Mpox often leads to severe illness, even death, in people with advanced HIV.

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In people with advanced HIV disease, the mpox virus — formerly known as monkeypox — often causes severe disease, with a mortality rate of about 15 percent, researchers reported Tuesday.

Researchers at the Conference on Retroviruses and Opportunistic Infections in Seattle said the severity of the infection warrants the inclusion of mpox in opportunistic situations that are particularly dangerous for people with advanced HIV.

Dr Chloe Orkin, an HIV expert at Queen Mary University of London who led the research, said: “These findings make it very, very clear that everyone with mpox should have an HIV test.” She and her colleagues described the results Tuesday in the journal The Lancet.

The Mpox outbreak began last May. Although the number of cases has slowed in most regions, it has so far affected nearly 86,000 people in 110 countries and killed 92. Several studies estimate that 40 percent to 50 percent of infected individuals were living with HIV.

When antiretroviral drugs keep HIV under control, mpox is almost as dangerous as it is for people without HIV infection. But a CDC study last year, along with observations from previous outbreaks in Nigeria, indicated that mpox was more severe and far more fatal in people with higher levels of HIV.

In the new study, an international group of physicians tracked 382 adults in 28 countries who had advanced HIV disease and were infected with mpox. They analyzed the amount of HIV and the number of CD4 cells, a type of immune system cell, in these patients.

The typical range for CD4 cells is 500 to 1,500 per cubic millimeter of blood. All 27 deaths in the study were among people who had fewer than 200 CD4 cells. Mpox kills about 30 percent of people with fewer than 100 CD4 cells.

The nature of the disease was also strikingly different in patients with weakened immune systems. While most people infected with mpox show lesions only at the site of exposure, people with advanced HIV develop large, ulcerated lesions with the virus spread throughout the body.

“It’s on the back, it’s on the legs, it’s in the eyes, it’s everywhere — it’s horrible,” Dr. Orkin said. “This is because the immune system is not able to contain the virus at all.”

He said that many patients also had lumps in the lungs, which led to acute respiratory distress.

Adding mpox to the list of opportunistic infections among people with advanced HIV would encourage health care workers to identify and prioritize patients at risk of severe illness and death.

Patients would need antibiotics to prevent other opportunistic infections, and would be offered two doses of the vaccine injected under the skin rather than into the folds of the skin, as is currently done, Dr. Orkin said.

The United States added mpox to its list of possible opportunistic infections in people with HIV in September. The World Health Organization plans to discuss doing the same in the next few months, said Dr. Meg Doherty, director of global HIV, hepatitis and sexually transmitted infections programs at WHO.

Dr. Doherty said the new data makes a “compelling case” for adding mpox to the list of opportunistic conditions.

In parts of the world where people living with HIV may not have access to the mpox vaccine or treatment for mpox and HIV, he said, “this should just raise awareness that we have more to do than we do in those areas. “

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