Bird flu has been invading the brains of mammals. Here’s why

In outpouring 2022, a fistful of reddish foxes successful Wisconsin were behaving oddly. Veterinary pathologist Betsy Elsmo learned that a section wildlife rehabilitation halfway was caring for foxes pinch neurological symptoms for illustration seizures, tremors, uncoordinated movements and lethargy.

But tests for communal pathogens for illustration canine distemper microorganism and rabies that typically origin nan symptoms came backmost negative. Then a reddish fox kit tested affirmative for influenza A. This group of viruses includes seasonal flus that origin respiratory illness successful group and galore different strains that commonly move among animals specified arsenic waterfowl and different birds.

“I was surprised,” says Elsmo, of nan University of Wisconsin–Madison. “And to beryllium honest, astatine first I benignant of wrote it off.”

That is, until a veterinary technician astatine nan rehab halfway sent Elsmo a study describing cases of avian influenza successful reddish foxes successful nan Netherlands. Examinations of nan Wisconsin kit’s tissues nether nan microscope revealed lesions successful nan brain, lung and bosom that matched what had been seen successful nan Netherlands animals. “And I thought, I deliberation it is [bird flu],” she recalls.

The ongoing vertebrate flu pandemic has revealed that H5N1 tin infect a wide array of mammal species, including nan reddish fox.S.J. Krasemann/Getty Images

Additional testing confirmed nan diagnosis successful nan kit and nan different foxes, Elsmo and colleagues reported successful nan December 2023 Emerging Infectious Diseases. The animals had contracted a lethal strain of H5N1 avian influenza that emerged successful precocious 2020 successful Europe and has since dispersed astir nan world. At nan clip infections were discovered successful nan Wisconsin reddish foxes, vertebrate flu was expanding its incursion into North America.

Since H5N1 arrived connected North American shores successful December 2021, it has infected animals arsenic wide-ranging arsenic polar bears, skunks, oversea lions, bottlenosed dolphins and cows (SN: 7/8/24). And 1 unwelcome revelation of nan ongoing outbreak is nan virus’s propensity to invade nan brains of myriad mammals.

Bird flu connected nan brain

H5N1 is now known to infect much than 50 mammal species. Scientists had documented vertebrate flu cases successful only astir a twelve type during erstwhile outbreaks.

There’s thing peculiarly typical astir this strain that allows it to plague truthful galore species, investigation suggests. Rather, because nan strain has dispersed truthful acold and wide among birds — both chaotic and domesticated — location are simply much opportunities for mammals to beryllium exposed, says virologist Emmie de Wit of nan National Institutes of Health’s Rocky Mountain Laboratories successful Hamilton, Mont. (SN: 1/25/24). For immoderate mammals, infected birds whitethorn beryllium connected nan menu. For others, their h2o aliases nutrient mightiness beryllium contaminated pinch virus-laden vertebrate excrement.

Water-loving birds are exposed to influenza A viruses truthful often that astir person immoderate shape of immunity that tin protect them from a superior lawsuit of H5N1. If nan birds do grounds symptoms, they are usually mild and respiratory aliases gastrointestinal successful nature. But different birds that don’t get infected arsenic frequently, specified arsenic eagles, vultures and pelicans, tin go severely sick and die.

In mammals, vertebrate flu typically causes respiratory symptoms specified arsenic congestion aliases shortness of breath. Neurological symptoms specified arsenic seizures aliases paralysis are among nan astir striking and communal signs that nan microorganism has reached nan brain. And erstwhile it does, nan infection is often fatal. In nan ongoing H5N1 outbreak, specified cases person been confirmed posthumously successful house cats, raccoons, oversea lions, dolphins and respective different type (SN: 5/31/24).

H5N1 and its adjacent relatives person an easier way to nan encephalon compared pinch different flu strains, de Wit says. And nan microorganism is exceptionally bully astatine making copies of itself wrong a assortment of cells, including nervus cells.

To break wrong a cell, flu viruses utilization a macromolecule called sialic acerb connected nan cell’s surface. Bird cells and mammalian cells successful nan precocious airway typically person different types of sialic acids, which intends that vertebrate flu viruses don’t easy infect mammals and vice versa.

But studies person shown that parts of nan quality brain, for instance, are covered successful birdlike sialic acerb proteins. Many cells, including nan nervus cells successful nan olfactory bulb — which is important for consciousness of smell and links straight to nan encephalon — besides person further proteins that let H5 viruses to replicate. Both of those traits mean that “there’s a nonstop introduction way [to nan brain] from your nose,” de Wit says. So avian flu viruses whitethorn airs a bigger threat to nan encephalon than, say, seasonal influenzas that mostly target cells successful nan nose, pharynx and lungs.

Since 2003, location person been 891 cases of H5N1 successful people, astir half of which were fatal according to nan World Health Organization. In nan existent outbreak, 15 group person been infected pinch nan strain hitting mammals hard. Four group had terrible illness — 2 developed pneumonia — and astatine slightest 1 died.

Symptoms successful group tin alteration from mild to severe, and immoderate individuals infected pinch avian influenza viruses during erstwhile outbreaks person developed neurological symptoms. Such cases are rare, nan agency says, though really often nan microorganism makes it to nan encephalon is unclear.

How often nan microorganism invades nan brains of different mammals is besides unknown. Most countries don’t regularly trial healthy-looking wildlife for viral infections, truthful specified cases apt correspond only nan sickest individuals, Elsmo says. It’s imaginable that immoderate infected animals ne'er create terrible illness and truthful flight nan announcement of people.

Avian influenza viruses tin impact different parts of nan assemblage arsenic well. “When you break it retired by species, it seems for illustration nan shape of tissues that are astir affected are small spot different,” Elsmo says.

For instance, foxes and raccoons look to person terrible encephalon lesions arsenic good arsenic lesions successful nan bosom and lungs. Striped skunks, meanwhile, thin to person fewer encephalon lesions, immoderate of which are mild compared pinch reddish foxes. Instead, astatine slightest successful Elsmo’s experience, H5N1 seems to target nan skunk livers and organs successful nan lymphoid system, which protects against infection and removes discarded from nan body.

Stranger things

Some animals that person been killed by H5N1 weirdly had signs of infection only successful nan brain. “Flu’s a respiratory virus,” says Richard Webby, a virologist astatine St. Jude Children’s Research Hospital successful Memphis, Tenn. “So [the respiratory strategy is] wherever it should beryllium earlier anyplace else.”

The encephalon of a reddish fox successful nan Netherlands, for instance, was infected pinch H5N1 but not nan animal’s lungs. Red foxes and mink successful Canada likewise had mostly neurological disease. And vertebrate flu didn’t look to touch nan lungs of a bottlenosed dolphin recovered disconnected nan seashore of Florida successful 2022, contempt nan animal having virus-infected encephalon tissue, Webby and colleagues reported April 18 successful Communications Biology.

It’s imaginable that nan dolphin had cleared nan respiratory infection earlier it died, but nan squad recovered nary grounds of pulmonary disease. Nor do dolphins person a consciousness of smell, which eliminates nan anticipation that nan microorganism could scope nan encephalon done nan nose.

Exactly really nan microorganism mightiness sometimes sidestep respiratory tissues and hitch a thrust straight to nan encephalon is unclear. One anticipation is that H5N1 already has that ability. “Maybe it was much common, but we conscionable ne'er had nan numbers [of mammalian infections necessary] to spot it,” Webby says.

Or possibly erstwhile a mammal for illustration a reddish fox aliases a dolphin takes a wound retired of an infected bird, viral particles successful nan nutrient summation a nonstop way to nan encephalon via nerves lining nan rima and digestive tract.

Virologist and veterinarian Víctor Neira thinks that’s what whitethorn beryllium happening successful immoderate oversea lions successful Chile. He and colleagues investigated an outbreak successful nan animals successful 2023 on nan seashore of Chile, including 2 animals that nan squad necropsied. Although 1 animal didn’t person neurological symptoms, nan different had paralysis, disorientation, deficiency of coordination and tremors, nan researchers reported successful October 2023 successful Veterinary Quarterly. The microorganism had infected aggregate organs including nan lungs, liver and kidneys, but nan highest viral counts came from nan brain.

Because H5N1 hasn’t been infecting oversea lions for very long, since conscionable 2023, location is still a batch to study astir really nan microorganism affects nan animals, says Neira, of nan University of Chile successful Santiago.

Just really galore mammal type are susceptible to infection is an unfastened question. “Usually, scientists study influenza microorganism successful swine, mice, humans and birds,” Neira says. “Different chaotic animals, moreover respective home animals, person not been studied pinch this virus.”

There’s an urgency to uncovering out. That’s because each infection gives nan microorganism an opportunity to accommodate successful ways that could let it to dispersed much readily among mammals, de Wit says. Some research suggests that nan virus whitethorn beryllium spreading from marine mammal to marine mammal successful parts of South America. But truthful far, infections successful different animals are mostly dormant ends, pinch nary dispersed to different individuals. The consequence to group remains low, though workers connected poultry and cattle farms who activity intimately pinch animals are astatine a higher consequence than nan wide public.

Still, “we cannot extremity paying attention,” de Wit says. The ongoing outbreak serves arsenic a reminder that viruses are versatile foes.