Scientific Opinion On The Possibility Of Occurrence And Spread Of West Nile Fever Disease In Bulgaria

The causative agent of the West Nile Fever disease (WNF) is a virus that belongs to the family Flaviviridae, genus Flavivirus. The West Nile virus (WNV) belongs to the antigenic group of the Japanese encephalitis, which also includes the viruses causing Murray Valley encephalitis and Saint Louis encephalitis. Most commonly, the disease affects birds, horses and humans. There are sporadic cases of WNF reported in other animal species such as chimpanzees, dogs, cats, squirrels, bats, deer, sheep, llamas, alligators, seals. The WNF is prevalent in Africa, the Middle East and West Asia. The disease was established in New York in 1999. In the next 5 years it was spread on the whole territory of USA, Canada, the Caribbean and Latin America. Birds are the natural reservoir host in which the virus multiplies in large quantities. The main route of transmission is through the mosquitoes bites. Due to the multi-staged and long lasting viremia (from 20 to 100 days), the infected birds are a primary source and a generator of the infection for the bloodsucking mosquitoes that by biting infect other birds, but also humans and horses. Mosquitoes and especially those who are primarily fed by bloodsucking from poultry are the principal vector of the WNF. The WNV was isolated from 43 mosquito species as the genus Culex dominates. Depending on the geographical area, vectors are different – in Europe the main competent vector of WNF is Culex pipiens. Vectors also may be Cx. modestus and Coquillettidia richiardii. Many authors define mammals as hosts at the end of the epizootic chain ("dead-end hosts"). The virus was isolated from at least 138 dead bird species belonging to the following orders of Sparrows (Passeriformes), Plovers (Charadriformes), Owls (Stringoformes) and Falcons (Falconiformes). The birds of the family Corvidae (crows and blue jays) are particularly susceptible to the WNV. Taking into account the geographical distribution of WNF in the recent years, as well as that the most countries with reported cases of WNF in 2012 had the infection also in 2010 and 2011 (Russia, Greece, Romania, Turkey, Israel, Italy, etc.), it may be concluded that WNF transforms from an emerging disease to a stationary disease with endemic definitions in the most parts of Europe. It should not be unexpected in front of the changes of environmental factors such as high density of local residential birds, the urbanization degree in the suburb areas where a contact between local residential birds and migratory birds can be made. WNDV can be transmitted by the migratory birds outside traditional ecologic niches and WND to be adapted to the local ‘bridge” vectors on background of existing of wet territories or high rainfalls with higher activities and abundance of mosquito vectors. <br> Further spread of WNF infections is expected in the coming years, including countries neighboring the already affected areas, where the environmental factors are favorable and there is a link among the migratory birds, residential birds, the competent mosquito vectors, other vertebrate animals (including agricultural animals) and humans. The serological surveys carried out in Bulgaria confirm that the WNV circulate in at least 4 regions of 3 administrative areas including different animal species. The surveillance of WNF includes and other regions of the country by elaborating of a surveillance program that is well planned and secured in time and space and makes use of sentinel animals of the residential birds and equines, as well as constant supervision of vectors. Particular attention should be paid to those regions that are considered the most risky and are characterized by close co-existence of susceptible animals near wetlands visited by large colonies of various species of migratory birds on Via Pontica and Via Aristiteles, that may be carriers and a source of WNF infection.

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