Experts fear the Zika virus, which has been linked to brain damage in unborn babies, may be transmitted through sex.
Zika is known to be spread by mosquitos bites and has spread rapidly through Brazil and other South American countries since late 2015.
While it manifests itself as a relatively harmless fever in most patients, it has also been linked to a spike in microcephaly – a condition causing babies to appear to have shrunken heads.
Health officials across the world have warned pregnant women in affected South American countries to take precaution to protect themselves from mosquito bites, with some advising women to put off becoming pregnant.
However, two cases of the virus in medical literature suggest that Zika may also be spread through during sex, prompting experts to call for further investigation into the possibility.
The only known cases of the virus being detected in a man’s semen involved a 44-year-old Tahitian man who contracted Zika during a drink to French Polynesia in 2013, The New York Times reported.
While his blood was clear, French investigators found traces of the virus in his semen and his urine.
The second case was that of Dr Brain D Foy, an expert in insect-borne diseases at Colorado State University, who unwittingly developed the virus after he travelled to Senegal to collect mosquitos for a study. Both he and his colleague who accompanied him fell ill with a fever when he returned to the US.
Days later, Dr Foy’s wife, a nurse, displayed similar symptoms including headache pains, a rash, and bloodshot eyes.
However, what had infected the three was unclear after the blood samples tested for malaria, dengue fever and yellow fever returned negative.
On the suggestion of another scientist, Dr Foy had the blood samples re-tested and found that he and his colleague, as well as his wife who had remained in the US, had been infected by Zika.
Dr Foy relayed his experience of the virus in the journal ‘Emerging Infectious Diseases’.
Research into Zika is further complicated by the fact that it does not infect common lab animals such as rats and mice, meaning controversial trials on monkeys may need to be used to investigate the condition.
Dr. William Schaffner, chief of preventive medicine at Vanderbilt University Medical School told The New York Times that while two suspected cases do not warrant a health warning from public health officials, he said: “it certainly should be studied.”
Battling the zika virus - in pictures
Battling the zika virus - in pictures
A worker of the Salvadorean Ministry of Health fumigates a house in Soyapango, 6 kilometers from San Salvador, El Salvador. Salvadorean authorities have began a three days campaign of fumigation to reduce the presence of the mosquito that transmit the Zika virus.
A Health Ministry employee fumigates a home against the Aedes aegypti mosquito to prevent the spread of the Zika virus in Soyapango, six km east of San Salvador. Health authorities have issued a national alert against the Aedes Aegypti mosquito, because of the link between the Zika virus and microcephaly and Guillain-BarrÈ Syndrome in foetuses.
AFP PHOTO/Marvin RECINOSMarvin RECINOS/AFP/Getty Images
A pediatric infectologist examines a two-months-old baby, who has microcephaly, on 26 January 2016 in Recife, Brazil.
A woman walks through the fumes as Health Ministry employee fumigate against the Aedes aegypti mosquito to prevent the spread of the Zika virus in Soyapango.
Marvin RECINOS/AFP/Getty Images
A health ministry employee sprays to eliminate breeding sites of the Aedes Aegypti mosquito, which transmits diseases such as the dengue, chicunguna and Zica viruses, in a Tegucigalpa cemetery on January 21, 2016. The medical school at the National Autonomous University of Honduras (UNAH) recommended that women in the country avoid getting pregnant for the time being due to the presence of the Zika virus. If a pregnant woman is infected by the virus, the baby could be born with microcephaly.
AFP PHOTO/Orlando SIERRA
A man walks away from his home with his son as health workers fumigates the Altos del Cerro neighbourhood as part of preventive measures against the Zika virus and other mosquito-borne diseases in Soyapango, El Salvador
A three-months-old, who has microcephaly, in Recife, Brazil.
A pregnant woman waits to be attended at the Maternal and Children's Hospital in Tegucigalpa. The medical school at the National Autonomous University of Honduras (UNAH) recommended that women in the country avoid getting pregnant for the time being due to the presence of the Zika virus. If a pregnant woman is infected by the virus, the baby could be born with microcephaly.
ORLANDO SIERRA/AFP/Getty Images
Army soldiers apply insect repellent as they prepare for a clean up operation against the Aedes aegypti mosquito, which is a vector for transmitting the Zika virus in Sao Paulo, Brazil.
AP Photo/Andre Penner
Workers disinfect the Sambadrome in Rio de Janeiro to fight the spread of the Zika virus
Dr. Vanessa Van Der Linden, the neuro-pediatrician who first recognized the microcephaly crisis in Brazil, measures the head of a 2-month-old baby with microcephaly in Recife
Mother Mylene Helena Ferreira cares for her son David Henrique Ferreira, 5 months, who has microcephaly, on January 25, 2016 in Recife, Brazil. In the last four months, authorities have recorded close to 4,000 cases in Brazil in which the mosquito-borne Zika virus may have led to microcephaly in infants
U.S. women who are pregnant from traveling to many South American countries
In the last four months, authorities have recorded close to 4,000 cases in Brazil in which the mosquito-borne Zika virus may have led to microcephaly in infants.
Dr. Vanessa Van Der Linden, the neuro-pediatrician who first recognized the microcephaly crisis in Brazil, examines a two-month-old baby with microcephaly on January 27, 2016 in Recife, Brazil
Brazil is one of the countries in South America where the Zika virus has taken hold
Health workers fumigating to combat Zika virus in Lima, Peru. The US have already issued a warning urging pregnant women to avoid travel to Latin American countries
Two-month-old Jose Wesley, born with microcephaly in Brazil, is nursed by his brother
However, the World Health Organisation has sought to quell fears and said that
“Zika has been isolated in human semen, and one case of possible person-to-person sexual transmission has been described" but added: "more evidence is needed to confirm whether sexual contact is a means of Zika transmission.”
“The role of Aedes mosquitoes in transmitting Zika is documented and well understood, while evidence about other transmission routes is limited,” it said.
Health officials at the US Centers for Disease Control and Prevention have also said the two apparent instances are a “theoretical risk” and there is insufficient evidence to issue a warning about any concern that the virus may be spread through sex.
Dr Márcio Nehab, a paediatrician and infectious disease specialist at the Fiocruz research institute in Rio de Janeiro said that researchers should focus their efforts on mosquitoes.
"We still need a lot of study to conclude that sexual transmission can happen because little is known about the Zika virus.
"At the moment, we have to care more about the known vector, which is the mosquito, as the virus transmission route,” he said according to MailOnline.