Monday, April 02, 2018

EID Journal: Hx of Mosquitoborne Diseases In the U.S. & Implications For The Future


Other than West Nile Virus, which arrived in the United States two decades ago and has since become endemic across the lower 48, we've been pretty lucky with mosquito borne diseases over the past 70 to 80 years.
Despite the yearly arrival of thousands viremic travelers carrying malaria, Dengue, Yellow Fever, Chikungunya, and (most recently) Zika Virus - only WNV has managed to take hold.
While our temperate climate may have something to do with their collective failures - not all that long ago, Malaria, Dengue and Yellow Fever were fairly widespread and caused major epidemics in the United States (see map below).

Outbreaks of yellow fever reported during 1693–1905 among cities comprising part of present-day United States. - Credit EID Journal

While mosquito control efforts have certainly been a big part of this victory, as a Floridian I can assure you there are still plenty of mosquitoes out there. What has really changed over the past 100 years is the way we live; largely indoors, with either screens on our windows, and/or air conditioning.  
In 2003, a CDC EID study concluded that economics and lifestyle probably have a lot to do to with our lack of locally transmitted Dengue (see Texas Lifestyle Limits Transmission of Dengue Virus).
Arboviruses like Malaria, Yellow Fever, and West Nile Virus are often maintained in non-human animal reservoirs, like monkeys or horses. This cycle - where mosquitoes infect and transmit the virus between non-human hosts, is called the Sylvatic (Jungle) cycle.

Credit CDC

Mosquito borne illnesses can also have an urban cycle, where no non-human host is required. We've seen this with Dengue, Chikungunya, and Zika in the Americas.  It usually requires a fairly dense population of susceptible human hosts in order to sustain an outbreak.

As you might imagine, keeping this sort of urban cycle going in the United States, where people spend more time indoors, and where control efforts keep the mosquito population down, is much more difficult than in the tropics.
West Nile Virus has become endemic here because it is hosted by birds, and horses, and the Sylvatic cycle ensures that the virus always has a host.  The same is true for EEE (horses) and La Crosse viruses (squirrels & chipmunks).
The non-human hosts for Yellow Fever and Malaria (usually non-human primates) are relatively rare in the United States, making it much harder for these diseases to become entrenched. 
Harder . . . but not impossible.  
And there are a number of exotic mosquito borne illnesses around the globe that could - if introduced here - find suitable non-human hosts; Rift Valley Fever (cattle), Japanese encephalitis (pigs), and Venezuelan equine encephalitis (horses) among others. 
All of which brings us to an EID Journal article that looks at the long history of Mosquito borne diseases in America's past, and examines the potential implications of the future introductions of pathogens like the ones listed above.
It's a fascinating historical read and perspective, and well worth the 20-30 minutes it will take to review. I'll have a bit more when you return.
History of Mosquitoborne Diseases in the United States and Implications for New Pathogens

Max J. Moreno-Madriñán and Michael Turell


The introduction and spread of West Nile virus and the recent introduction of chikungunya and Zika viruses into the Americas have raised concern about the potential for various tropical pathogens to become established in North America.
A historical analysis of yellow fever and malaria incidences in the United States suggests that it is not merely a temperate climate that keeps these pathogens from becoming established. Instead, socioeconomic changes are the most likely explanation for why these pathogens essentially disappeared from the United States yet remain a problem in tropical areas.
In contrast to these anthroponotic pathogens that require humans in their transmission cycle, zoonotic pathogens are only slightly affected by socioeconomic factors, which is why West Nile virus became established in North America. In light of increasing globalization, we need to be concerned about the introduction of pathogens such as Rift Valley fever, Japanese encephalitis, and Venezuelan equine encephalitis viruses.
        (Continue . . .  )
The authors concluded with a cautionary note:
Given that the decreased exposure of humans to mosquitoes in the United States is primarily driven by changes in socioeconomic conditions, it is important to note that these very conditions could be threatened by massive natural disasters or any other similarly disruptive event. Consequently, appropriate disaster preparedness plans need to be in place to address this potential threat.
After last year's triple whammy from Hurricanes Harvey, Irma, and Maria millions of people in the United States and the Caribbean suddenly saw a very abrupt change in their socioeconomic condition.
In Florida, and in Texas, interruptions in electricity, water, and essential services lasted for days or weeks. In Puerto Rico, and the islands of the Caribbean, some people are still waiting for repairs.  
While it might not be at the top of your list of stock up items for your Hurricane/Disaster kit, having a supply of mosquito repellent for you and your family is highly recommended.

For some earlier mosquito-borne illness blogs, you may wish to revisit:
CDC Vital Signs - Zika Virus Update
Arboviruses: (Already) Coming To America
Grand Rounds: Preventing Aedes Mosquito-Borne Diseases

PNAS: Asymptomatic Humans Transmit Dengue Virus To Mosquitoes

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