West Nile Virus : From Uganda to America

Richard HorvitzAug 3, '18

We have been receiving many calls recently from people asking about mosquitoes in general, and West Nile Virus in particular.  So, I decided to put on my scientist hat and share with you what I have learned over the past few years about this virus. 

What is it? 

You may have heard of West Nile Virus before.  It’s a nasty Flavivirus that is transmitted from host to host to you via your friendly mosquito (or 43 species of mosquitoes, to be exact).  It was first isolated in Uganda, in 1937, arrived in New York City in 1999, and in five years West Nile Virus has been found in birds and mosquitoes in every continental state in the USA.  Other viruses in this family include Yellow Fever, Dengue Fever, Japanese encephalitis, and St. Louis encephalitis. 

The interesting thing about Flaviviruses like West Nile Virus is the actual way in which they are transmitted.  First, a mosquito bites a host, in this case an indigenous bird.  Then, the virus actually reproduces in the host, and is spread to the next mosquito when it bites the host.  The second mosquito then gets the West Nile Virus, and bites another bird (perpetuating the spread of the virus), or perhaps bites a human or a horse (or an exotic bird breeding outside).  When the virus reproduces to a critical level, the host dies.  Birds can thus transmit the disease as a host, but humans can not.  Suffice it to say the reason is that we are so much larger than birds it would take thousands upon thousands of mosquito bites on a human to produce the level of virus in the bloodstream required for transmission to another mosquito, then to another human. 

How Bad is it? 

West Nile Virus seems to have peaked in 2003 with 9,858 reported cases (262 deaths).  Last year, there were significantly fewer case and deaths:  2,282 and 77, respectively (New York Times, November 13, 2004).  An interesting development is that the greatest number of cases last year was in places that had little activity in prior years.  Namely, California (757 case/20 deaths) and Arizona (381 cases, 10 deaths) led the country in cases and deaths per state. 

Since it is spread via mosquito, it was originally thought that it was carried by mosquitoes in swamps and salt marshes.  However, it seems that they are more likely to breed in storm drains and catch basins than in swamps (New York Times, May 26, 2005).  Therefore, it makes sense, as any Floridian knows, not to keep standing water around your house. 

The good news is, less than 1% of all people bitten develop serious symptoms, and only 10% of those infected have died.  Most people affected are over 50 years old. Symptoms include stiff neck, mental confusion, muscle weakness,  and sensitivity to light 5 – 15 days after exposure.  The most serious conditions turn into West Nile Meningitis (inflammation of the brain or area around the brain) which is very, very painful. 

What’s Happening this year with infections?

In 2005 thus far, however, there have been very few cases.  In fact, there have been no reported human cases in the United States thus far this year.  There have been 105 bird deaths in California, 45 in Louisiana, and no dead birds testing positive in Florida thus far in 2005.  However, there have been 18 cases of Sentinels (chickens or horses) that have tested positive in Florida – nearest us would be 2 cases in Sarasota, 5 cases in Hillsborough County, and 1 case in Orange County.  California has had only two sentinels testing positive.  You can track the above statistics on a nationwide and statewide basis on www.fwc.state.fl.us/bird/  or call the Department of Health at 561-355-3070.  If you handle a dead bird, use gloves or an inverted plastic bag. 


I hope this brief summary of West Nile Virus has made you feel more comfortable about the disease, as well as giving you a basic understanding of West Nile Virus.  It is interesting that it has significantly dropped off all over the country, and it is great that a vaccine will be tested by the end of this year.  However, caution is till in order.

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