Avian Flu Diary: Nature: Mapping The Spread & Risk Of H7N9 In China

# 7822

With H5N1 still in the wings, and the upstart H7N9 and MERS coronavirus making menacing overtures, infectious disease watchers and public health authorities have their hands full right now, just trying to keep track. This week we’ve seen the CDC issue new guidance on MERS-CoV along with an IHR Emergency Committee meeting, and warnings from the Chinese government to remain alert for the H7N9 avian flu virus.

Despite these warnings and preparations, no one knows if any of these viruses will pose a serious public health risk this fall and winter. Officials just know they could. And while we watch this trio of emerging diseases, we could easily get blind-sided by something else.

As Yogi Berra famously said - Prediction is very hard, especially about the future.

But hard or not, scientists are working to model and predict the spread of these diseases in order to help guide surveillance and containment efforts. Which brings us to an open-access report, published this week in Nature’s Scientific Reports, that attempts to map and predict future outbreaks of H7N9 in China.

These researchers, from China and from the University of Florida, found that human cases last spring tended to occur near irrigated land, suggesting that ducks and geese may play a role in its transmission, even though the virus has not been isolated in either species.

A bit surprisingly, they also found - `high humidity and an atmospheric temperature around 15°C were predictive factors for the risk of avian influenza A’.

This runs counter to other studies on influenza viruses that show moderately low relative Humidity (< 40%) as being more conducive to viral survival and transmission (see PLoS One: High Humidity Reduces Flu’s Infectivity & NIH Study: Climate & Influenza Transmission).

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Although the methods section of this study will likely be tough sledding for those without a background in statistics, the Introduction and Discussion are both particularly well written, and well worth reading in their entirety.

Scientific Reports | Article Open

Mapping Spread and Risk of Avian Influenza A (H7N9) in China

Li-Qun Fang, Xin-Lou Li, Kun Liu, Yin-Jun Li, Hong-Wu Yao, Song Liang, Yang Yang, Zi-Jian Feng, Gregory C. Gray & Wu-Chun Cao

doi:10.1038/srep02722

Published 26 September 2013

The outbreak of human infections with an emerging avian influenza A (H7N9) virus occurred in China in early 2013. It remains unknown what and how the underlying risk factors were involved in the bird-to-human cross-species transmission. To illustrate the dynamics of viral spread, we created a thematic map displaying the distribution of affected counties and plotted epidemic curves for the three most affected provinces and the whole country. We then collected data of agro-ecological, environmental and meteorological factors at the county level, and used boosted regression tree (BRT) models to examine the relative contribution of each factor and map the probability of occurrence of human H7N9 infection. We found that live poultry markets, human population density, irrigated croplands, built-up land, relative humidity and temperature significantly contributed to the occurrence of human infection with H7N9 virus. The discriminatory ability of the model was up to 97.4%. A map showing the areas with high risk for human H7N9 infection was created based on the model. These findings could be used to inform targeted surveillance and control efforts in both human and animal populations to reduce the risk of future human infections.

First, from the Introduction (and reparagraphed for readability), a concise summary of where things stand right now with the H7N9 virus:

A novel avian influenza A (H7N9) virus infecting humans has emerged in mainland China¹, causing global concerns about its potential to start an influenza pandemic^{2, 3}. Since the National Health and Family Planning Commission of China (NHFPC) announced the emerging infectious disease on March 31, 2013, a total of 131 confirmed cases, with 39 deaths have been reported in eight provinces and two municipalities as of May 30⁴. Fortunately, thus far there has been no evidence of sustained person-to-person transmission⁵.

However, unlike the high pathogenic avian influenza A viruses such as H5N1 and H7N7, in which outbreaks in poultry precede human infections and imply where the public health threat lies^{6, 7}, the novel H7N9 virus causes no or only mild disease in birds^{8, 9}. This means that the virus is likely to spread silently in birds or other animal reservoirs. Human infections are therefore the sentinel events, and the quick geographical expansion of human cases indicate that a hidden epidemic in birds is well underway³, and many parts of the country offers a favorable breeding ground for the virus to circulate.

So far, however, apart from birds and the contaminated environments at the live poultry markets^{9, 10}, the sources of infection remain elusive¹¹. It is unclear how the emerging avian influenza A (H7N9) virus is spreading in China³, and what and how the underlying risk factors are involved in the cross-species transmission.

From the discussion section, a brief excerpt (but follow the link to read it in its entirety).

The outbreak of human infections with the novel avian influenza A (H7N9) virus lasted for over three months since it emerged in mainland China. Fortunately, thus far there has been no sustained human-to-human transmission⁵, although the virus has genetic characteristics that suggest it could effectively replicate in mammals^{1, 15}. Currently, H7N9 virus infection is primarily zoonotic. In this study, we used GIS-based spatial analysis to map the spatial distribution of human infections with H7N9 virus. The thematic map displaying the distribution of human cases indicates that although most human cases were concentrated at the Yangtze River delta on China's eastern seaboard, sporadic cases were distributed in large areas of adjacent provinces, even spreading northward to Beijing and being exported to Taiwan (Fig. 1).

As a final note, last March – and published before we learned of the outbreak of H7N9 in China – the CDC’s EID Journal carried a study (see Predicting Hotspots for Influenza Virus Reassortment) that attempted to map those areas of the world most likely to spawn new influenza strains.

This study identified a number of key geographic locations - the northern plains of India, the coastal and central provinces of China, the western Korean Peninsula and southwestern Japan in Asia, and the Nile Delta in Egypt - as likely hotspots.

Their H5N1 risk map of China (see below), matches pretty closely to the H7N9 risk map in today’s study.

Of course, the last pandemic reassortment originated in the Americas, and no one is quite sure where the devastating 1918 `Spanish’ flu evolved. While scientists can find and predict patterns, Nature often goes its own way.