The coronavirus invades cells in the penis and testicles of monkeys, researchers discover.

The coronavirus may infect tissue within the male genital tract, new research on rhesus macaques shows. The finding suggests that symptoms like erectile dysfunction reported by some Covid patients may be caused directly by the virus, not by inflammation or fever that often accompany the disease.

The research demonstrated that the coronavirus infected the prostate, penis, testicles and surrounding blood vessels in three male rhesus macaques. The monkeys were examined with whole body scans specially designed to detect sites of infection.

Scientists — who expected to find the coronavirus in spots like the lungs but did not know where else they would find it — were somewhat surprised by the discovery.

“The signal that jumped out at us was the complete spread through the male genital tract,” said Thomas Hope, the paper’s senior author and a professor of cell and developmental biology at Northwestern University Feinberg School of Medicine in Chicago. “We had no idea we would find it there.”

When his team initially reviewed a scanned image from the first animal, one of the scientists asked, “What sex was the animal again?” Dr. Hope recalled.

“I said, ‘I think female.’ She said, ‘I don’t think it’s a female.’ I went down to the bottom of the image, which was almost cut off, and the testes were brightly lit up. And the signal in the penis was off the radar,” Dr. Hope said.

The paper was based on findings in just three monkeys, but the findings were consistent, Dr. Hope said. The study has not yet been peer reviewed for publication in a journal, and was posted Monday on the site bioRxiv.

The work was carried out at the Tulane National Primate Research Center in Louisiana. The researchers do not know whether the monkeys had symptoms corresponding to the viral infection of the male genital tract, such as low testosterone levels, low sperm counts, pain or sexual dysfunction, Dr. Hope said.

About 10 to 20 percent of men infected with the coronavirus have symptoms linked to male genital tract dysfunction, studies have reported.

Men infected with the virus are three to six times as likely as others to develop erectile dysfunction, believed to be an indicator of so-called long Covid.

Patients have also reported symptoms such as testicular pain, reduced sperm counts and reduced sperm quality, decreased fertility and hypogonadism, a condition in which the testes produce insufficient amounts of testosterone, leading to low sex drive, sexual dysfunction and reduced fertility.

Other viruses are known to take a toll on fertility, Dr. Hope noted. “Mumps is most famous historically, for causing sterility,” he said. “The Zika virus goes to the testes and infects the testes, and Ebola can also do that.”

Even if just a small fraction of men experience such complications after a coronavirus infection, millions may suffer from impaired sexual and reproductive health in the aftermath of the pandemic, simply because the virus has infected so many people around the world, Dr. Hope warned.

He urged men to get vaccinated, and to seek a medical evaluation if they are concerned about their sexual or reproductive health.

The positron emission tomography technology that was used in the new study was designed to identify the sites of coronavirus infection in a living animal. The technology makes it possible to do repeated, sequential scanning of an animal, tracking how the virus works its way through the body and how it is cleared.

Dr. Hope next plans to determine whether the testicles are a reservoir for the coronavirus, as has been hypothesized by some scientists. He will also look at whether the virus infects tissue in the female reproductive system.

The hope is to use the information to develop treatments that will mitigate the pandemic’s impact on fertility. The scans could also potentially detect the location of the virus in patients and to tailor treatments appropriately.