Human activity drives the spread of flaviviruses, and further climate change could aid the spread of these diseases additionally. These diseases, their clinical properties, their behavior in infected populations, and human effects on their evolution and propagation are explored in a seminar in the February 8, 2008 issue of The Lancet.

Flaviviruses are mosquito or tick borne viruses that can be transmitted to humans or other mammals, and infection is often characterized by hemorrhagic disease, encephalitis, biphasic fever, flaccid paralysis, and jaundice. Some diseases caused by these viruses include West Nile fever, yellow fever, dengue, Japanese encephalitis, and Omsk hemorrhagic fever. Dr Ernest Gould, Centre for Ecology and Hydrology (CEH) Oxford, Mansfield Road, Oxford, UK, and Professor Tom Solomon, Walton Centre for Neurology and Neurosurgery and University of Liverpool, Liverpool, UK, examined the available data on flaviviruses, hoping to highlight new findings and discuss unanswered questions and controversies that exist surrounding these diseases.

"Although the characteristics of these viruses are well defined, they are still unpredictable with increases in disease severity, unusual clinical manifestations, unexpected methods of transmission, long-term persistence, and the discovery of new species." State the authors. Each of the above mentioned diseases is discussed at length in the Seminar.

In their discussion of control of the viruses, the authors discuss issues involved with vaccination against these diseases. This includes a current debate regarding yellow fever's vaccination: should it be reserved for control of outbreaks, or should it become a part of the Expanded Programme on Immunization in regularly affected areas. They also relate the success of the immunization campaign in China in protecting children from Japanese encephalitis, and describe further funding awarded by the Bill & Melinda Gates Foundation to expand this program into India. According to the authors, additional challenges are created by dengue fever and dengue hemorrhagic fever, as neither has an available vaccine and is thus liable to spread further. "A major challenge is to produce a multivalent vaccine for dengue virus that is effective against the four serotypes...the successful development of an effective vaccine for this disease would be a major achievement." They emphasize that the financial power of developing countries should be considered for available vaccines, and accounted for in the development of future vaccines.

Most of these viruses can employ several different types of hosts and methods of transmission. In conclusion, the authors discuss various effects that may come into play in the future. "With increasing temperatures worldwide, movement of people, increasing human population densities, wider dispersal of competent mosquitoes or ticks, and transportation of goods, animals, and agricultural products, the continuing spread of these arboviruses into new regions seems probable. Furthermore, we might expect increasing numbers of epidemics due to recognised flaviviruses such as West Nile virus in southern Europe and perhaps eventually in Northern Europe. Moreover, Japanese Encephalitis could spread further west."

Pathogenic flaviviruses
E A Gould, T Solomon
The Lancet, February 9, 2008, Vol 371: 500-509
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Anna Sophia McKenney

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