Potentially fatal mosquito-borne West Nile fever (WNF) can become much more widespread in Europe than previously thought, say scientists in a new report just out in the journal BMC Evolutionary Biology.
The disease in temperate climates is carried by a population of Culex pipiens mosquitoes that only bites birds - the disease reservoir host - but Bruno Gomes and colleagues from the Centre for Malaria and Tropical diseases and Institute of Hygiene and Tropical Medicine in Portugal found high numbers of hybrids between this population and another one that bites on humans. These hybrids, by feeding on both humans and birds, can act as a bridge transmitting the disease to humans. The study was done in Portugal and follows recent reports of infected mosquitoes and several disease cases in this country.
The brain infection WNF is originally from Africa and Asia where is usually rare and mild. While birds are the virus' reservoir host - only in them can the virus develop and grow - the infection can be transmitted, through the bite of contaminated mosquitoes, to humans where usually causes mild febrile symptoms although, in some cases, can lead to severe neurological problems such as meningitis and encephalitis (acute inflammation of the brain) or even death.
In the last decade the virus has emerged in Europe and the United States (US) carried by Culex pipiens, the most abundant species of mosquito in temperate climates, particularly of urban (overpopulated) areas. Despite the fact that the virus seemed to be carried by a specific population of the species - the pipiens or Culex pipiens pipiens population - which feeds on birds but not on humans, several human outbreaks have been recorded, particularly in the US, where epidemics have spread everywhere in the country. Finally, just a few years ago, the US cases were linked to a new hybrid population - a breed of Culex pipiens pipiens (that bites birds and carries the virus) and Culex pipiens molestus (that bites humans) - able to feed on both species and, as result, acting as disease vector transmitting the infection to humans.
In Europe, however, no evidence of these hybrids was found and studies of the European pipiens and molestus populations suggested they were genetically too different to breed easily. Even more, on the north of Europe, the two populations were shown to occupy distinct habitats - with molestus living underground (in the London tube for example) and pipiens aboveground - further limiting their potential danger to humans. All this, associated with the fact that no outbreaks have ever occurred in this part of the world has reassured the European health authorities.
Recently, however, recent reports in the south of Europe - where the two populations live together aboveground - of infected mosquitoes and sporadic cases of the WNF have raised the alarm and led to the study now published by Bruno Gomes, João Pinto and colleagues, which attempts to understand the real risk of WNF in this part of the world.
In the study several Culex pipiens mosquitoes were collected in Portugal, close to an area of birds natural reserves, analysed and then allowed to breed so their first set of descendants could be studied. The idea was to determine the genetic closeness between the south European molestus and pipiens populations, as well as their potential to create the epidemiologically relevant hybrids.