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Men infected with Zika virus should take extra caution to prevent transmission

The Zika virus is usually spread by mosquito bites, but can also be spread by sexual intercourse. A recent study measured the amount of active Zika virus in the semen of infected men.

The Zika virus can cause serious birth defects if it infects pregnant women. It is usually spread by a bite from an infected mosquito. Infected mosquitos are usually found in the warmer parts of the world, including Central and South America. Despite this, almost 6,000 cases of infections have been reported in the USA. These are mostly among travelers returning from affected countries, but more than 50 cases were due to sexual transmission.

Sexual transmission could potentially increase the risk of birth defects in infected pregnant women. For this reason, it is important to better understand how easily Zika virus can be spread by sexual transmission.

A group of researchers at the American Centers for Disease Control and Prevention (CDC) recently measured the amount of Zika virus in the urine and semen of infected men. The results of their study were published in the New England Journal of Medicine.

Infected men were asked to submit samples

The researchers contacted American residents who had been recently infected with the Zika virus. They were asked to prove urine and semen samples twice monthly. They tested these samples for Zika virus at the CDC. They also asked them to continue submitting samples for at least six months or until Zika could no longer be detected in their urine or semen.

A total of 184 men submitted samples for the study. The most common symptoms in the study participants were rash, fever, and joint pain. Half of the men also experienced conjunctivitis (i.e., pink eye). All but one of the participants had picked up the disease while travelling outside the USA.

Zika virus found in samples

The most common method of identifying a virus in a sample is to look for its genetic material. For the Zika virus, this is RNA. Usually, more RNA means that more of the virus is present. The researchers found Zika virus RNA in the semen of a third of the men in the study.

Zika RNA was very common in semen samples submitted within a month of infection and very rare in samples submitted more than three months after infection. The amount of viral RNA tended to decrease over time for each man in the study, but the rate of decrease was not very steady.

For a few men, Zika disappeared in their semen, only to re-appear in the next sample. Four study participants had measurable Zika RNA in their semen for longer than six months; it persisted for nine months in one man.

The researchers calculated that it took, on average, two months for Zika RNA to vanish from semen. However, they estimated that 1% of infected men would still carry Zika in their semen eight months after infection.

Some of the study participants had received a vasectomy before they were infected. Their semen was just as likely to have detectable virus, but the amount of viral RNA in their semen was significantly less than in non-vasectomized men.

Younger men, and men who reported more frequent ejaculations tended to be rid of the virus faster than older men. The virus tended to remain longer in men who had reported conjunctivitis or had not reported joint pain.

Zika virus RNA was only found in eight urine samples. There was less Zika RNA in the urine samples compared to the semen samples. Active Zika virus was not detected in any of the urine samples.

The most commonly-used tests do not reveal if the virus is active and infective

Measuring genetic material, such as RNA, is the most rapid and specific method for detecting individual viruses. However, it does not tell you if the detected virus is actually active and infective. For that, you need to isolate and grow the virus from the samples. This method is time-consuming and potentially hazardous, so it is rarely used in the clinic anymore.

The researchers attempted to grow Zika from the semen samples that had tested positive for Zika RNA. However, they only found active Zika in three of 19 samples taken within a month of infection. The three semen samples with active Zika were also the ones with the highest concentration of viral RNA. They found no active Zika in any of the 59 samples taken after a month.

Infected men need to take precautions

This study shows that Zika virus is present in the semen of infected men for a few months after infection. However, it appears that it is active and infectious in only some of those semen samples. This does not invalidate current guidelines calling for infected men to take precautions in their sexual activity.

The current detection methods accurately detect people who have been infected. However, Heinz Feldmann, a senior researcher who has worked with infectious viruses such as Ebola, responded to this study by pointing out that “on the research front, we need more rapid approaches for detection that measure virus infectivity rather than genome presence.”

Future research will need to address some of the shortcomings of this study. For example, this study didn’t include any of the many infected people who have mild or no symptoms. This study also did not determine how likely it is for the Zika virus to be sexually transmitted.

Researchers are developing vaccines that may prevent infection, but in the meantime this study helps us to better understand how the virus can be transmitted between people. This could help slow the spread of the disease.

Written by Bryan Hughes, PhD


(1) Mead, P. S., Duggal, N. K., Hook, S. A., Delorey, M., Fischer, M., Olzenak McGuire, D., Becksted, H., Max, R. J., Anishchenko, M., Schwartz, A. M., Tzeng, W.-P., Nelson, C. A., McDonald, E. M., Brooks, J. T., Brault, A. C. & Hinckley, A. F. Zika Virus Shedding in Semen of Symptomatic Infected Men. New England Journal of Medicine 378, 1377-1385 (2018)
(2) Zika Virus. Centers for Disease Control and Prevention, U.S. Department of Health & Human Services. Accessed June 26, 2018.
(3) Feldmann, H. Virus in Semen and the Risk of Sexual Transmission. New England Journal of Medicine 378, 1440-1441 (2018)

Bryan Hughes PhD
Bryan Hughes PhD
Bryan completed his Ph.D. in biology at McGill University, where he studied metabolism and the mechanisms of aging. He then worked at the University of Alberta as a Postdoctoral Research Fellow, investigating the causes of heart disease. After publishing many articles in scientific journals, he welcomes the opportunity to share the latest research findings with the wide audience of the Medical News Bulletin.


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