Early this week, the World Health Organization announced that the Zika virus, a mosquito-borne illness that in the past year has swept quickly throughout equatorial countries, is expected to spread across the Americas and into the United States.
The disease, which was discovered already back in 1947 but had since been seen in only small, short-lived outbreaks, causes symptoms including a rash, headache and small fever. However, a May 2015 outbreak in Brazil led to nearly 3,500 reports of birth defects linked to the virus, even after its symptoms had passed, as well as an increase in cases of the immune disorder Guillain-Barre syndrome.
Zika virus is transmitted by the mosquito species Aedes aegypti, which is also a carrier of two other tropical diseases, dengue fever and chikungunya. This mosquito species is not native to North America and restricted to tropical and subtropical regions of the world and not found farther north in the United States than Alabama, Mississippi, Georgia and South Carolina.
Analysis of a 441-bp fragment of the mitochondrial cytochrome oxidase I gene sequence identified the same two haplotype sequences during 2011–2014, and placed these within two discrete groups known to be derived from lineages resident in the Americas. Analysis of 10 microsatellite loci for specimens collected during 2011–2014 revealed no evidence for introgression of new alleles across years.
These findings are evidence that these mosquitoes not only occur in more northern areas of the USA but also that they have overwintered for at least the past four years, meaning they are adapting for persistence in a northern climate well out of their normal range.
The Washington population is currently disease-free but the fact that some mosquito species are finding ways to survive in normally restrictive environments by taking advantage of underground refugia means that there is a real potential for active transmission of mosquito-borne tropical diseases in the future.
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