case report. However, a CT scan showed no signs of swelling or hemorrhage in her head. The doctors attempted a lumbar puncture (also called a spinal tap) but failed to collect any cerebrospinal fluid that they could test for irregularities.

The doctors judged the girl to be too medically unstable to be referred to the hospital's psychiatric ward, so she was admitted to the general pediatric department. Pediatric neurologists suggested an electroencephalogram (EEG), a test that measures electric signaling in the brain, but this revealed no further clues.

On her second day in the hospital, the girl entered a complete catatonic state. Her specific symptoms were not described in the report, but in general, catatonia disrupts how a person processes and reacts to the world around them. Catatonic patients often stop moving and speaking. Doctors treated the patient with lorazepam, which slows down nervous-system signaling, and her catatonia subsided in days.

Meanwhile, interventional radiologists performed a successful second lumbar puncture, but the girl's cerebrospinal fluid appeared normal in the initial panels they ran.

After the patient had spent eight days in the hospital, psychiatrists recommended that the girl be transferred to their ward. However, this was not possible because she developed problems with feeding herself. Her continued psychotic symptoms were treated with the antipsychotic drug olanzapine.

The diagnosis: While initial tests of the girl's cerebrospinal fluid did not show signs of meningitis or other abnormalities, a later test came back positive for autoantibodies — immune proteins that target healthy tissues instead of foreign invaders. These antibodies targeted neuronal intermediate filament (NIF), a protein that helps give nerve cells structural support.

Based on this finding, the patient was diagnosed with autoimmune encephalitis, a condition in which the body produces antibodies that attack nervous tissue, leading to harmful inflammation that can cause psychiatric symptoms.

Autoimmune encephalitis is often triggered by a tumor; proteins produced by the growth can resemble proteins within nervous tissue, and the immune system develops a harmful response against the latter while attempting to fight the tumor. The doctors performed MRIs and a PET scan to look for tumors, but they did not find any.

The treatment: The girl received intravenous infusions of a steroid to reduce inflammation, as well as immunoglobulin — antibodies from healthy donors that are often used to calm autoimmune reactions. She also got an infusion of rituximab, a drug that kills B lymphocytes, a type of white blood cell that makes antibodies like the ones attacking the girl's neurons.

In addition to these infusions, the girl underwent five sessions of plasmapheresis over the next month. In that procedure, blood is drawn from the body, a machine separates plasma (the liquid portion of blood) from blood cells, and the extracted plasma is substituted with a replacement fluid before that blood is returned to the patient. This treatment can help rid the body of autoantibodies.

The girl was discharged after having spent almost two months in the hospital. She was treated with ongoing prescriptions of olanzapine and lorazepam, as well as prednisone and monthly immunoglobulin treatments. As her symptoms improved over the following year, she stopped taking the psychiatric drugs but continued her immunotherapy.

What makes the case unique: Autoimmune encephalitis is rare. One study conducted in the U.K. suggests it affects only 5 to 8 out of every 100,000 people. Additionally, most data about the condition in pediatric patients comes from cases caused by autoantibodies that target NMDA receptors, which detect a key chemical messenger in the brain, rather than NIF.

The case report did not name a specific trigger for the girl's condition, but the authors noted that anti-NIF autoimmune encephalitis has been documented in adults following infections, including certain tick-borne bacterial infections, HIV and COVID-19. Cases associated with these infections are rarer than those linked to tumors.

The doctors did not attribute the teen's case to a specific infection, but her family did report that she'd had a cough three weeks before going to the emergency room. The family had visited the Philippines, and the girl and multiple relatives came down with the same illness.

For more intriguing medical cases, check out our Diagnostic Dilemma archives.

This article is for informational purposes only and is not meant to offer medical advice.

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'> Teenager's rare autoimmune disorder caused her to hallucinate a flying green dinosaur

Laboratory analyses of specimens that died this year tested positive for the pathogenic bacterium Erysipelothrix rhusiopathiae, environmental authorities in Chubut province told Live Science. E. rhusiopathiae was first identified in southern right whale calves in 2013, but scientists don't know what might have changed this year to cause an infection-driven die-off of this magnitude.

Researchers say they need to quickly find the answer to better understand the changing threat this species faces, the risk to the wider ecosystem and humans, and what can be done to manage it.

"The battle between living organisms and pathogens is constant," said Carla Fiorito, a researcher at the Center for the Study of Marine Systems (CESIMAR) who previously identified the bacterium in right whales. "But for this to happen now, there must have been a shift in the balance between the agent, the host, and the environment," she told Live Science.

Record-breaking number

The first dead calves appeared on the beaches of Península Valdés in June, at the start of the breeding season, which runs through to December. Península Valdés is located in Chubut province, on the coast of northern Patagonia.

In a typical year, calf mortality reaches 20% to 30% of a cohort from various causes, including infections. But this year's losses have already far surpassed that.

In September, 409 calves were counted among a total of 1,375 southern right whales during a survey conducted by CESIMAR's Marine Mammal Laboratory. With more than 300 deaths already recorded, the die-off represents an exceptional proportion of these calves. The final figure will be released by the Southern Right Whale Health Monitoring Program in December.

Scientists have responded by expediting routine research to determine what's different this year to previous years.

"Samples were sent for analysis earlier than usual — typically, they are analyzed at the end of the year — because the higher-than-usual mortality is a cause for concern," said Víctor Fratto, the director of the Native Wildlife Rescue Program and a contributor with the whale monitoring program. "What those samples are showing is that the calves died from erysipelas," he told Live Science.

A known bacterium full of unknowns

Erysipelas is a bacterial infection that affects the skin. In its most severe form, it can spread through the bloodstream and cause a systemic infection, damaging vital organs and sometimes leading to death. Animals that catch the disease, for example through a wound, may develop characteristic skin lesions, which are rhomboid or rectangular in shape, caused by alterations in the blood vessels.

The bacterium that causes erysipelas, E. rhusiopathiae, is not exclusive to whales. It can be found in marine environments and in a wide variety of animals, including fish, crustaceans and birds. Pigs are one of its natural reservoirs.

"It is a ubiquitous and opportunistic bacterium," Fiorito said. (The human illness of the same name is different and caused by Group A streptococcal bacteria, although humans can also catch the E. rhusiopathiae form of the illness.)

The presence of E. rhusiopathiae does not necessarily mean an animal is diseased. Yet many questions remain about E. rhusiopathiae in whales.

"We don't know much about the bacterium itself; we don't know which serotypes are present in the whales, or how common it is in healthy animals or other species," Fiorito said. "There is also no data on the environment."

Calves are especially vulnerable because their immune systems are still developing and they spend more time at the surface than adults do, exposing them to more attacks by gulls, which can transfer the disease.

"The working hypothesis is that the gull is the vector: Pecking causes wounds on the back, allowing the bacteria to enter," Fratto said. "The bacteria might already be present on the skin, or they could be carried on the beak of a gull that had previously pecked another whale suffering from erysipelas."

The potential role of gulls

Attacks by kelp gulls (Larus dominicanus) on southern right whales were first recorded at Península Valdés in the 1970s. The behavior began with just a few individuals in Patagonia's San José Gulf and became increasingly frequent over time, Ricardo Bastida, a marine biologist and professor emeritus at the National University of Mar del Plata who has studied these cetaceans for more than four decades, told Live Science. The birds also began scavenging the carcasses of stranded whales, Bastida said

Gull populations have grown and new breeding colonies have emerged in the region thanks to increased food availability linked to fishing activities and discarded catch, he noted. However, the attacks alone do not explain the 2026 mortality event: They have been occurring for decades, and gulls are a regular part of the ecosystem.

Fratto said that swabs will be taken from gulls to check for the presence of E. rhusiopathiae and assess whether the results could help develop a management strategy for the species.

Diego Rodríguez, a marine biologist at the National University of Mar del Plata, said that research must go beyond the dead calves and examine the ecosystem's fauna to determine where the bacterium is present and how it might be circulating. "It is far more likely that [the die-off event] is linked to cross-infection," Rodríguez told Live Science.

What changed this year?

The major question for scientists is what might have changed this year to cause a bacterium already present in the ecosystem to trigger a die-off of this magnitude. Experts told Live Science they believe the phenomenon could be the result of several factors acting simultaneously.

"The bacterium was present, but this year it resulted in a widespread infection, making this event unusual," Mariano Coscarella, a researcher at the Marine Mammal Laboratory at CESIMAR, told Live Science. "It is a major step forward to have a fairly clear idea of what caused the disease, but we do not know what changed."

One factor could be the environment. Changes in water temperature, salinity, pH or rainfall could alter the conditions under which infection occurs. There could also be changes in food availability, affecting the mothers' body condition and the vulnerability of their calves. Another possibility is that the pathogen itself has changed — for instance, through the emergence or increased circulation of a more virulent strain or a different serotype, Fiorito said.

The El Niño phenomenon could also play a role by altering oceanographic conditions and food availability, although the researchers said there's insufficient evidence to link it to the die-off.

It is too early to determine what impact this die-off will have on the southern right whale population. Rodríguez suggested that losing such a large proportion of a season's calves could have demographic consequences. "It is very risky to predict what will happen in the future at this stage, but clearly, this year's cohort of live-born calves will essentially be absent from the adult records of future populations," he said.

Coscarella is more cautious. Southern right whales are extremely long-lived, and calves represent only a fraction of the population. Therefore, a single season of exceptionally high mortality does not necessarily mean the population will go into decline, he argued. "What would be concerning is if there were two, three or four episodes like this," he noted.

The event also has public health implications. Fiorito pointed out that erysipelas can infect humans, usually through skin wounds. The whales' carcasses could also pose a risk of exposure to anyone coming into contact with them, as could seawater for people swimming in the area.

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'> Hundreds of whale calves have mysteriously died in Patagonia — and scientists are racing to find out why