Timing of influenza A(H5N1) in poultry and humans and seasonal influenza activity worldwide, 2004-2013

Abstract

Co-circulation of influenza A(H5N1) and seasonal influenza viruses among humans and animals could lead to co-infections, reassortment, and emergence of novel viruses with pandemic potential. We assessed the timing of subtype H5N1 outbreaks among poultry, human H5N1 cases, and human seasonal influenza in 8 countries that reported 97% of all human H5N1 cases and 90% of all poultry H5N1 outbreaks. In these countries, most outbreaks among poultry (7,001/11,331, 62%) and half of human cases (313/625, 50%) occurred during January-March. Human H5N1 cases occurred in 167 (45%) of 372 months during which outbreaks among poultry occurred, compared with 59 (10%) of 574 months that had no outbreaks among poultry. Human H5N1 cases also occurred in 59 (22%) of 267 months during seasonal influenza periods. To reduce risk for co-infection, surveillance and control of H5N1 should be enhanced during January-March, when H5N1 outbreaks typically occur and overlap with seasonal influenza virus circulation.

Figure 1

Shading indicates countries that reported confirmed human cases of highly pathogenic avian influenza (HPAI) A(H5N1) infection and outbreaks of H5N1 among poultry during 2004–2013. Black shading indicates the 8 study countries that reported 97% of all human H5N1 cases and 90% of all poultry H5N1 outbreaks: Bangladesh, Cambodia, China, Egypt, Indonesia, Thailand, Turkey, and Vietnam.

Figure 2

Outbreaks of highly pathogenic avian influenza A(H5N1) infection by month (white bars) and mean temperature (black lines) for the 8 countries that reported 90% of worldwide poultry H5N1 outbreaks during 2004–2013. Stars indicate month with highest average number of outbreaks for each country. Temperature data were not available for Cambodia.

Figure 3

Monthly average number of highly pathogenic avian influenza A(H5N1) infection outbreaks among poultry (black line) and human H5N1 cases (white bars) for 8 study countries (Bangladesh, Cambodia, China, Egypt, Indonesia, Thailand, Turkey, and Vietnam) that reported 90% of all poultry H5N1 outbreaks and 97% of all human H5N1 cases during 2004–2013.

Figure 4

Human cases of highly pathogenic avian influenza A(H5N1) infection and seasonal influenza rates for 8 study countries, by month. Bars indicate the proportion of human respiratory samples that tested positive for seasonal influenza viruses; black bars indicate months during which seasonal influenza was epidemic and a human H5N1 case was reported. Years covered for each country are provided along x-axes.