European surveillance shows West Nile virus transmission is under way again this season, with 81 locally acquired human cases across six countries and 35 affected areas reported in the most recent seasonal figures. The word doing the work in that sentence is “locally acquired”.

Locally acquired means the virus lives there

An imported case is someone who was infected abroad and diagnosed at home. It tells you about travel. A locally acquired case means transmission happened where the person lives — the mosquitoes, the birds and the virus are all present in that place.

West Nile virus circulates between birds and mosquitoes, principally of the Culex genus. Humans and horses are what epidemiologists call dead-end hosts: we can be infected but do not carry enough virus to pass it on. So human cases are not the epidemic; they are the visible edge of one happening in birds.

Most people infected have no symptoms at all. A minority develop fever, and a small fraction — concentrated among older people and those with weakened immune systems — develop neurological disease, which can be severe.

Why the counts understate it

Eighty-one cases sounds small against a continent of hundreds of millions, and it is not the number of infections.

Because the great majority of infections produce no symptoms, only a small proportion ever reaches a doctor, and only some of those are tested for an uncommon virus. Surveillance figures for West Nile therefore track severe cases and testing effort as much as they track transmission. A country that tests more finds more.

That makes year-on-year comparisons within a country more informative than comparisons between countries — a caution that applies to a good deal of infectious disease reporting.

The climate question, stated carefully

West Nile is not new to Europe — it has been recorded there for decades, and outbreaks have occurred periodically. So the accurate claim is about frequency and range rather than arrival.

The mechanism researchers point to is specific and testable. Mosquito development, biting rate and the speed at which virus replicates inside the insect are all temperature-dependent. Warmer seasons mean more generations of mosquitoes, more bites, and a shorter interval before an infected mosquito becomes infectious. Milder winters affect overwintering survival. Standing water from irrigation or heavy rainfall provides breeding sites.

What that supports is a claim about conditions becoming more suitable more often. What it does not establish on its own is that any particular season’s case count was caused by climate change — that requires attribution work, and surveillance figures alone cannot do it. The same discipline we applied to the Swiss drought and to storm categories applies here.

What actually reduces cases

There is no licensed human vaccine, though one exists for horses. Control is therefore mosquito control and personal protection: removing standing water, larval treatment of known breeding sites, repellent, and screens.

The other measure is invisible to the public and arguably the most important. Because the virus can be transmitted through blood and organ donation, affected areas introduce donor screening or deferral during transmission season. That is a large administrative undertaking triggered by a small number of detected cases — and it is one of the practical reasons surveillance of a mostly asymptomatic infection is worth funding.

Sources

• Global Health Press, citing ECDC seasonal surveillance — West Nile virus transmission in Europe