London School of Hygiene & Tropical Medicine

Twice a year, millions of birds traverse our planet in search of food and better nesting conditions. Although avian migration is a spectacular event, an increasing number of studies indicate that these birds may be carrying unwanted guests on their travels- antimicrobial resistant bacteria. Antimicrobial resistance (AMR) is a property of bacteria which develops as they evolve to survive drugs once effective at killing them. The threat of AMR is difficult to overstate- the WHO have described it as a ‘silent pandemic’ with the potential to cause thirty-nine million deaths by 2050, as common and once treatable infections become fatal.

Resistant bacteria were first identified in wildlife in 1975 with the detection of resistant Escherichia coli (more commonly known as E-coli) within pigeons. More recent studies have identified migratory birds as potential hosts of a range of resistant bacteria, including Salmonella and the particularly dangerous Staphylococcus aureus- responsible for life-threatening methicillin-resistant (MRSA) infections- among other bacterial species.

Although AMR occurs in the environment naturally, human activities are increasing its prevalence and therefore the likelihood of birds picking up these bacteria during migration. Farmlands in particular can act as AMR hotspots; antibiotics are frequently prescribed to farmed animals to prevent disease and promote growth, leading animals to excrete a proportion of these antibiotics unmetabolized. Antibiotic residues can then enter soil and water, exposing existing bacteria and giving them an opportunity to develop resistance. As migratory birds often utilise farmlands as food and water sources, they may either ingest these resistant bacteria or pick them up on their feet or feathers.

Farmlands are certainly not the only source of resistant bacteria for wild birds, however; landfills and water treatment plants, particularly those receiving hospital waste, are other key sites. Additionally, a rise in ecotourism- nature-based activities such as hiking and bird watching- bring opportunities for humans to introduce new bacterial strains into previously untouched habitats.

Genomic research supports this idea. Scientists have analysed bacterial samples from wildlife, and found strains that are genetically very similar, or identical, to those in suspected contaminated environments. While these findings certainly suggest strong links, they cannot always indicate the direction of transmission- that is, whether birds are acquiring bacteria from these environments, introducing the bacteria themselves, or a combination of both.

Once birds acquire these bacteria, there is evidence they can survive on or inside of these birds’ bodies for long enough to be deposited far from their original sources. Birds can shed bacteria through excretion, or through direct contact with new environments. In some cases, this contamination may pose a direct threat to humans- if bird droppings containing resistant bacteria land on crops or in rivers, for example, humans may be directly exposed. Additionally, migratory birds often interact with resident birds at shared food and water sources, potentially increasing AMR prevalence in resident species like pigeons who live in close contact with humans.

So, how do we go about tackling this problem? Encouraging changes in human behaviour, such as responsible antibiotic use in farmed animals and proper waste management to reduce environmental contamination, could be effective in preventing wild birds from picking up resistant bacteria. Beyond this, wider improvements in AMR monitoring are needed, since wildlife is consistently overlooked in global health security plans and AMR surveillance. Existing surveillance must expand to include migratory birds within its scope, such as through the inclusion of satellite-tracking of migration routes and bacterial sampling of birds in transmission hotspots. This will be crucial in allowing scientists to identify emerging resistance patterns. Since climate change has been indicated to affect the migration routes of various bird species, AMR surveillance must be ready to adapt.

Addressing the issue will ultimately require collaboration across multiple disciplines, including epidemiology, veterinary medicine, and conservation science. Understanding the role of migratory birds in AMR bacteria spread may be a crucial step towards tackling one of the most urgent public health challenges of our lifetimes.