Sentinel chickens guard against the spread of mosquito-borne viruses

To the casual viewer, the 15 chickens scratching about inside this coop might look just like that: A flock of ordinary chickens.

But in reality, these chickens are on the frontlines of the battle to stop the spread of mosquito-borne viruses.

The birds are brought to farms when they are just weeks old and every week over the mosquito-breeding season, they are bled to determine whether viruses including Japanese encephalitis, Kunjin virus, and Murray Valley encephalitis, are circulating the area.

"So these viruses are bird viruses," said associate professor Linda Hueston.

"They live their life cycle between mosquitoes and wild birds. It's usually the water birds, but it can be a number of others."

Professor Hueston is the head of the Arbovirus Emerging Diseases Unit at NSW Health Pathology's Institute of Clinical Pathology and Medical Research (ICPMR) labs in Westmead. She is the principal scientist overlooking the sentinel chicken program.

"When the breeding season starts and it's warm and it's wet and there are lots of mosquitoes around, we usually see these viruses have everything they need to circulate," Professor Hueston said.

"And so we pop chickens out, the young chickens, they're only about 12 weeks old, and that's because we want to make sure they're young and never have never had the possibility of seeing these viruses before."

Programs like these have been running since the 1970s, and provide an early warning system for health services to know when potentially life-threatening mosquito-borne viruses are circulating in the community.

"Now in the past, we've only tested for two viruses, Murray Valley encephalitis virus and Kunjin virus," Professor Hueston said.

"But last year we added Japanese encephalitis virus because of the incursion, that's the first time Japanese encephalitis virus has been seen in the southern states of NSW, Victoria, and South Australia."

As well as these sentinel flocks, the health department also throws out a mosquito trap in the area once a week.

"The mosquitoes are sent in here to the lab and they're analysed for the presence of the virus," Professor Hueston said.

"But that tells us only that the virus was present on one night of the week. And if you got the wrong night or there weren't very many mosquitoes in your trap, then you might miss it.

"The mosquitoes that transmit these viruses have a preference for birds, although they will certainly feed on humans. And so once the virus is circulating these birds then are sitting out there, 24/7 being bitten by mosquitoes."

It's estimated each of the 15 birds will be bitten up to a thousand times per night. So, there's a 15x1000 chance of discovering the virus in the flock on any given night for six months of the year.

And the best thing is, the chickens themselves will never develop any symptoms at all.

"They're not they're not causing any damage to the wild birds either," Professor Hueston said.

"It's just when they get into people, humans who have not seen it before or in other production animals that can be successfully infected, that it becomes a problem.

"So nothing happens to the chickens. They just produce antibodies. So if chickens have produced antibodies to Murray Valley, they're protected for life by their antibodies, but they're still able to be infected with contagion and Japanese encephalitis."

When infected, most people will barely notice a change in their health. Some might experience a mild febrile illness, similar to a cold.

But in rare and extreme cases, these diseases can cause severe reactions and death.

Once the birds have completed their six months of life-saving sentinel work, they are retired to live out their lives as ordinary farm chooks.

"They live for many, many years producing eggs for the carers," Professor Hueston said.

"And some of them have become almost like pets. They, all have names and they're very well looked after."