The simple answer is: we don't know.
Urban chickens continue to be in the news, with debate about instituting (or enforcing) bylaws banning or restricting the raising of chickens in backyards running next to articles on how to raise your own backyard flock.
People have various concerns, including:
- Attracting wildlife, including predators such as coyotes
- Infectious diseases
- Animal welfare
But what is the real risk?
The simple answer is: we don’t know.
We know what the concerns are, but how do those correspond to tangible risks to people? That’s the hard part to answer.
A recent paper in the journal Zoonoses and Public Health (Pohjola et al 2016) doesn’t answer the question, but provides some more information.
In the study, the researchers looked at 51 backyard chicken “farms” in Finland in 2012-2013. Ninety-four percent (94%) of these “farms” had less than 50 birds, so probably similar to the urban chicken situation in North America. Cloacal (rectal) samples were collected from 457 chickens from these farms and tested for Salmonella, Listeria, Yersinia and Campylobacter, bacteria that can cause gastrointestinal disease in people (and animals). They also collected environmental samples from the properties and tested them for the same bugs.
Here are the highlights of their findings:
- Campylobacter was found on 45% of farms.
- Listeria was found on 33% of farms.
- Salmonella and Yersinia were isolated on only 1 farm each. Salmonella was only cultured in the environment, not chickens, on the positive farm. However, Salmonella was detected by PCR (which looks for the bacterium’s DNA instead of trying to grow the bugs) in two chickens.
- Overall, 13% of chickens were positive for Campylobacter.
The (reasonable) conclusion was the backyard flocks represent a potential source of pathogens, particularly Campylobacter.
Does that mean they cause disease?
A reasonable question is: why don’t we know how often people get sick from urban chickens?
To answer the question, we need to think about how diseases are diagnosed and reported.
Let’s say I decide to raise some chickens in my backyard. (Very hypothetical since it’s not going to happen. Raising chickens at my place would simply be creating a poultry buffet for coyotes and foxes, but let’s pretend.) If I get a disease from them, who will find out?
Here’s what’s involved if I get Salmonella from my imaginary chickens.
- I have to get sick (the easy part)
- I have to be sick and/or motivated enough to go to a doctor.
- The doctor has to ask for a fecal sample for testing.
- I have to actually collect the sample and drop it off.
- The lab has to grow Salmonella from the sample.
- That gets reported to Public Health.
- Public Health investigates and finds out I have chickens.
Then what? Even if all these steps occur, which happens in the vast minority of cases, will this be tracked?
I would be a single case on a line-listing of people with salmonellosis. Single events like this tend not to attract much attention. Typically, attention is paid to situations where the same strain of Salmonella is isolated from a lot of people or where a lot of people get sick at once. Sporadic disease doesn’t get the same attention (for good reasons). Disease caused by a range of different strains also gets less attention since it’s unlikely to have a common source that needs to be addressed. In the study I talked about, there were 31 distinct Campylobacter types among the 72 samples, showing how much diversity can be present.
So, there could be a lot of people getting infected with Salmonella, Campylobacter or other zoonotic pathogens from backyard chickens. Or there could be none.
Reality is probably somewhere in the middle. Unless someone specifically goes searching for backyard chicken-associated disease (and has access to databases to do this), we won’t know. Even then, without knowing much about how common backyard chickens are, putting results into context is difficult.
If there are four infections identified by a health unit in a summer, what does that mean?
If it’s four infections from 4,000 houses with chickens, that’s different than if it’s four infections from eight houses with chickens.
It’s also much different if it’s four chicken owners vs four neighbours of chicken owners (who didn’t choose to have chickens).
As is common, a lack of good data doesn’t help the decision-making process.
(Reprinted with permission from Worms and Germs Blog.)