While it may seem a pathetic attack, it is a low cost, easy delivery method to spread death from a distance, Just populate an airtight container with swarms of infected insects and ship it to the US.
That is very low maintenance from the perpertrator's perspective and has almost no chance of tracing the culprits.
164 deaths is certainly "insignificant" when stacked up against the various other things that kill people on a ragular basis. Heck, before Rudy Giuliami became mayor, this would represent a typical monthly murder total for the City of New York.
People get infected by West Nile mosquitoes all the time. For 99% of these people, the symptoms are either miniscule or cannot be distinguished from a mild case of the flu.
Also, keep in mind that Americans are better able to resist disease than almost any racial or ethnic group in the world. Any disease that is capable of killing large numbers of Americans will wipe out most of the planet in the process.