Immaterial. The mosquitos borne of those eggs wouldn't be carriers of WNV...until they bit an infected bird or human.
Accordingly, the accidental introduction of a single mosquito, or just a few, would be insufficient to sustain and spread an outbreak. Presumably, a significant colony of infected mosquitos would be necessary to do that.
Recall, as well, where the first two outbreaks occurred: New York City and Washington, D.C.
We'd need one WN-carrying mosquito to get here and bite one bird. One infected bird can be bitten by many mosquitoes, who will then be transmitting it to many other birds.
And those birds fly.
Meanwhile, the mosquitoes who are carriers can survive the winter and get right back to work on the rest of the birds come the next spring.
Recall, as well, where the first two outbreaks occurred: New York City
Major port town, as well as tourist destination
and Washington, D.C.
Birds flying south.
I'm afraid West Nile got here the same way that the Med fly and that beetle worm/larvae thing that was killing all the trees in the outer boroughs here did--on a boat or plane, in other cargo.
West Nile Encephalitis
WNV is a flavivirus belonging taxonomically to the Japanese encephalitis serocomplex that includes the closely related St. Louis encephalitis (SLE) virus, Kunjin and Murray Valley encephalitis viruses, as well as others. WNV was first isolated in the West Nile Province of Uganda in 1937 (2). The first recorded epidemics occurred in Israel during 1951-1954 and in 1957. Epidemics have been reported in Europe in the Rhone delta of France in 1962 and in Romania in 1996 (3-5). The largest recorded epidemic occurred in South Africa in 1974 (6).
An outbreak of arboviral encephalitis in New York City and neighboring counties in New York state in late August and September 1999, was initially attributed to St. Louis encephalitis virus based on positive serologic findings in cerebrospinal fluid (CSF) and serum samples using a virus-specific IgM-capture enzyme-linked immunosorbent assay (ELISA). The outbreak has been subsequently confirmed as caused by West Nile virus based on the identification of virus in human, avian, and mosquito samples. See also these MMWR articles:
Outbreak of West Nile-Like Viral Encephalitis -- New York, 1999. MMWR, 1999:48(38);845-9
Update: West Nile-Like Viral Encephalitis -- New York, 1999. MMWR, 1999:48(39);890-2
The virus that caused the New York area outbreak has been definitively identified as a strain of WNV. The genomic sequences identified to date from human brain, virus isolates from zoo birds, dead crows, and mosquito pools are identical. SLE and West Nile viruses are antigenically related, and cross reactions are observed in most serologic tests. The isolation of viruses and genomic sequences from birds, mosquitoes, and human brain tissue permitted the discovery of West Nile virus in North America and prompted more specific testing. The limitations of serologic assays emphasize the importance of isolating the virus from entomologic, clinical, or veterinary material.
"Although it is not known when and how West Nile virus was introduced into North America, international travel of infected persons to New York or transport by imported infected birds may have played a role. WNV can infect a wide range of vertebrates; in humans it usually produces either asymptomatic infection or mild febrile disease, but can cause severe and fatal infection in a small percentage of patients. Within its normal geographic distribution of Africa, the Middle East, western Asia, and Europe, WNV has not been documented to cause epizootics in birds; crows and other birds with antibodies to WNV are common, suggesting that asymptomatic or mild infection usually occurs among birds in those regions. Similarly, substantial bird virulence of SLE virus has not been reported. Therefore, an epizootic producing high mortality in crows and other bird species is unusual for either WNV or SLE virus. For both viruses, migratory birds may play an important role in the natural transmission cycles and spread. Like SLE virus, WNV is transmitted principally by Culex species mosquitoes, but also can be transmitted by Aedes, Anopheles, and other species. The predominance of urban Culex pipiens mosquitoes trapped during this outbreak suggests an important role for this species. Enhanced surveillance for early detection of virus activity in birds and mosquitoes will be crucial to guide control measures.