Hypothesis: WNV found its way into the United States as a consequence of increased opportunities for intercontinental travel.
West Nile Virus (WNV) is an emerging zoonotic arbovirus that has recently undergone intercontinental expansion (Nemeth et al, 2010). The main route of transmission through female common house mosquitoes (Culex pipiens) though other species of mosquitos can also carry the virus. When the mosquito bites an infected animal usually a bird it acquires the virus from the blood meal. Following this the newly infected mosquito then acts as a primary vector and can go on to infect further birds and other vertebrates (NIH, 2012). Commonly found in Africa, West Asia and the Middle East. Though in 1999, the first reported case of WNV in the hemisphere was reported in New York City.
Numerous theories exist around the introduction of WNV into the western hemisphere. Mostly revolving around the increased incidence of intercontinental travel. Though this is a plausible explanation for its introduction into the United States it is an unlikely one. Considering that an otherwise healthy individual was a carrier of the virus, the asymptomatic nature of the viral infection means it would go completely unnoticed. Although for the individual to infect others they would need to be bitten by a mosquito in a foreign country and then by another in the United States. Only then would the WNV infection cycle be able to recommence. This is due to humans being terminal hosts. (Hecht; 2009)
A much more viable introduction of WNV is through viral vectors such as mosquitos or through infected birds; introduced by stowaway birds on cargo ships entering the USA, through migration, as many species of birds that breed in central Africa and Asia often winter in the United States or via illegal import of foreign birds.
Mosquitos act as viral vectors as they feed on the blood of infected birds or other vertebrates. Though parasites carried on birds such as ticks are also a viable pathway for the passage of WNV into the USA. Moskvitina et al (2008) found that within four Ixodid tick species collected from Tomsk, Russia. It was found that 5.2-11.7% of ticks carried the virus depending on locality. They concluded that ticks play an important role in the distribution of WNV, mainly being distributed by domestic birds. Similar studies have been carried out in Israel with similar results in the examination of Argasid ticks (Mumcuoglu; 2005)
From the phylogram, it is evident that the highest level of sequence similarity to the WNV sample taken from the USA is also found in Israel. Malikinson et al (2002) first discovered in a domestic goose, it was hypothesised that the introduction of WNV into the Middle East was due to migrating fledgling white storks which had hatched in Europe. As sequence analysis of WNV isolates from their brains showed that envelope gene matched exactly those extracted from Israeli domestic geese. It was concluded that the storks had become exposed to WNV in their passage over Europe. Genomic sequence analysis also showed that the stork isolates shared a 0.25% nucleotide and 0.3% amino acid differences with the 1999 case of the Chilean flamingo in Bronx zoo, this being the origin of WNV within the United States. Rappole et al (2000) suggested the most likely way for introduction of WNV into the United States is likely to be introduction by birds that may have been illegally imported or that have migrated and have been deviated from their course of flight by storms. This contradicts the initial hypothesis that WNV found its way into the United States as a consequence of increased opportunities for intercontinental travel. It is much more likely that migrating or illegally imported birds are the culprit for WNV introduction into the western hemisphere.
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