First human case of Keystone virus discovered in Florida

J. Glenn Morris, Jr.

Researchers from the University of Florida said they have discovered the first human case of Keystone virus, a mosquito-borne disease previously only found in animals but thought to be fairly common in people.

It took almost 2 years for the researchers to determine that the virus was the cause of a rash and mild fever in a teenage boy in Florida.

“This was, to a degree, a back-burner project — we thought it was likely that a virus caused the illness, but standard screening approaches were all negative,” J. Glenn Morris, Jr., MD, MPH, TM, professor of medicine and director of the Emerging Pathogens Institute at the University of Florida, told Infectious Disease News.

“This was a virus discovery — there are no tests for Keystone, and as the virus had not been seen for close to 50 years, we weren’t thinking about it,” added Morris, an Infectious Disease News Editorial Board member. “Our virology team, which does outstanding work in the area of virus discovery, worked intermittently on the sample, and after more than a year of effort, identified Keystone virus.”

In August 2016, with Florida in the midst of a Zika outbreak, a boy aged 16 years presented to an urgent care clinic in north-central Florida with a rash and low-grade fever. Saliva and urine samples tested negative for Zika and other vector-borne infections.

According to the case study, the rash — which started on the teenager’s chest and progressively spread to his abdomen, arm, back and face — was painless, nonpruritic and had no present vesicles. Additionally, the patient did not report chills, headache, neck stiffness or gastrointestinal symptoms. Although he did experience mild fatigue and ankle discomfort, it was attributed to wearing new shoes while attending concurrent marching band summer camps. The patient also reported being bitten several times by mosquitoes during camp, despite the use of DEET.

According to Morris and colleagues, Keystone virus was first isolated in 1964 in mosquitoes in Keystone, Florida, near Tampa Bay, and has also been found in animal populations, such as raccoons, squirrels and whitetail deer, in coastal regions from the Chesapeake Bay to Texas. Although no human infection had been reported until now, serologic testing decades ago in humans identified antibodies of the virus in up to 20% of the sampled population, Morris said.

“We would hypothesize that the virus is actually fairly common, but cases have not been recognized because of the lack of diagnostics,” Morris explained. “When the results were negative, we switched over to tissue culture to see if anything grew in culture using a variety of tissue culture lines. We ultimately saw cytopathic effects in some of the cell lines, which led us to undertake a series of steps to try to identify which virus might be present.”

Aedes a tlanticus appears to be the primary vector of Keystone virus, which is part of the so-called California serogroup of viruses that are known to cause encephalitis, the researchers said.

Morris urged the public to remember to wear mosquito repellent at times and places where mosquitoes are present to minimize the risk of bites. The researchers also suggest that that there are other common but previously unidentified pathogens in the United States and further research into vector-borne diseases is imperative.

“There are a lot of viruses out there causing human disease which are still not recognized — including mosquito-borne viruses such as Keystone,” Morris said. “We need to keep looking for these viruses and develop appropriate diagnostic tests when new viruses are identified. And we need to continue to respect the ability of mosquitoes (and other arthropods) to serve as vectors for a wide range of diseases.” – by Marley Ghizzone

References:

Morris GJ, et al. Clin Infect Dis. 2018;doi:10.1093/cid/ciy485.

UF Health. Virus found in Florida resident may be widespread throughout the Southeast. https://ufhealth.org/news/2018/virus-found-florida-resident-may-be-widespread-throughout-southeast. Accessed June 27, 2018.

Disclosures: Morris reports receiving grant funding from the NIH and the European Commission.