The announcement Tuesday that Dr. Shannon Hader, director of the city’s HIV/AIDS administration, had resigned after serving three years at an agency she is credited with greatly improving drew expressions of puzzlement and concern among LGBT and AIDS activists.

Speculation that the resignation could be linked to disagreements between her and the director of the Department of Health, Dr. Pierre Vigilance, under whom she served, surfaced when the department omitted any mention of Hader in a statement announcing her interim replacement.

“I share the concern of the community about the abrupt departure of Dr. Hader,” said Peter Rosenstein, a local gay activist. “We have to make sure we don’t fall back into the hole again on AIDS,” he said, referring to the AIDS administration’s reputation for being trouble-plagued prior to Hader’s tenure there.

Mayor Adrian Fenty formally announced Hader’s resignation at an impromptu news conference Tuesday afternoon, when he introduced Dr. Nnemdi Kamanu Elias, the AIDS administration’s chief medical director, as Hader’s temporary replacement.

“Our city has made considerable strides in the fight against communicable diseases like HIV/AIDS, increasing the number of HIV testing programs and investing in HIV/AIDS awareness and research,” Fenty said. “I am confident that Dr. Kamanu Elias and the District will continue to ensure that city residents are armed with the necessary tools to protect and improve their health.”

The mayor’s office said a nationwide search would be conducted to find a permanent replacement.

Fenty told the Blade on Thursday that Hader, whom he described as having done “a fantastic job” at the city’s HIV/AIDS administration, felt it was time to “move on” in her career.

“Just like everything else in life, there’s a time when you move on and do something bigger, better and/or different,” he said. “And I think it was that time for her.”

Fenty said he had no comment on speculation of friction between Hader and Vigilance.

Hader will leave her current post July 15 and will begin a new job sometime this summer as vice president of the D.C.-based Future’s Group, an international consulting firm that works on public health issues.

Hader told the Blade in an e-mail that she would leave it to the mayor’s office to comment on details surrounding her resignation rather than discuss the matter herself.

“I will continue to be involved in the fight against HIV/AIDS, just changing hats from time to time,” she said. “I look forward to putting on the new hat of ‘expert & engaged community member’ with regards to the fight against HIV/AIDS in D.C. I am a resident, I am a constituent, I am passionate about our community and feel privileged to be (and continue to be) a part of it!” she said in her e-mail.

Gay D.C. Council members David Catania (I-At Large) and Jim Graham (D-Ward 1), who have been outspoken advocates for city programs on behalf of people with HIV/AIDS, could not be immediately reached for comment.

The Washington Post, which contacted Catania, reported he declined to comment on speculation that he was deeply troubled that Fenty and Vigilance did not take adequate steps to keep Hader in her post as D.C.’s AIDS administration head.

“Her loss is catastrophic,” the Post quoted Catania as saying.

Catania told the Blade in an interview last week, before news of Hader’s resignation surfaced, that she played an instrumental role in improving the city’s AIDS programs, helping to transform the AIDS administration from what he called an agency in “total chaos.”

Although few people use its full name, the administration was merged two years ago with other health department agencies and renamed the HIV/AIDS, Hepatitis, Tuberculosis & Sexually Transmitted Disease Administration.

As chair of the Council’s Committee on Health, Catania is also credited with focusing attention on the problems at the AIDS administration through numerous committee oversight hearings.

Before becoming head of the city’s AIDS administration, Hader had served as an epidemiologist for the U.S. Centers for Disease Control and Prevention’s Global AIDS Program in Zimbabwe.

Kamanu Elias became chief medical officer for the D.C. AIDS administration in February 2009. The statement from the mayor’s office says her duties included ensuring “the technical and programmatic quality of HIV/AIDS and related programs throughout the DOH.”

Before joining the D.C. government, she worked as senior manager at the European and Developing Countries Clinical Trials Partnership in the Netherlands. The organization is an arm of the European Commission and was created to support the development of “clinical interventions for HIV and other diseases in Africa and to build the capacity of developing country scientists,” the statement from the mayor’s office says.