By Bill Berkrot

(Reuters) - It could take years to learn how long men infected with Zika are capable of sexually transmitting the virus, which can cause crippling birth defects and other serious neurological disorders.

In the meantime, health officials have warned couples to refrain from unprotected sex for six months after a male partner is infected. The extraordinary recommendation, based on a single report of Zika surviving 62 days in semen, could affect millions.

The grave risks associated with Zika, along with its potential reach, are driving U.S. health authorities to pursue research even though funding is mired in Congressional gridlock. A study of sexual transmission risk is one example of science that health officials said can’t wait for politics.

Borrowing money earmarked for other programs, the U.S. National Institute of Allergy and Infectious Diseases has started enrolling men infected with Zika in Brazil and Colombia in the study to determine how long the virus remains transmittable in semen. The study could take years to complete, but interim results could help public health officials fine-tune their recommendations on sex.

"We are going out on a limb, but we have to," Dr. Anthony Fauci, director of the U.S. institute, said in an interview. "We can't say we're going to wait until we get all the money.”

Public health officials are alarmed by Zika’s transmission versatility, which has the potential to expand its reach. It is primarily spread by Aedes aegypti mosquitoes, as are the dengue and chikungunya viruses.

But at least 10 countries, including the United States and France, have reported Zika infections in people who had not traveled to an outbreak area but whose sexual partners had. This ability to spread through sex could help Zika gain a further foothold outside the warm habitats of its most effective agent, the mosquito.

    CAUTION IN LIEU OF ANSWERS

To protect women who are pregnant or trying to conceive, the U.S. Centers for Disease Control and Prevention recommended couples refrain from unprotected sex for six months - triple the 62 days the virus survived in the semen in one British case study. The World Health Organization recently issued similar guidance.

But such strict advice is not ideal, Dr. Anne Schuchat, a CDC deputy director, said in an interview.

"To tell people not to have sex until we get back to you is not a very satisfying recommendation," she said. "We would like to have some more understanding of the sexual risk."

In the U.S. territory of Puerto Rico, where more than 2,100 cases of infection have been reported since the start of the year, health officials are passing out Zika protection kits that include bug spray and condoms, along with the recommendation.

But the warning against unprotected sex isn’t going over very well, said Dr. Chris Prue, a CDC behavioral scientist who has studied the response.

"Condoms are not popular in a lot places," she said. "There's religious and personal preferences and lots of personal factors in that."

U.S. lawmakers deadlocked over funding to fight the Zika virus on Tuesday, as Senate Democrats blocked a Republican proposal they said fell short of the challenge posed by the virus and hurt other health priorities. It was unclear when Congress would revisit the request by President Barack Obama for $1.9 billion.

FUNDING PRIORITIES

In the meantime, the White House has diverted more than $500 million earmarked for other projects for urgent Zika initiatives, including those where scientific opportunities will be lost if not acted upon immediately.

One such study will follow children born to women infected with Zika to identify the development of any disabilities not detected at birth. Other projects on the priority list include vaccine development and mosquito eradication.   One study underway will assess whether the risk of transmission is greater from men who experience Zika infection symptoms, such as fever and rash, than from those who don't. This information is considered vital since most people experience no symptoms.

The study of infected men in Brazil and Colombia will test semen from thousands of men over time to determine how long Zika poses a risk to sexual partners. As long the virus can be grown in a laboratory from semen cell samples, infectious disease experts believe it is potentially contagious.

Zika typically clears the bloodstream about a week after infection, but it has been detected in urine for at least twice as long. Its persistence in semen in the British case study has caused some researchers to draw comparisons to other viruses.

HIV can last in blood and semen indefinitely, and the mosquito-borne West Nile virus can reside in the kidneys and urine for years, researchers said. One patient who survived the deadly Ebola outbreak had evidence of that virus in his semen for 18 months.

"We got very surprised by Ebola that it was hanging around for so long," said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine.

"One of the big questions we have to ask is does Zika also cause a similar type of latency?”

(Additional reporting by Julie Steenhuysen; Editing by Michele Gershberg and Lisa Girion)