The highly contagious avian flu is now infecting marine mammals. In a troubling new study, University of Florida scientists discovered the dangerous bird flu virus in a bottlenose dolphin found dead in the Sunshine State. This marks the first known case of HPAI A(H5N1) infecting any cetacean species. Since late 2021, HPAI A(H5N1) has been causing widespread disease outbreaks in wild birds and poultry across the United States.

According to the study, published in the journal Communications Biology, the juvenile male dolphin was discovered in distress between a seawall and dock in a canal in Dixie County, Florida, in late March 2022. It died shortly after being found. A necropsy was performed to determine the cause of death.

Although bird flu was not initially suspected, a microscopic examination of the dolphin’s brain revealed telltale signs — including inflammation and destruction of brain and nerve cells. This prompted University of Florida researchers to test tissues for the presence of HPAI A(H5N1).

The brain, spinal cord, and some other organs tested positive for genetic material from the virus. The brain contained the highest viral levels, which aligned with the observed brain lesions and indicated the virus had a strong affinity for invading the central nervous system.

To confirm an active infection, researchers isolated live HPAI A(H5N1) virus from the dolphin’s brain tissue. Genetic sequencing showed the virus was closely related to strains circulating in American wild bird populations at the time. However, it appears wild birds in the immediate area were not the direct source, as viruses isolated from local bird die-offs had slightly different genetics.

So, how did this land-based bird virus end up in a dolphin? Researchers suspect the dolphin may have been exposed through close contact with an infected bird in the water, possibly while feeding on fish. Seabirds like gulls congregate around dolphins to snag an easy meal, potentially contaminating the water with infectious feces or secretions.

“We still don’t know where the dolphin got the virus and more research needs to be done,” says corresponding study author Dr. Richard Webby, who directs the World Health Organization Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds at St. Jude’s Children’s Research Hospital, in a media release.

While concerning, this singular case doesn’t necessarily mean bird flu has gained a foothold in dolphin populations quite yet. No common mutations associated with mammalian adaptation were found in the dolphin virus. However, it did contain a few unique changes with unclear significance that merit close monitoring.

One mutation slightly reduced the virus’ susceptibility to the antiviral drug oseltamivir, though other flu antivirals remained effective. Another mutation in the H5 hemagglutinin protein, which the virus uses to enter host cells, has been shown to potentially increase binding to mammalian cell receptors. The impact of these changes on the virus’ ability to infect dolphins is still unknown.

Although the current health risk to humans remains low according to the CDC, the dolphin case highlights the importance of vigilant surveillance for HPAI A(H5N1) in all marine mammals. If the virus gains the ability to efficiently spread between dolphins, it could potentially devastate these already vulnerable populations.

“This investigation was an important step in understanding this virus and is a great example where happenstance joins with curiosity, having to answer the ‘why’ and then seeing how the multiple groups and expertise took this to a fantastic representation of collaborative excellence,” explains Dr. Mike Walsh, an associate professor of aquatic animal health at the University of Florida.

As of now, any dolphins or whales displaying abnormal neurological symptoms should be tested for bird flu. Examination of the brain is critical, as it appears to be a favored target of HPAI in dolphins and other infected mammals. Expanding and sustaining monitoring for this virus in dolphin populations is an urgent wildlife conservation priority.

In early 2024, a human case of HPAI A(H5N1) was identified after the person was exposed to dairy cattle that were presumably infected with bird flu. Because of the possibility that bird flu viruses could change and gain the ability to spread easily between people, monitoring for human infection and person-to-person spread is extremely important for public health.