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LANCASTER, Pa. — If you see a chicken with a runny nose or conjunctivitis, you might well guess that it has a respiratory illness.

But which one?

Several common chicken diseases — including coryza, infectious bronchitis and infectious laryngotracheitis — have similar symptoms, making them hard to tell apart.

But the treatment for each disease is different, so getting a laboratory diagnosis is important, said Gino Lorenzoni, a Penn State poultry science professor.

Lorenzoni spoke about respiratory diseases at a poultry management meeting on Nov. 13 at the Farm and Home Center.

Infectious bronchitis can affect the kidneys and reproductive system in addition to the upper respiratory system.

It spreads quickly in the house but generally kills few birds.

In addition to the common respiratory symptoms, it can cause rough eggshells and uneven coloration of brown eggs.

If the chickens are infected when they’re very young, the disease can destroy their oviducts. The birds will still ovulate, but they are unable to lay eggs.

As the egg material builds up inside the birds, they become bloated and sit with an unusually upright posture.

“Those animals will not recover, and we’ve seen this in Pennsylvania,” Lorenzoni said.

A bronchitis vaccine usually has to be matched to the specific strain of the disease to be effective, but the pathogen is fairly easy to kill in the environment.

A standard cleaning program — wash, disinfect and dry — should do the trick, Lorenzoni said.

Unlike bronchitis, infectious laryngotracheitis spreads slowly and may only produce symptoms when triggered by stress.

In severe cases, sick birds will spit blood and gasp for air.

But in many cases, birds will exhibit only mild symptoms like facial swelling, tracheitis, almond-shaped eyes and conjunctivitis — nothing distinctive enough to make a diagnosis.

Laryngotracheitis, a type of herpes virus, can be inactivated in the environment by disinfecting, or by heating the house to 100 degrees for three days.

Laryngotracheitis vaccines are available, but the chicken-embryo-origin version can become pathogenic and attack other birds, Lorenzoni said.

That isn’t a concern, though, for most coryza vaccines, which use dead samples of the pathogen.

Coryza, which has mushroomed into a problem for Pennsylvania farms over the past year, is typically most serious in older birds.

Like the other diseases, coryza is easy to kill when it’s outside the chicken, but it’s at its strongest when causing disease.

“It gets protected by the environment that the chicken is offering,” Lorenzoni said.

Coryza can cause conjunctivitis, coughing, and a drop in egg production of 30 to 50%. It can also produce airsacculitis, which leads to broiler carcasses being condemned.

A drop in feed and water consumption is often the first sign of the disease that people notice.

When coryza is suspected, it’s best to send the whole bird, or at least the head, to the lab. Select an animal in the early stages of the disease, Lorenzoni said.

Coryza will spread widely in the flock, but it usually doesn’t kill many birds unless another disease is present.

Poor environmental conditions, such as dust and ammonia, can worsen the symptoms, Lorenzoni said.

Treatment efficacy varies by strain.

A coryza vaccine made for one strain in classification group A is generally effective against other strains in that group, but a vaccine based on one group C strain will be less effective against other C strains.