A Systematic Review of the Comparative Epidemiology of Avian and Human Influenza A H5N1 and H7N9 - Lessons and Unanswered Questions

Abstract

SummaryThe aim of this work was to explore the comparative epidemiology of influenza viruses, H5N1 and H7N9, in both bird and human populations. Specifically, the article examines similarities and differences between the two viruses in their genetic characteristics, distribution patterns in human and bird populations and postulated mechanisms of global spread. In summary, H5N1 is pathogenic in birds, while H7N9 is not. Yet both have caused sporadic human cases, without evidence of sustained, human-to-human spread. The number of H7N9 human cases in the first year following its emergence far exceeded that of H5N1 over the same time frame. Despite the higher incidence of H7N9, the spatial distribution of H5N1 within a comparable time frame is considerably greater than that of H7N9, both within China and globally. The pattern of spread of H5N1 in humans and birds around the world is consistent with spread through wild bird migration and poultry trade activities. In contrast, human cases of H7N9 and isolations of H7N9 in birds and the environment have largely occurred in a number of contiguous provinces in south-eastern China. Although rates of contact with birds appear to be similar in H5N1 and H7N9 cases, there is a predominance of incidental contact reported for H7N9 as opposed to close, high-risk contact for H5N1. Despite the high number of human cases of H7N9 and the assumed transmission being from birds, the corresponding level of H7N9 virus in birds in surveillance studies has been low, particularly in poultry farms. H7N9 viruses are also diversifying at a much greater rate than H5N1 viruses. Analyses of certain H7N9 strains demonstrate similarities with engineered transmissible H5N1 viruses which make it more adaptable to the human respiratory tract. These differences in the human and bird epidemiology of H5N1 and H7N9 raise unanswered questions as to how H7N9 has spread, which should be investigated further.

Serological evidence for exposure to avian influenza viruses within poultry workers in southern China

“…Avian influenza A(H7N9) virus has been detected mainly in poultry from LPM, which therefore are considered important drivers of the zoonotic infections. This was confirmed by the observed reduction in human infections following temporary closure of LPM (Bui et al., ; Wu et al., ). However, only 6 per cent of the 139 confirmed H7N9 patients detected before 1 December 2013 were poultry workers, suggesting that other modes of transmission may apply, including infection after contact with persons with mild illness (Li et al., ).…”

An overview of the epidemiology and emergence of influenza A infection in humans over time

“…However, exposure details are missing for a large proportion of cases (see Additional file 1: Table S1 and [3, 7]). We have shown that a history of close poultry contact is far more common for human H5N1 cases than H7N9 cases [7].…”

“…For all zoonotic avian influenza virus AIV serotypes except H5N1 and H7N9 (for which methods have been previously described ([3]), we reviewed epidemiological features of outbreaks or cases. We searched publications using the Scopus database, and grey literature using the World Health Organisations (WHO), Centres for Disease Control and Prevention (CDC), Flutrackers and ProMed websites, using the subtype as the keyword.…”

“…We previously compared the novel H7N9 which emerged in humans in Shanghai, 2013 to the more extensively studied H5N1 and highlighted some puzzling differences in the epidemiology [3]. In this brief report, we compared the epidemiology and emergence of all influenza A serotypes known to cause human infections.…”

Geldanamycin Reduces Acute Respiratory Distress Syndrome and Promotes the Survival of Mice Infected with the Highly Virulent H5N1 Influenza Virus

“…Pandemic influenza outbreaks of HPAIV in poultry pose significant threats to public health, as highlighted by the emergence of HPAIV H5N1 (Ferguson et al, 2004); thus, pandemic preparedness is a global priority to combat these threats. The HPAIV H5N1 first emerged in Hong Kong in 1997 with direct transmission from chickens to humans, and then re-emerged in Mainland China in 2003 (Bui et al, 2016). This infection leads to an acute respiratory distress syndrome (ARDS), the destruction of inflamed lung tissues, and an overwhelming inflammatory response, known as the “cytokine storm,” releasing numerous cytokines, such as interleukin-1 (IL-1), IL-6, tumor necrosis factor-α (TNFα), and other cytokines, which have been linked to fatal outcomes in humans (Subbarao et al, 1998; Ku and Chan, 1999; Chan, 2002; de Jong et al, 2006; Kash et al, 2006; Xu et al, 2006; Kobasa et al, 2007; Kawachi et al, 2009; Uyeki, 2009; Zhou and He, 2009; Li et al, 2010).…”