CHICAGO • The Zika virus may be particularly adept at entrenching itself in parts of the body that are shielded from the immune system, making it harder to fight off and possibly lengthening the timeframe in which it can be transmitted, top US experts said on Friday.

Researchers reported that Zika virus can be detected in semen for 62 days after a person is infected, adding to evidence of the virus' presence in foetal brain tissue, placenta and amniotic fluid.

Their work is part of an international race to understand the risks associated with Zika, a rapidly spreading mosquito-borne virus thought to be linked to thousands of cases of birth defects in Brazil.

"Right now, we know it's in the blood for a very limited period of time, measured in a week to at most 10 days. We know now, as we accumulate experience, it can be seen in the seminal fluid. We're not exactly sure after the infection clears, where else it would be," Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said. "These are all things that need to be carefully examined in natural history and case-control studies," he said.

Dr Fauci said that Zika's persistence in the body recalled findings during the 2014 Ebola outbreak, the worst on record. In patients, the highly deadly virus remained in semen and eye fluid for months.

Several organs in the body, including the testes, the eyes, the placenta and the brain, are "immune privileged" - protected from attacks launched by the immune system to neutralise foreign invaders. These sites are safeguarded from antibodies to prevent the immune system from attacking vital tissues.

But if a virus enters these protected sites, it is much harder to fight it off. "The virus can continue to persist and/or multiply," said Dr William Schaffner, an infectious disease expert at Vanderbilt University Medical Centre in Nashville.

Last week, researchers in Slovenia published a paper in the New England Journal Of Medicine describing a severely brain damaged foetus from a mother who was infected with Zika in Brazil and later terminated the pregnancy.

They suggested that Zika may persist in the foetal brain because it is an immunologically privileged site.

That is true of many other viruses, such as toxoplasmosis, rubella, cytomegalovirus and herpes, which can also cross the placenta and cause microcephaly, a birth defect marked by small head size and underdeveloped brains.

REUTERS