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

New research on rhesus monkeys shows that the coronavirus can infect tissue within male genital system. This finding suggests that symptoms such as erectile problems reported by Covid patients could be directly caused by the virus and not inflammation or fever, which often accompany the disease.

The coronavirus infected three male rhesus monkeys with their prostate, penis and surrounding blood vessels. The monkeys were subject to whole-body scans that were specifically designed to detect infection sites.

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.

Dr. Hope said that although the paper was based on only three monkeys, the findings were consistent. The study was posted Monday to bioRxiv, but has not been peer reviewed for publication.

The research was conducted at the Tulane N Primate Research Center in Louisiana. Dr. Hope stated that the researchers don’t know if the monkeys suffered from symptoms related to viral infection of the male reproductive tract.

Studies have shown that around 10-20% of coronavirus-infected men have symptoms related to male genital dysfunction.

Three to six times as likely are men infected with the virus to experience erectile dysfunction than others, which is believed to be an indicator for so-called long Covid.

Patients have also reported symptoms including testicular pain, reduced quality and sperm counts, and hypogonadism. Hypogonadism is a condition in the which the testes don’t produce enough testosterone. This can lead to low sex drive and sexual dysfunction as well as reduced fertility.

Dr. Hope pointed out that other viruses can also affect fertility. “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.”

Dr. Hope warned that even though only a small percentage of men are affected by coronavirus infections, millions of people may experience impaired sexual and reproductive health after the pandemic.

He advised men to get vaccinated and to seek medical evaluation if they have concerns about their sexual or reproductive health.

The new study used positron emission technology to detect coronavirus infection in living animals. The technology allows for repeated, sequential scanning in an animal. This allows the tracking of how the virus moves through the body, and how it is cleared.

Dr. Hope plans to next determine if the testicles can be a reservoir for coronavirus as some scientists have suggested. He will also examine whether the virus infects the tissue of 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 be used to detect the virus’s location in patients and tailor treatment accordingly.