A West Nile virus recipe for a the whole neighborhood

Ingredients

• Impervious surfaces (roads and roofs work best)
• Breeding habitat for mosquitoes (ditches and sewer drains) 
• Mosquitoes (Culex or Aedes) 
• Simplified bird community (robins work best but you can substitute house  sparrows) 
• Mild winters followed by extended dry periods with periodic heavy rains
• People
• Serves tens of thousands!

If you have trouble putting together the ingredients, no problem!  

Kits are available 

Just add an urban environment! 

Details: The prevalence (how many people have it at one time) of West Nile increases with increasing urbanization. But why?


West Nile virus is carried by female mosquitoes that have typically bitten a bird with West Nile virus but has not had a full meal. The disease is spread when the still-hungry mosquito now carrying West Nile bites an unexposed bird. West Nile virus (WNV) is a typical arbovirus (=ARthropod BOrne VIRUS) that into the mosquito's salivary glands. When a mosquito bites, into the host (you) the mosquito injects saliva containing anticoagulants to keep the blood flowing. Saliva can also virus particles and a host of other organisms including protozoans (e.g., the hemosporidian parasite Plasmodium that causes malaria). The mosquito injects the virus into the bird where the virus either (1) amplifies and kills the host quickly, (2) gets cleared from the host's blood stream by the host's immune system or (3) the virus copies itself like crazy (amplification) and hangs out in the blood.  If there are lots of copies of viruses in the blood (high viremia) then the mosquitoes that bite this bird will get the virus.  If the bird is a species that dies or can clear the virus, that species represents a dead end for the virus. The third case represents a bird that is highly competent. The more competent individual in an area, the greater the probability that a mosquito will pick up West Nile virus. When a mosquito bites a person, the mosquito injects a soup of anticoagulants (to keep the blood flowing), and viruses (and, possibly, a number of protozoans that parasitize birds but die in people). 


We call this spillover - when a "non-target" species gets infected. Humans (and horses) are the unfortunate recipients of West Nile virus. Mortality (death) rates are not very high for West Nile virus but morbidity (being sick) is. The virus arrived in 1999 in the New York city area and has spread throughout the US since then. 


Urban areas have a high prevalence of West Nile virus for several reasons including a large number of breeding sites for mosquitoes. 

Small ditch created by vehicles.  This body of water  has been around for weeks and is full of mosquito larvae. 

The bird community also affects WNV prevalence. More diverse bird communities have more dead ends - birds either die quickly or can clear the virus before it spreads. These species that have low competence include woodpeckers and warblers. In urban areas, there are fewer species of birds and they are often highly competent (able to tolerate high WNV particles in the blood). Highly competent species include jays, crows, house sparrows and robins. These species are also good spreaders of West Nile because they are social. Most people of seen flocks of crows (called a murder of crows!) but robins are also social! At night they roost in a common tree so imagine a mosquito jumping from bird to bird like an evil West Nile virus toting fairy. 

Mosquito larva from a south Wilkes Barre puddle. Note the siphon on the back end in the upper right hand of the picture.  30x

A family portrait with an early larval instar (top), a later instar (middle), and pupa (bottom). 

How to ruin the recipe! 


If more diverse avian communities have lower prevalence then increase the biodiversity of an urban site. How can this be done? Trees - big old trees that attract warblers, vireos, orioles, etc. 


Reduce breeding sites. Litter just is not just an eye-sore, old tires, beer bottles, paint cans provide great habitat for mosquitoes.  Make sure gutters drain completely.  



References 


ALLAN, B., R. LANGERHANS, W. RYBERG, W. LANDESMAN, N. GRIFFIN, R. KATZ, B. OBERLE, M. SCHUTZENHOFER, K. SMYTH, A. ST. MAURICE, L. CLARK, K. CROOKS, D. HERNANDEZ, R. MCLEAN, R. OSTFELD and J. CHASE. 2009. Ecological correlates of risk and incidence of West Nile virus in the United States. Oecologia, 158: 699-708.


BRADLEY, C. A., S. E. J. GIBBS and S. ALTIZER. 2008. Urban land use predicts West Nile Virus exposure in songbirds. Ecological Applications, 18: 1083-1092.


Despommier, D. 2001. West Nile Story. Apple Trees Production. [Amazon]


WALSH, M. G. 2012. The role of hydrogeography and climate in the landscape epidemiology of West Nile virus in New York State from 2000 to 2010. PLoS ONE, 7: e30620.