West Nile virus is spreading across the US and Europe. This deadly disease has been around for decades, but there is still no vaccine and no cure in humans.

After a distinguished career as one of the world's leading HIV researchers, and then a role as the face of the US government's response to the Covid-19 pandemic, it was a very different virus which recently hospitalised Anthony Fauci.

Last month, the 83-year-old began showing symptoms of fever, chills and fatigue after contracting West Nile virus, a mosquito-borne pathogen which was first discovered in Uganda in the 1930s. But Fauci didn't contract the virus in East Africa, instead he was reportedly bitten by an infected mosquito in his back garden in Washington DC, incidents which are becoming progressively more common.

The Centers for Disease Control and Prevention (CDC) told the BBC that 2,000 Americans fall ill from West Nile virus every year, leading to 1,200 life-threatening neurological illnesses and over 120 deaths.  "Anybody can be at risk," says Kristy Murray, a professor of paediatrics at Emory University in Atlanta, Georgia, who has been studying West Nile virus for the best part of two decades. "A simple mosquito bite is all it takes to become infected. And while it's mostly older individuals who we see getting severe disease, young people get it too," she says.

It was late August 1999 when an infectious diseases physician in the New York City borough of Queens reported two cases of viral encephalitis, or brain inflammation, to the city's Department of Health and Mental Hygiene. An urgent investigation commenced after similar cases were identified at neighbouring hospitals. Estimates showed that in total, this mysterious epidemic ultimately infected approximately 8,200 people across the city. It was the first known outbreak of West Nile virus in the western hemisphere.

No one knows exactly how the virus was introduced to the US from parts of Africa, the Middle East, southern Europe and Russia where it had circulated for decades, but research has since shown that birds are the main carriers of the virus. Mosquitoes contract the virus when feeding on infected birds, before passing it onto humans.

Since that initial outbreak in 1999, there have been more than 59,000 US cases and more 2,900 deaths from West Nile virus, although some estimates place the real number of infections as exceeding three million.

Now, there are growing concerns that West Nile outbreaks in the US and around the world will become more frequent due to climate change. Studies have shown that warmer temperatures can accelerate mosquito development, biting rates and viral incubation within a mosquito. In Spain, where the virus is endemic, an unprecedented outbreak in 2020 has been followed by a prolonged period of escalating circulation.

This has led to particular concern because while infections are predominantly asymptomatic, with only one in five people experiencing mild symptoms, severe cases can result in lifelong disability. In around 1 in 150 people, the virus can invade the brain and central nervous system, causing life-threatening inflammation, and in many cases, brain damage.

 

In particular, people who are immunocompromised in some way, over the age of 60, or have diabetes or hypertension, are particularly vulnerable. "With hypertension, we think that increased pressure in the brain allows the virus to cross the blood brain barrier more readily," says Murray.