West Nile Virus Transmission Cycle
# 8737
As a kid growing up in Florida, mosquito bites were as common as freckles; everybody had ’em. Sure, they were irritating. . . sometimes even maddening . . . but other than exceedingly rare cases of arboviral encephalitis (EEE, SLEV, Lacrosse, etc. ), they were not considered to present a serious health hazard.
At least not since dengue, malaria and yellow fever were successfully eradicated in the 1920s and 1930s.
But in recent years, mosquitoes have returned as a public health concern, due to the introduction (or in some cases, re-introduction) of exotic diseases into local mosquito population. And not just in Florida, but around the nation.
While right now all eyes are on the burgeoning Chikungunya epidemic in the Caribbean, and the ever expanding range of Dengue, fifteen years ago another exotic disease – West Nile Virus – arrived (likely via infected travelers from the Middle East) in New York City, and since then it has become endemic across the lower 48 states and into Canada (see WNV: The Economic Costs Of An Invasive Arbovirus).
From the USGS Factsheet on West Nile Virus
Unlike Chikungunya – which primarily affects humans and non-human primates – WNV is primarily a disease of birds. Humans and horses are considered `incidental’ infections (see WNV vs CHIKV: A Host Of Differences).
Incidental or not, West Nile Fever, the least severe form of the disease probably infects more than 100,000 Americans every year, although most are so mildly affected the person has no idea they have the virus.
Neuroinvasive cases (which present with meningitis, encephalitis, or flaccid Paralysis), while less common, are severe enough that they nearly always result in hospitalization and diagnosis, and so they are considered the best indicator of the scope of each year’s epidemic.
In 2012, the United States saw nearly 3,000 cases of neuroinvasive WNV, and 286 deaths (see DVBID: 2012 Record Number Of West Nile Fatalities).
Each summer the CDC produces weekly surveillance reports on WNV activity (in humans, birds, and animals).
These reports tend to lag behind actual events by a week or so (fatalities even more so, since some cases may be hospitalized for weeks before succumbing), but they give us an excellent idea of how the WN season is progressing around the country.
Although the West Nile season is just beginning, we’ve our first look at the CDC’s 2014’s surveillance:
While vector-borne diseases don’t often the the respect that `airborne’ pathogens get, they undoubtedly sicken and kill more people every year than any other infectious disease.
The UN’s Millennium Project estimates 300 to 500 million Malaria infections every year, with a up to 2.7 million deaths.
Which is a greater disease impact each and every year than the last two influenza pandemics (1968 & 2009) put together.
Add in millions more Dengue and Chikungunya cases, and the recent sharp increase in tick-borne diseases as well, and the case can be made that emerging vector-borne diseases represent a major, and often underappreciated, challenge to public health.
The CDC offers specific advice on preventing West Nile and other mosquito-borne diseases, including Chikungunya, Dengue, and EEE:
The most effective way to avoid West Nile virus disease is to prevent mosquito bites. Be aware of the West Nile virus activity in your area and take action to protect yourself and your family.
