Could wildfire smoke spread infectious diseases?

Wildfire smoke has the potential to carry and spread infectious bacteria and fungi up to “hundreds of miles”, but more research needs to be done to understand the risk, write two US researchers.

Their article in Science says wildfires can allow for pockets where microbes can survive the heat, and could potentially travel “hundreds of miles” depending on the fire and atmospheric conditions at play. Given that wildfire seasons are getting longer and more severe, the authors write that more research needs to be done to understand how far and which microbes are transported by smoke under different conditions.

The SMC asked experts to comment on the paper.

Professor George Perry, School of Environment, University of Auckland, comments:

“Large wildfire events have significant public health costs. For example, the estimated health-related cost of the 2019-2020 Australian fire season was AUD $1.95 billion, driven by respiratory illnesses and premature mortality.

“In their study, Kobziar & Thompson describe a novel health issue associated with wildfire: the spread of infectious microbes (bacteria and fungi). In a trial simulation, they demonstrate that microbes could potentially spread tens of kilometres with smoke as well as potentially being entrained in connective updrafts beyond the fire zone.

“Few studies have directly measured microbial load associated with smoke, but one that has reported microbial concentrations five-times higher than background levels. As fire activity changes rapidly across the planet, there has been considerable focus on the direct effects – loss of ecosystems and infrastructure – but the indirect effects of fire, such as the health costs, have the potential to be as socially and economically severe.”

No conflicts of interest

Dr Joel Rindelaub, Aerosol Chemist, University of Auckland, comments:

“This is an interesting study that reinforces the hazards of breathing in smoke from forest fires. We already know that such smoke contains high risk particulate matter containing cancer-causing chemicals, and exposure to this smoke can lead to a range of negative health effects that impact lung, heart, and immune system function. Adding tiny microbes to the mix makes this airborne soup all the more toxic.

“Finding living fungi and bacteria in wildfire smoke isn’t completely unsurprising, as we already know that they are present in the atmosphere. For instance, these types of particles make up 20% of the particulate matter in the sky, and up to 75% of the particles above forests.

“What’s more surprising is that not only do these tiny living creatures survive in smoke, but the smoke itself can act like an airborne fishing net that collects microbes and transports them hundreds of kilometers.

“As climate change continues, we should expect more bush fires and more exposure to these types of harmful species.”

No conflicts of interest

Dr Tara Strand, General Manager Forests and Landscapes, Scion, comments:

“This is an interesting article. I’m glad to see the demonstrated link between viable microbes and wildland fire. It relates to biosecurity.

“Plant disease and insect outbreaks in forest ecosystems have often been associated with wildfire as a cascade effect, occurring after wildfire. For our own part of the world, vast tracts of Australian forests are regenerating from last summer’s wildfires, providing the right conditions for the pandemic myrtle rust (Austropuccinia psidii) to flourish. The new branches and seedlings provide myrtle rust with its favourite place to grow. This pandemic myrtle rust has proven devastating to Australian indigenous myrtaceae plant species and it is here in New Zealand attacking our indigenous flora, including the iconic pohutukawa. This myrtle rust arrived here on the wind – blowing from Australia to New Zealand.  A large outbreak in Australia is concerning as a large load of viable pathogens could hit New Zealand, increasing New Zealand’s outbreak, or it could come ashore in a new location.

“As the authors point out, for human health, particulate matter (2.5 micrometers and smaller, PM2.5) is known to cause health effects even at low levels of exposure. Essentially, the particulates irritate the lung tissue and, at 2.5 micrometers and smaller, these particles make it past all our natural defences, like hairs in our nose, and into our blood stream. When exposure of PM2.5 is combined with a virus, i.e., COVID, then there is a combined stress. This past summer in North America, there was a high level of alert and concern about the double exposure of smoke and COVID among firefighters, as well as communities experiencing wildfire smoke events.

“The demonstrated link between emissions of viable microbes and wildland fire and their transport within smoke plumes is of concern for biosecurity reasons. One of the primary tools in a biosecurity response to a pathogen attack is to remove and burn. To date, the majority of these burns have been in the open atmosphere. This begs the question of, when doing a remove-and-burn biosecurity response, what is the sensitivity of these pathogens to heat, and do they remain viable, and what is their inoculum load, in the smoke plume?

“New Zealand is tracking with the rest of the world in increased wildfires and increased occurrence of highly impactful wildfires (house loss, evacuations). Over the last five to six years, New Zealand has also seen an increase in extreme wildfire behaviour, rapid fire spread, spotting and fire whirls. This type of fire spread makes it very difficult for responders to slow and contain and often results in an impactful wildfire.

“This year has already seen a wildfire in winter, significant house loss, and a small evacuation of an urban neighbourhood (recent Port Hills fire).  The conditions for wildfires are not at their worst; the heart of the summer is still to come.  (The conditions we look at for wildfire risk are dryness of fuels, drought, temperature, and atmospheric moisture content along with wind, terrain and fuel type.)

“Unfortunately, unless climatic conditions change, going forward New Zealand is likely to see an increase of wildfire impacts upon small rural communities and, as we see overseas, on suburban neighbourhoods at the interface of large urban environments.

“We have an opportunity now to weave together New Zealand’s many knowledge sources (governance, science, mātauranga, operations, immigrant experience with wildfire, etc.) to bring forward solutions to mitigate and prepare for this type of risk.”

No conflicts of interest

Dr Richard Winkworth, Senior Lecturer in Plant Genetics, Massey University, comments:

“This paper focuses on the human health aspects of pathogens being spread as the result of wildfires. Yet, there is potentially a much wider range of impacts. If human pathogens are capable of being aerosolised and carried hundreds of kilometers, then those affecting our native biota or primary production could also be spread this way.

“The recent arrival of myrtle rust in New Zealand is attributed to natural dispersal from Australia, so we already know that pathogens can be carried across the Tasman Sea on air currents. Although, as the authors point out, there are still many unknowns, anything that increases the chances of pathogens finding their way into the air currents flowing from Australia to New Zealand can only increase the potential for further incursions.”

No conflicts of interest