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.
While it will come as no surprise that they found the incidence of H5N1 infection – both in poultry and people – peaks during the months of January–March, there’s more to the synopsis which appeared yesterday in the CDC’s EID Journal on H5N1 activity over the past decade.
First, a link and the abstract, along with a few snippets from the discussion section (bolding mine) of this detailed, well-timed report, then I’ll return with more.
Volume 21, Number 2—February 2015
Lizette O. Durand, Patrick Glew, Diane Gross, Matthew Kasper, Susan Trock, Inkyu K. Kim, Joseph S. Bresee, Ruben Donis, Timothy M. Uyeki, Marc-Alain Widdowson, and Eduardo Azziz-Baumgartner
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.
Our study reaffirms that, in Southeast Asia, H5N1 outbreaks among poultry and human H5N1 cases often occur seasonally, during months when temperatures are relatively cool. Even when accounting for H5N1-endemic countries outside Southeast Asia, most (>50%) poultry H5N1 outbreaks and human H5N1 cases of H5N1 infection occurred during January–March.
Our analysis of 2004–2013 data from 8 countries also suggests that lower ambient temperatures are associated with H5N1 outbreaks among poultry, even though half of our data came from tropical countries, where annual temperature variations are often small. These results are similar to those described by Park and Glass, who observed poultry H5N1 outbreaks during 1997–2006 in Southeast Asia and China and concluded that these outbreaks most often occurred during colder months (10). Other studies have found similar associations (5,25,26). A decrease in temperature can make poultry more susceptible to H5N1 because lower ambient temperature can decrease poultry immunity (27–29). Moreover, cold weather may enable prolonged viral survival in the secretions and feces of infected poultry, and anticipation of seasonal holidays (e.g., Chinese New Year) often results in increases in population density of domestic poultry and in trafficking of poultry (27–34).
Human H5N1 cases were almost 5 times more common in months during which poultry H5N1 outbreaks occurred. These findings reaffirm reports that human H5N1 virus infection is typically preceded by exposure to sick or dead poultry (35) and suggest that human and animal health officials in affected countries should explore the effectiveness of education and outreach efforts before and postexposure prophylaxis during anticipated H5N1 epidemic periods.
Our data also suggest that one fifth of human H5N1 cases occurred in months during which seasonal influenza was epidemic. Concurrent H5N1 and human seasonal influenza activity provides opportunities for humans and other animals (e.g., swine) to become co-infected with these co-circulating viruses and for the viruses to reassort. Reassortment may generate novel influenza A virus strains with the ability to cause sustained human-to-human transmission.
The final paragraph excerpted above discusses the dangers of having H5N1 co-infecting humans (or other hosts) with an already humanized seasonal flu virus (i.e. H3N2, H1N1).
Reassortment is the mechanism where two different flu viruses infect the same cell simultaneously, and swap genetic material, producing a new, hybrid virus. - Credit AFD
We know this can happen in the wild, as reassortment in birds and/or swine is the route by which many new subtypes of influenza are created. It is how the H5N1 virus originally evolved in the 1990s, and how H7N9 abruptly appeared in the spring of 2013.
Over the past year we’ve seen several additional reassortants of concern: including H5N6, H5N8, and H5N3, H10N8, and based on reports from Taiwan this week, a new H5N2.
We’ve also seen this process (albeit, rarely) in humans. In 2011 an influenza co-infection in Canada led to the creation of a unique hybrid reassorted virus (see Webinar: pH1N1 – H3N2 A Novel Influenza Reassortment), although it was not passed on to anyone else.
Influenza co-infections in humans are rarely documented, but probably occur more frequently than we suppose. Luckily, the creation of successful reassortant viruses is the exception, not the rule.
In the summer of 2013, in the Lancet: Coinfection With H7N9 & H3N2, we saw the first evidence of co-infection with the newly emerged H7N9 virus and a seasonal flu virus in a human. While last October, in EID Journal: Human Co-Infection with Avian and Seasonal Influenza Viruses, China, we looked at co-infections in 2 patients in Hangzhou, in January 2014. In these cases, no reassortant virus was detected.
While an influenza co-infection leading to the creation of a biologically `fit’, and competitive, novel virus is the viral equivalent of hitting the lottery, when you have a growing number of viral players (H5N1, H7N9, H5N6, H5N8 . . .), buying huge numbers of tickets (infecting hosts) every week, the chances of hitting the `right’ genetic combination go from being astronomically bad to being reasonably good over the long run.
Which is why we watch H5N1, H7N9, and the bevy of newly emerged avian viruses carefully for any signs that they are playing too well with others.