Virology Updates: Influenza and Yellow Fever on the Rise

by Robert Buckheit III

February 1, 2017

As we move into the heart of influenza season, data from a recently published study suggests a close link between the start of the seasonal influenza virus epidemic and the weather, as reported in the Journal of Clinical Virology. It was observed over a three-year period that the first outbreaks of the flu appeared in close proximity with the first "cold spell" of the winter season. It was hypothesized that the decrease in temperature to levels below freezing, in combination with reduction in humidity, resulted in an increased frequency of influenza cases. These data are not entirely novel, as it has been known that aqueous droplets from coughing and sneezing travel more effectively in cold, low humidity environments. However, these results could help provide guidance to public health organizations as they attempt to control and predict when seasonal epidemics might appear. Other respiratory virus infections, such as those caused by respiratory syncytial virus and coronavirus also exhibited a temperature dependent shift in incidence rates.

The CDC tracks cases of influenza throughout the season, and the 2017 incidence rate currently falls within the seasonal baseline; however, the flu season is still reaching its peak and there is evidence that the incidence rate may soon surpass the epidemic threshold. The current 2016-2017 season has a higher incidence rate of influenza-life illness (ILI) compared to last year, but the incidence rate is reduced relative to the 2014-2015 influenza season. Most laboratory confirmed cases have been observed in patients older than age 65, with all other age demographics having similar incidence rates. The majority of cases observed have been for influenza virus type A. These data highlight the need for high levels of influenza vaccination as we reach the peak in the seasonal flu epidemic in the United States, to protect those most at risk for influenza infection associated morbidity and mortality. Separately, the World Health organization is calling for increased vigilance against avian influenza virus, with 24 countries in the WHO European Region reporting outbreaks of highly pathogenic avian influenza A (H5N8) since June 2016.

In other seasonal epidemic news, Brazil has seen an alarming increase in the number of cases of yellow fever, specifically in the Minas Gerais region of Brazil. As of January 30th, more than 100 cases of yellow fever have been reported with 40 deaths out of 97 total cases reported in Minas Gerais. This is the largest outbreak of yellow fever in Brazil since 2000 and the WHO has reported that they expect the mosquito borne disease to spread to other Brazilian states. Similar to zika virus, the yellow fever virus is carried by the Aedes aegypti mosquito and there are fears that the virus could be carried by this vector into more densely populated urban areas.

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