Overview of the Usutu Virus

The Americas haven't seen many of the viruses that are more common in Africa and Asia, but the Americas do have mosquitoes and other vectors. These are often the same or similar mosquitoes as found around the world; just as globalization spreads viruses, it also spreads mosquitoes. As such, when a new virus lands, if it can spread through one of the mosquitoes (or other vectors) found in the Americas, it spreads fast. Summer after summer, there have been new viruses in the Americas carried by mosquitoes, and as climate change creates more habitats for mosquitoes, ticks, and other vectors, these infections can spread further.

Do we know what the next virus will be? The simple answer is no, we don’t. And before there's another virus that takes us by surprise, it would be good to know as much as possible about the many viruses that could possibly spread, like Zika, Chikungunya, and different strains of Dengue that we've seen in recent years in the Americas. As we look more, we find more, too. Better diagnostics help us identify the causes of fevers, headaches, and rashes that previously we had only been able to identify through a bit of guesswork.

There are also viruses that we are just beginning to recognize as they begin to spread further and further. One of these viruses, which seems mild but may hold surprises, is the Usutu virus.

A mosquito on the skin of a person

Joao Paulo Burini / Getty Images

What Sort of Virus Is Usutu?

Usutu is a flavivirus. This is like many other viruses spread by mosquitoes. It is more closely related to Japanese encephalitis and Murray Valley encephalitis. Other slightly more distant relatives include other viruses we know, like Dengue, Powassan, West Nile, and Zika, all flaviviruses that have certainly jumped into our headlines.

Where Is Usutu Found Now?

Usutu virus was first found in South Africa in 1959. It was found in mosquitoes and named for a river nearby (also known as the Maputo River which runs through South Africa, Swaziland, and Mozambique). There weren't any illnesses noted to be associated with this virus.

The virus has also been found in other parts of Africa: Burkina Faso, Cote d’Ivoire, Morocco, Nigeria, Uganda, Central African Republic (CAR), and Senegal. However, there was only one patient who was seen to have the virus (in CAR) who seemed to do well and just had a fever and rash.

The virus was later recognized to be in Europe. It was likely introduced multiple times in Europe. In hindsight, before it was first noted in Austria in 2001, it likely had spread elsewhere in Europe, such as in Italy, which testing of birds has shown.

Like West Nile Virus, Usutu virus has flown, quite literally, with birds. Large numbers of dead blackbirds alerted authorities in Austria to the problem; this was how the virus was first detected as having spread to Europe. Like West Nile, the virus can kill birds but also spread to us. It has been seen in other countries in Europe (or at least antibodies have been found) and has continued to be associated with bird deaths—Italy, Germany, Spain, Hungary, Switzerland, Poland, England, Czech Republic, Greece, and Belgium.

How Do You Catch Usutu Virus?

You catch it from a mosquito bite. But not all mosquitoes can (or will) carry the virus. The virus seems to be carried by Culex mosquitoes, especially Culex pipiens. This is a lot like West Nile Virus, also spread by Culex mosquitoes, so Usutu virus may be found where there is West Nile Virus. In fact, screening for West Nile Virus led to the identification of Usutu virus in some patients in Italy.

This virus also may be carried by other mosquitoes like Aedes albopictus. Aedes albopictus is found in the U.S., particularly in the South and also in areas throughout the Americas that have just seen large outbreaks of Zika, which is also carried by this mosquito.

The virus appears to cycle through birds, in particular blackbirds, but also some crows, jays, magpies, owls, and chickens. The birds experience liver, spleen, and neurologic damage, among other effects from the virus.

The birds in which the virus is found often fly long distances, even as far as Asia. Similar birds are found in the Americas. More regions than just Africa and Europe may be at risk.

Other animals appear to be infected. The virus has also been found in bats (in Germany) but it's not known yet what role bats play (if any). Other animals as diverse as horses and rodents have been found to be infected, but these do not seem to be centrally involved in virus spread. Most of the virus transmission appears to be between mosquitoes and birds.

What Does Usutu Virus Cause?

The virus may just cause a rash and a fever. The infection may be mild and may pass quickly. However, there is a worry that for a small number, the infection may be more severe.

Initially, little was known about the virus. There wasn't much known about the infection in Africa. There was a case identified in the Central African Republic in 1981 and the patient seemed in good condition, except for a rash and fever that went away. However, there was little surveillance or testing for the virus and no one really knew how common it was and no one knew whether severe and undiagnosed fevers and illnesses were actually infections like Usutu for which there weren't the labs and the tests needed to identify it.

When the virus was noted in Austria in 2001 it was because there were blackbirds dying. There weren't human infections noted. Studies of the general population showed only a very small number had been infected in Europe. Some blood donors have been found to have antibodies in Europe, but this is rare. However, a study of those who had a rash in areas where the virus had circulated then showed that actually among those likely to have been infected as many as 1 in 4 had antibodies, but all reported only a mild illness. So it seemed this may be a more common virus than realized, but it didn't seem to have serious consequences.

However, there were then two cases in Italy which were quite serious in immunocompromised patients in 2009 (one was on chemotherapy; the other had a serious blood disorder). In these cases, the virus seems to have led to meningoencephalitis, but given the severity of their already existing illnesses, it isn't entirely clear. This points to an infection and inflammation of the brain and of the meninges which surrounds the brain and the spinal cord. In one of the cases, the patient developed liver failure as well.

These cases raised the concern that the virus might be much more serious for some individuals. Most people are well with West Nile Virus and with Zika, but a small subset faces serious consequences. It may be that Usutu can present a real risk for some, even if mild in most.

How Do You Test for Usutu Virus?

Most labs would not be able to test for it. A healthcare professional would need to contact a specialty referral laboratory to handle a specimen.

Is There Treatment for Usutu Virus?

Not yet. Fortunately, most infections in healthy people appear to be mild so far.

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By Megan Coffee, MD
Megan Coffee, MD, PhD, is a clinician specializing in infectious disease research and an attending clinical assistant professor of medicine.