How humans started a bacterial pandemic in chickens
The prospect of infections spreading from animals to humans has become all too real with the onset of the current swine flu pandemic, and the threat of a bird flu still looming. But infections can jump the other way too. Decades before the world’s media were gripped with panic over bird flu, humans transferred a disease to chickens and it has since caused a poultry pandemic right under our noses.
The infection in question is a familiar one – Staphylococcus aureus, a common human bacterium that’s behind everything from mild skin infections to life-threatening MRSA. It causes chicken diseases too, including septic arthritis and ‘bumblefoot‘. But in the 1970s, broiler chickens began developing a new type of S.aureus infection called ‘bacterial chrondronecrosis with osteomyelitis’ or, more simply, BCO. It’s a bone infection and it’s a major cause of lameness in broiler chickens.
This new disease had human origins. Bethan Lowder from the University of Edinburgh has shown that all of the bacteria behind BCO share a common ancestor, which jumped from humans to chickens in Poland, around 38 years ago. From that point on, the bacterium’s travel itinerary was set. Just as air travel has facilitated the spread of swine flu among humans, a global distribution network for chickens made it easy for S.aureus to spread all over the world aboard its new feathery hosts.
Lowder traced the common ancestry of S.aureus in chickens by analysing the genes of 57 samples. Of these, 48 came from healthy and diseased chickens across eight countries and four continents, and 9 were taken from different species of wild and domesticated birds. Amazingly, she found that two-thirds of all the broiler chicken samples came from a single strain of the bacterium called ST5.
ST5 infects humans all over the world and is one of the most successful strains of S.aureus to do so. But Lowder found that all of the chicken samples were more closely related to each other than they were to any of the human bacteria from the same strain. They all shared a common ancestor – a lineage of ST5 found only in Poland. Around 38 years ago, this pioneering bacterium made the leap from humans to chickens and its descendants have spread from Poland to countries as far as the US and Japan.
Since then, the ST5 strain has adapted to its new host. It has lost many of the genes it needs to cause disease in humans but it has picked up others that allow it to better infect chickens. A complete sequence of the bacterium’s genome reveals that since its human days, it has picked up five new genes from other bird sources, none of which are found in humans or other mammals. In fact, Lowder thinks that the ST5 strain may be particularly good at picking up mobile genes from other sources. That might explain why both human and chicken versions are so successful, and why the human one often picks up genes that allow it to shrug off powerful antibiotics.
It’s not clear how exactly these changes benefit the bacteria, but certainly, they’re much better at resisting a chicken’s immune system than their human predecessors. When faced with chicken heterophils – a type of white blood cell – the poultry strains were much more likely to survive than the human equivalents.
Lowder thinks that globalisation was the key to the new pandemic. In just the last fifty years, the broiler chicken industry has shifted from one dominated by small farms to a multi-billion dollar leviathan controlled by a small number of multinationals. These companies transport a relatively few breeding lines of chickens all over the world, and the low genetic diversity of these birds makes them vulnerable to infections as opportunistic as S.aureus.
She recommends that livestock are screened regularly so that emerging diseases can be picked up, and that stocks should often be cleansed of S.aureus, to nip potential new threats in the bud. Better regulations for international transport wouldn’t go amiss either – it’s no surprise that Australia, a country with stringent regulations on importing livestock, has no trace of the pandemic S.aureus strain.
Reference: PNAS: 10.1073/pnas.0909285106
More on bacteria:
- Space flight turns Salmonella into super-bug
- The bacterial zoo living on your skin
- MRSA in pigs and pig farmers
- Top Ten Bacteria
- An ecosystem of one in the depths of a gold mine
- When bacteria merge – two species are turning into one
Go Further
Animals
- Octopuses have a lot of secrets. Can you guess 8 of them?
- Animals
- Feature
Octopuses have a lot of secrets. Can you guess 8 of them? - This biologist and her rescue dog help protect bears in the AndesThis biologist and her rescue dog help protect bears in the Andes
- An octopus invited this writer into her tank—and her secret worldAn octopus invited this writer into her tank—and her secret world
- Peace-loving bonobos are more aggressive than we thoughtPeace-loving bonobos are more aggressive than we thought
Environment
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- U.S. plans to clean its drinking water. What does that mean?U.S. plans to clean its drinking water. What does that mean?
- Food systems: supporting the triangle of food security, Video Story
- Paid Content
Food systems: supporting the triangle of food security - Will we ever solve the mystery of the Mima mounds?Will we ever solve the mystery of the Mima mounds?
- Are synthetic diamonds really better for the planet?Are synthetic diamonds really better for the planet?
- This year's cherry blossom peak bloom was a warning signThis year's cherry blossom peak bloom was a warning sign
History & Culture
- Strange clues in a Maya temple reveal a fiery political dramaStrange clues in a Maya temple reveal a fiery political drama
- How technology is revealing secrets in these ancient scrollsHow technology is revealing secrets in these ancient scrolls
- Pilgrimages aren’t just spiritual anymore. They’re a workout.Pilgrimages aren’t just spiritual anymore. They’re a workout.
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- This ancient cure was just revived in a lab. Does it work?This ancient cure was just revived in a lab. Does it work?
- See how ancient Indigenous artists left their markSee how ancient Indigenous artists left their mark
Science
- Jupiter’s volcanic moon Io has been erupting for billions of yearsJupiter’s volcanic moon Io has been erupting for billions of years
- This 80-foot-long sea monster was the killer whale of its timeThis 80-foot-long sea monster was the killer whale of its time
- Every 80 years, this star appears in the sky—and it’s almost timeEvery 80 years, this star appears in the sky—and it’s almost time
- How do you create your own ‘Blue Zone’? Here are 6 tipsHow do you create your own ‘Blue Zone’? Here are 6 tips
- Why outdoor adventure is important for women as they ageWhy outdoor adventure is important for women as they age
Travel
- This royal city lies in the shadow of Kuala LumpurThis royal city lies in the shadow of Kuala Lumpur
- This author tells the story of crypto-trading Mongolian nomadsThis author tells the story of crypto-trading Mongolian nomads
- Slow-roasted meats and fluffy dumplings in the Czech capitalSlow-roasted meats and fluffy dumplings in the Czech capital