Microbes that cause disease in humans can travel thousands of miles on high-level winds, scientists have revealed for the first time.
The winds studied carried a surprising diversity of bacteria and fungi, including known pathogens and, some with genes for resistance to multiple antibiotics. Some of the microbes were shown to be alive – in other words, they had survived the long journey and were able to replicate.
The researchers said this intercontinental transport route was unlikely to cause disease in people directly, because the concentration of microbes was low. However, they said it was a cause of concern that microbes could be seeded into new environments and that antibiotic-resistance genes could travel in this way.
The study showed the microbes hitched a 1,200-mile (2,000km) ride on dust particles blown from farm fields in north-eastern China to Japan. Similar patterns of winds exist around the world. More than 300 types of bacteria and about 260 types of fungi were found in the samples collected over Tokyo. Other microbes not yet known to science are thought to be present.
Prof Xavier Rodó at the Barcelona Institute for Global Health, who led the research team, said: “Around 30-40% of the microbes were potentially pathogenic species, either well-recognised human pathogens or opportunistic pathogens [which affect people with weakened immune systems].”
The study “is a word of caution that we should be changing our view of the air”, Rodó added, particularly the idea that air at higher altitudes is almost sterile.
“We should be making use of the new methods to take samples and see what’s there. These bacteria and fungi are capable of withstanding very high and strong conditions in the environment.”
The analysis, published in the journal Proceedings of the National Academy of Sciences, used an aircraft to collect 22 samples of dust from the air between 0.6 and 1.9 miles above Japan. The higher samples were above the planetary boundary layer (PBL), the atmospheric layer closest to the ground. The winds above the PBL travel faster and further, as they are not slowed by friction with the ground.
Analysis of the long-range air currents on the days the samples were collected, combined with chemical analysis, showed the dust particles had travelled 1,243 miles and came from China.
The chemical analyses of the samples showed signatures characteristic of agricultural areas, including animal manure, pesticides and fertilisers, and also rare elements such as zirconium and hafnium, which are mined in that part of China.
The microbes were embedded in the particles, which protected them from ultraviolet light and dehydration, allowing some to remain viable. The human pathogen species included bacteria such as E coli, Staphylococcus saprophyticus and Clostridium difficile.
Rodó said the study team was surprised to find so many different microbes, as the initial aim of the research was to analyse the chemistry of the dust particles. Such particles can reach the ground by falling or in raindrops.
“The identification of pathogenic organisms above the PBL indicates that large portions of the troposphere can become potential reservoirs and act as long-distance spreaders of a rich variety of microbes,” the researchers concluded.
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Rodó said: “We are talking about ultra-low concentrations, and in most cases they wouldn’t elicit infection. But we cannot rule that out in immunocompromised individuals.”
Viable bacteria and fungi have been shown to travel long distances in soil dust before, for example from Africa to the Caribbean. But the researchers said: “The isolation of harmful species to humans had never been reported before for such long distances [until now].”
Dr Allen Haddrell, at the University of Bristol, UK, who was not part of the research team, said: “Many studies have reported antimicrobial-resistance genes in the air. The [new study] shows there is a physical means by which AMR genes can spread extremely long distances.
“Moreover, the genes are transported in living organisms so they have a greater likelihood of passing once the aerosol settles. Over the long term, this is going to be a problem.” Many experts have warned that antibiotic resistance is a grave threat to humanity.
Prof Chris Thomas, at the University of Birmingham, UK, said: “The chances of getting an infectious dose [via high-level winds] must be considerably less than when encountering an infected person on an aeroplane, or even just going on holiday to a foreign country. The study also implies that the dust and chemicals transported in the air may be more harmful [as air pollution] than the microbes.”