NAVLE Nuggets: Viral Poultry Diseases (pt. 1)

Case number one.

Let's start with a flock of chickens.

The producer notices that several birds are becoming weak and uncoordinated.

One bird is sitting with one leg extended forward and the other leg extended backward.

Another bird is having difficulty walking.

At necropsy, you notice that several peripheral nerves are enlarged.

You also find lymphoid tumors in visceral organs.

What disease should immediately come to mind? The answer is Merrick's disease.

Marek's disease is caused by Marek's disease virus.

The virus is a highly contagious alpha herpes virus.

Chickens are the most important natural host.

Here's an important point.

Merrick's disease is primarily spread through dust and dander from infected chickens.

The virus becomes infectious in the feather follicle epithelium, and it can survive for months in poultry house litter and dust.

So once a flock is infected, birds can remain carriers and continue to serve as sources of infection long into the future.

And here's an important NAVLE distinction.

Merrick's disease is not vertically transmitted.

So what is the pathogenesis of Merrick's disease? This is where the disease gets particularly interesting.

The virus has lymphotropic properties and can cause T-cell lymphomas.

The virus can also produce several other syndromes, including transient paralysis, persistent neurologic disease, early mortality, and immunosuppression.

The neoplastic phase is responsible for the classic lymphoid tumors associated with Marek's disease.

So what is the classic clinical presentation? The answer is paralysis.

Affected chickens may become ataxic, weak, paralyzed, and the paralysis can prevent birds from reaching food and water and ultimately result in death.

And here's another major NAVLI clue.

On necropsy, you'll see enlarged peripheral nerves.