Prevention of avian influenza spread through wet markets is a must

Wet markets in Asia have a high risk potential for the spread of avian influenza. Photo: ANP

Research published by an international team reveals that the speed at which a subtype of avian influenza can spread in Asia’s live bird markets is high. Based on actual data, the scientists conclude that prevention, including vaccination of birds destined for sale at wet market, is needed.

The research team included scientists from the Royal Veterinary College and University of Oxford in the UK, City University of Hong Kong, Chattogram Veterinary and Animal Sciences University in Bangladesh and the French National Research Institute for Agriculture, Food and the Environment. They belong to the GCRF One Heath Poultry Hub, an interdisciplinary R&D programme supporting the growth of poultry meat and egg production in Asia while minimising risk to public health.

Previous attempts to model avian influenza spread in Asia have only been theoretical but this team’s model includes actual data from chickens in live bird markets (also known as wet markets). Previous research has shown that these markets are hotspots for avian influenza transmission. And they are very numerous. “Poultry is the main source of meat in Bangladesh and almost all poultry marketed in the country transits through live-bird markets,” notes team member Dr Francesco Pinotti of the University of Oxford. “Recent field surveys carried out by our collaborators revealed that 4,300 birds were sold on average in a single day in the market looked at in this study. This number increased to 61,000 when all live bird markets located in the city of Chattogram are considered.”

Influenza types

The team modelled the H9N2 avian influenza subtype. It is zoonotic, causing mild symptoms in people but able to cause substantial production losses for chicken farmers. This variant has also been involved as a gene donor in the emergence of new avian influenza variants. Pinotti explains that, at present, the majority of flu infections due to highly-pathogenic strains reported in Asia are associated with H5N1 viruses and found mostly in wild birds. The presence of H5N1 viruses in markets appears to be associated with the presence of ducks that constitute a tiny fraction of birds traded in some markets, such as those found in Bangladesh. “In contrast, low-pathogenic strains, especially H9N2, are much more common in marketed poultry throughout Asia,” he says. “Available evidence suggests there is a remarkably high prevalence of this virus (20-30%) among chickens traded in live bird markets.”

The team’s model found that more than 90% of chickens that enter these markets without having been previously exposed to the H9N2 subtype become infected with it if they remain there for the day. About 10% of birds arrive already exposed to H9N2. The time between a bird being infected with H9N2 and that bird becoming contagious can be less than 5.5 hours.

Preventing transmission

The model also evaluates potential control measures to reduce H9N2 in live bird markets. The findings point to the usefulness of considering multi-pronged interventions, including vaccination strategies, for poultry destined for sale in these markets. “Widespread vaccination of poultry against H9N2 viruses has already been implemented in some Asian countries, such as China and South Korea,” Pinotti explains. “H9-specific vaccines are available in Bangladesh as well but their use appears to be confined to layer and breeder chickens due to the virus’s impact on production. Broiler chickens, which represent the majority of chickens sold in live bird markets, remain largely unvaccinated. Unfortunately, there is little incentive for broiler farmers to vaccinate their birds against H9N2.”

Pinotti notes that eventually, government regulations, as well as training and awareness campaigns, will be required to improve vaccine coverage. “Crucially, an effective vaccine strategy will also require developing an appropriate infrastructure to, (a) closely monitor viral evolution and, (b) update vaccine composition in response to key mutations of the virus,” he says.

Other interventions

The team also identified other interventions besides vaccination, such as early removal/culling of unsold chickens from markets and reducing the number of exposed chickens entering markets by enhancing farmers’ and traders’ compliance with biosecurity practices. One of these practices is banning overnight storage of live poultry at markets. Pinotti explains that these bans have already been implemented in some places in Asia. However, while past research shows this is useful, it’s just a part of what’s needed to eliminate infection completely. This and similar interventions help to disrupt transmission chains within the market but do not prevent frequent viral introductions.

“We have explored how a reduced influx of infected birds would affect viral circulation in the market, but how can we achieve such a reduction in the first place?” Pinotti asks. “Reasonably enough, we would need to target the steps along the poultry supply chains that precede markets, notably farms and upstream transport (i.e., trucks belonging to middlemen and dealers). We believe that improving hygiene conditions and biosecurity in these settings is fundamental in this context. However, we also need to understand and act upon the motivations and incentives behind the actions of stakeholders in the poultry sector.”

Expanding analysis

Going forward, Pinotti and his colleagues aim to expand their analysis to understand better how this virus spreads at the scale of the entire poultry production and distribution system. “To this end, we plan to use a recently developed computer model, EPINEST, that we designed specifically for this task [the first computer tool for tracking viruses and bacteria as they travel through livestock ‘from farm to fork’],” he says. “Taking the extensive empirical data generated by the One Health Poultry Hub as input, this new model generates realistic poultry movements while also enabling epidemic simulations. Ultimately, we will be able to show how production and distribution systems affect pathogen transmission and which veterinary public health interventions would work best.”