatest concern to both human and animal health.’’