The American Red Cross on Tuesday asked prospective donors to hold off giving blood for at least 28 days if they've travelled to areas with Zika outbreaks amid growing concerns about the virus's spread.

"Self-deferral" for blood donors should apply to anyone who has visited Mexico, the Caribbean or Central or South America during the past four weeks, the organization said in a statement.

The request comes the same day the U.S. announced that a patient in Texas had been infected with Zika through sexual transmission and Brazil said suspected and confirmed cases of newborns with abnormally small heads — believed to be linked to the virus — had increased to 4,074 as of Jan. 30 from 3,718 a week earlier.

Federal Health Minister Jane Philpott said Canada is paying close attention to the spread of Zika virus.

"At this point we believe there's a very low risk of possible transmission in Canada," Philpott told CBC's Power & Politics on Tuesday.

"I think Canadians primarily need to be concerned with respect to their possible travel plans," she said.

The Texas case is the first case of the virus being transmitted in the U.S. during the current outbreak of Zika, which has been linked to birth defects in the Americas.

"It's very rare, but this is not new; we always looked at the point that this could be transmitted sexually," Zachary Thompson, director of the Dallas County Health and Human Services, told WFAA-TV in Dallas.

The confirmed patient did not travel but was infected after having sexual contact with an ill person who had returned from Venezuela, Dallas County health officials said in a tweet. They did not release any details about either patient, citing privacy issues.

CLARIFICATION: Dallas County's 1st case of <a href="https://twitter.com/hashtag/Zika?src=hash">#Zika</a> thru sex was acquired from someone who traveled to Venezuela, confirmed case did not travel —@DCHHS

Zika virus is usually spread through mosquito bites. Investigators have been exploring the possibility the virus also can be spread through sex. It was found in one man's semen in Tahiti, and there was a report of a Colorado researcher who caught the virus overseas and apparently spread it to his wife back home in 2008.

"That gives you the plausibility of spread, but the science is clear to date that Zika virus is primarily transmitted to people through the bite of an infected mosquito," Dr. Anne Schuchat of the Centers for Disease Control said during a recent news conference about Zika.

The CDC said it will issue guidance in the coming days on prevention of sexual transmission of Zika virus, focusing on the male sexual partners of women who are or may be pregnant. The CDC has already recommended that pregnant women postpone trips to more than two dozen countries with Zika outbreaks, mostly in Latin America and the Caribbean, including Venezuela. It also said other visitors should use insect repellent and take other precautions to prevent mosquito bites.

In the epidemic in Latin America and the Caribbean, the main villain identified so far is called Aedes aegypti — a species of mosquito that spreads other tropical diseases, including chikungunya and dengue fever. It is found in the southern U.S., though no mosquito-borne transmission has been reported in the continental United States to date.

Texas health department officials said that the case announced Tuesday is the first one contracted in the state but noted that there are seven other cases of the virus in Texas related to foreign travel.

WHO declares emergency

The World Health Organization (WHO) on Monday declared a global emergency over the rapidly spreading Zika virus, saying it is an "extraordinary event" that poses a threat to the rest of the world.

The declaration was made after an emergency meeting of independent experts called in response to a spike in babies born with brain defects and abnormally small heads in Brazil since the virus was first found there last year.

Officials in French Polynesia also documented a connection between Zika and neurological complications when the virus was spreading there two years ago, at the same time as dengue fever.

WHO officials said it could be six to nine months before science proves or disproves any connection between the virus and babies born with abnormally small heads.

The CDC said that in the recent Texas case, there's no risk to a developing fetus.

Zika was first identified in 1947 in Uganda. It wasn't believed to cause any serious effects until last year; about 80 per cent of infected people never experience symptoms.

The most common symptoms are fever, rash, joint pain and red eyes. The illness is usually mild with symptoms lasting several days to a week. Symptoms usually start two days to a week after being bitten by an infected mosquito.

Canadians researching vaccine

Last week, Dr. Theresa Tam, Canada's deputy chief public health officer, said one published case described a male traveller to Africa who came back home, and subsequently his wife developed a Zika virus infection without having travelled.

There's been some detection of the virus in semen for up to two weeks, Tam said.

Tam suggested "precautionary measures" for travellers, but didn't offer specifics. The risk is low, she said.

Philpott said officials at the Public Health Agency of Canada are in "daily contact" with the WHO, and that Canada is ready to help international efforts.

"There are already Canadian researchers involved in some projects around vaccine development for Zika virus," she said.