By Jonathan Stempel

NEW YORK (Reuters) - A federal appeals court on Friday revived an effort to block a New York City regulation that requires people who perform a Jewish circumcision ritual on infants that involves oral suction to first obtain parental consent.

A unanimous panel of the 2nd U.S. Circuit Court of Appeals said a trial judge had been too deferential to the city, which had linked the ritual to a deadly form of herpes, in rejecting a request by rabbinical groups for a preliminary injunction.

The three-judge panel directed U.S. District Judge Naomi Reice Buchwald in Manhattan to instead use "strict scrutiny" to see if the regulation infringed the plaintiffs' free exercise of religion, violating the First Amendment.

At issue was the ritual metzitzah b'peh (MBP), in which a mohel who performs a circumcision uses oral suction to draw blood away from a wound on an infant's penis. The procedure is sometimes performed in ultra-Orthodox communities.

In September 2012, the New York City Board of Health voted to require mohels to obtain advance consent in which parents acknowledged the risk of herpes infection linked to the ritual.

This came after city health officials identified 11 cases since 2000 in which infant boys contracted herpes simplex virus (HSV) following circumcisions believed to involve oral suction. Two of the boys died.

The Central Rabbinical Congress of the United States and Canada, the International Bris Association and some rabbis sought to halt enforcement, but Buchwald denied the request, saying the regulation addressed "legitimate societal concerns."

Circuit Judge Debra Ann Livingston, however, wrote for the 2nd Circuit that the regulation was not neutral toward religion because it "purposefully singles out religious conduct performed by a subset of Orthodox Jews," and applies exclusively to them.

As a result she said Buchwald should have not reviewed simply whether there was a rational basis for the regulation.

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"The Department (of Health) may have legitimate reasons for addressing HSV infection risk among infants primarily, if not exclusively, by regulating MBP," Livingston wrote. "On the present record, however, the plaintiffs have made a sufficient case for strict scrutiny by establishing that the risk of transmission by reason of metzitzah b'peh has been singled out."

The 2nd Circuit did not rule on the regulation's constitutionality.

The city's law department had no comment on the decision.

In a joint statement, the plaintiffs called the decision a "great victory," and said they remain ready to work with city officials "to protect our children's health while fully respecting and accommodating our religious practice."

The case is Central Rabbinical Congress of the United States and Canada et al v. New York City Department of Health & Mental Hygiene et al, 2nd U.S. Circuit Court of Appeals, No. 13-107.

(Reporting by Jonathan Stempel in New York, editing by Tom Brown and Andrew Hay)