Emerging viruses: Usutu virus ("oo-soo-too")


A fun name to say, also an interesting virus- Usutu virus was discovered in Africa in 1959 but has only ever caused 2 human infections there. The virus like other related flaviviruses (Zika, Dengue, West Nile virus) that I have discussed, is transmitted by mosquitoes. This virus seemed rather benign, rarely infection humans. Mysterious mass bird deaths started to occur across Europe. First in Italy in the early fall of 1996. The bird carcasses were subjected to necropsy, they showed swollen livers and spleens, with no definitive results. Then in the late summer of 2001, in neighbouring Austria. More and more birds were dying year after year. Followed by Hungary, Switzerland, Italy, and Germany.

This drew more attention and the causative pathogen was isolated and determined as Usutu virus. Then in 2009, the first human infection was reported outside of Africa. Two immunocompromised individuals with meningoencephalitis (swelling of the brain and membranes) were shown to be infected with Usutu virus. In the summer of 2016, a mass bird epidemic occurred in Northern Europe. The virus appeared to be spreading dramatically and becoming endemic to Europe. Is Usutu virus capable of causing severe disease and human outbreaks in healthy populations? Or it is just merely a virus that kills birds?


The mass bird die-offs were ominously reminiscent of another well-known pathogen: West Nile virus. The primary host of which is also birds and mosquitos. First identified in Uganda in 1937, then circulating in Europe since the 1950’s, mass bird fatalities often indicate the arrival of this virus. In the US, West Nile virus wipes out millions of birds every year. It is the leading cause of mosquito-borne disease in the US and is life-threatening as it can invade the central nervous system. Most cases of West Nile virus are mild, with many known to be asymptomatic. However, when the virus invades the brain and spinal cord, paralysis can occur and acute respiratory failure as a result.


As these viruses share similar host species, it was no surprise that in 2018, with Usutu spreading rapidly in Western Europe, it was found to be co-circulating with West Nile virus. What the implications of both these viruses spreading together will have- we don’t know. Worryingly, Usutu has also been found to infect dogs, dear, wild boars, bats, squirrels, and rodents through the bite of an infected mosquito. These animals could also act as secondary reservoirs. Will Usutu emerge and cause similar outbreaks across Europe in the future? It seems very likely we’ll be hearing more.



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