A high-profile porn star nicknamed "patient zero" is being blamed for the syphilis outbreak which has halted porn production in Los Angeles.

The male actor, who has been named on social media sites, allegedly concealed a positive test result last month and continued filming, leading to the infection of at least half a dozen colleagues before the industry called a moratorium.

The outbreak will intensify pressure on porn makers to use condoms in sex scenes, a proposal which health authorities wish to make mandatory.

The Free Speech Coalition, a porn industry trade group, called for the temporary nationwide production halt on August 18, a day after Los Angeles county's public health department said it was investigating a cluster of possible syphilis cases within the industry. It had received reports of at least five cases last week.

The Free Speech Coalition said all performers should be tested for the sexually transmitted disease. It is also organising antibiotic treatment for LA's more than 1000 performers, because a negative test result did not give the all clear.

"It is important to note that this test has a large window period — as much as 90 days — and therefore cannot be relied upon to diagnose an acute (recent) syphilis infection," the group said on its website.

The industry's Adult Production Health and Safety Services experts had recommended that all performers take antibiotics, it said. "The shots have been ordered from the pharmacy and within the next couple of days, APHSS will have set up a network of medical professionals to administer the shots."

Performers will be allowed to return to work 10 days after receiving the shot. "Clearly our industry's priority is the health and well-being of our performers," Diane Duke, the executive director of the Free Speech Coalition, said in the statement.

Joanne Cachapero, a spokesperson for the group, said antibiotic supplies for performers had arrived on Wednesday and were available in Los Angeles and Florida. They will be available in Las Vegas from Monday.

The main production companies in Los Angeles, which makes most US porn, have reportedly complied with the moratorium, putting the billion-dollar industry on hold.

A well-known actor who has won multiple adult entertainment awards has been named on Twitter and gossip sites as the source of the outbreak. Some accounts say he photocopied and amended a positive test result so as to continue working. No evidence has been supplied, and the actor has not publicly commented.

Capachero declined to comment on the allegations about "patient zero",

saying the online atmosphere of gossip and speculation was harmful.

"Without facts from parties involved, comments that have appeared on

social networks and industry blogs should be disregarded until there

is clarification from parties involved or confirmed information from

definitive sources."

Syphilis is transmitted through direct contact with syphilis sores found mostly on the external genitals, vagina, anus or inside the rectum. Once a killer, now it can easily be treated with penicillin.

However, the Aids Healthcare Foundation, which has led the campaign to make condoms obligatory in adult films, criticised the call to give antibiotics to all performers. "They just want to throw an antibiotic blanket over the whole thing," the group's president, Michael Weinstein, told MSNBC. It would be better, he said, to take the time to properly investigate the outbreak. The industry's rushed response showed it was incapable of policing itself, he said.

California regulations in theory mandate the use of condoms by porn actors but few do so, saying it would hurt sales of films.

The Aids Healthcare Foundation successfully lobbied LA city officials to toughen the regulations in January but it remains unclear what impact, if any, that has had.

A ballot initiative known as Measure B, due in November will go further and require LA county health officials to impose requirements for condoms on porn sets, including studio sound stages which currently are exempt.

Porn production was briefly halted in 2010 after the actor Derrick Burts was diagnosed with HIV. There was another brief moratorium in 2004 when five performers tested positive for the virus.